v3.26.3
Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($)
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Segment Reporting [Line Items]        
Earned insurance premiums $ 517,700,000 $ 557,800,000   $ 1,328,000,000
Earned insurance premiums     $ 1,287,200,000  
Net investment income 70,600,000 59,200,000 212,900,000 180,200,000
Net investment income     212,900,000  
Net realized and unrealized investment gains (losses) (123,000,000.0) 83,900,000 31,600,000 251,200,000
Interest income from BAM Surplus Notes 6,800,000 7,500,000 20,600,000 22,500,000
Change in fair value of BAM Surplus Notes (16,100,000)   (16,100,000) 0
Commission and fee revenues 59,300,000 82,300,000   193,700,000
Commission and fee revenues     162,900,000  
Product and service revenues 121,100,000 68,800,000 285,100,000 138,700,000
Net gain on sale of the Bamboo Group     2,400,000 0
Other revenues 4,700,000 4,700,000 16,900,000 16,900,000
Total revenues 641,100,000 864,200,000 2,003,500,000 2,131,200,000
Loss and LAE 230,400,000 258,500,000   686,800,000
Loss and LAE     613,100,000  
Insurance acquisition expenses 115,300,000 111,800,000   308,400,000
Insurance acquisition expenses     323,600,000  
Cost of sales 91,800,000 55,300,000 214,600,000 105,200,000
Broker commission expenses 22,000,000.0 27,000,000.0 62,500,000 62,300,000
Amortization of other intangible assets 12,500,000 7,800,000 30,800,000 18,100,000
General and administrative expenses 141,600,000 169,000,000.0 420,100,000 392,200,000
Change in fair value of contingent consideration 30,000,000.0 36,100,000 71,700,000 74,200,000
Interest expense 20,700,000 19,900,000 59,800,000 56,300,000
Total expenses 664,300,000 685,400,000 1,796,200,000 1,703,500,000
Pre-tax income (loss) (23,200,000) 178,800,000 207,300,000 427,700,000
Product and service revenues        
Segment Reporting [Line Items]        
Product and service revenues 121,100,000 68,800,000 285,100,000 138,700,000
Other Operations        
Segment Reporting [Line Items]        
Earned insurance premiums 300,000 2,900,000   19,100,000
Earned insurance premiums     4,400,000  
Net investment income 13,500,000 6,200,000 42,500,000 24,500,000
Net investment income     42,500,000  
Interest income from BAM Surplus Notes 0 0 0 0
Change in fair value of BAM Surplus Notes 0   0  
Commission and fee revenues 4,100,000 4,300,000   12,400,000
Commission and fee revenues     11,500,000  
Net gain on sale of the Bamboo Group 0 0 2,400,000 0
Other revenues 0 500,000 (100,000) 1,000,000.0
Total revenues 4,000,000.0 51,300,000 146,800,000 150,000,000.0
Loss and LAE 300,000 600,000   18,800,000
Loss and LAE     700,000  
Insurance acquisition expenses 100,000 1,000,000.0   7,000,000.0
Insurance acquisition expenses     1,700,000  
Cost of sales 7,700,000 8,200,000 21,800,000 22,900,000
Broker commission expenses 0 0 0 0
Amortization of other intangible assets 1,000,000.0 1,300,000 2,900,000 3,400,000
General and administrative expenses 26,900,000 63,300,000 112,600,000 141,100,000
Change in fair value of contingent consideration 0 0 0 0
Interest expense 700,000 700,000 2,000,000.0 1,700,000
Total expenses 36,700,000 75,100,000 141,700,000 194,900,000
Pre-tax income (loss) (32,700,000) (23,800,000) 5,100,000 (44,900,000)
Other Operations | Product and service revenues        
Segment Reporting [Line Items]        
Product and service revenues 14,300,000 14,800,000 41,800,000 41,900,000
Investments, Excluding MediaAlpha        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) (69,400,000) 76,200,000 92,000,000.0 249,600,000
Investments, Excluding MediaAlpha | Other Operations        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) 25,400,000 14,900,000 104,700,000 49,500,000
MediaAlpha        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) (53,600,000) 7,700,000 (60,400,000) 1,600,000
MediaAlpha | Other Operations        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) (53,600,000) 7,700,000 (60,400,000) 1,600,000
P&C Insurance and Reinsurance (Ark/WM Outrigger)        
Segment Reporting [Line Items]        
Loss and LAE 230,100,000 254,600,000 612,400,000 652,100,000
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments        
Segment Reporting [Line Items]        
Earned insurance premiums 508,800,000 542,200,000 1,258,800,000 1,264,400,000
Net investment income 27,800,000 27,200,000 84,500,000 77,000,000.0
Net realized and unrealized investment gains (losses) (40,300,000) 17,800,000 (42,700,000) 98,400,000
Loss and LAE 230,100,000 254,600,000 612,400,000 652,100,000
Insurance acquisition expenses 113,100,000 106,800,000 315,600,000 287,500,000
Change in fair value of contingent consideration 30,000,000.0 36,100,000 71,700,000 74,200,000
Interest expense 4,200,000 4,400,000 12,400,000 12,900,000
Total expenses 436,900,000 463,600,000 1,163,500,000 1,180,800,000
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Operating Segments        
Segment Reporting [Line Items]        
Earned insurance premiums 509,600,000 498,600,000   1,201,700,000
Earned insurance premiums     1,256,100,000  
Net investment income 27,200,000 24,800,000   70,200,000
Net investment income     81,500,000  
Interest income from BAM Surplus Notes 0 0 0 0
Change in fair value of BAM Surplus Notes 0   0  
Commission and fee revenues 0 0   0
Commission and fee revenues     0  
Net gain on sale of the Bamboo Group     0  
Other revenues 4,500,000 2,300,000 16,100,000 10,800,000
Total revenues 501,000,000.0 543,500,000 1,311,100,000 1,381,200,000
Loss and LAE 230,100,000 252,400,000   628,000,000.0
Loss and LAE     612,200,000  
Insurance acquisition expenses 113,400,000 92,400,000   272,000,000.0
Insurance acquisition expenses     314,700,000  
Cost of sales 0 0 0 0
Broker commission expenses 0 0 0 0
Amortization of other intangible assets 0 0 0 0
General and administrative expenses 59,500,000 61,700,000 151,400,000 154,000,000.0
Change in fair value of contingent consideration 30,000,000.0 36,100,000 71,700,000 74,200,000
Interest expense 4,200,000 4,400,000 12,400,000 12,900,000
Total expenses 437,200,000 447,000,000.0 1,162,400,000 1,141,100,000
Pre-tax income (loss) 63,800,000 96,500,000 148,700,000 240,100,000
Other underwriting expenses 40,400,000 33,600,000 111,600,000 108,400,000
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Operating Segments | Product and service revenues        
Segment Reporting [Line Items]        
Product and service revenues 0 0 0 0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Investments, Excluding MediaAlpha | Operating Segments        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) (40,300,000) 17,800,000 (42,600,000) 98,500,000
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | MediaAlpha | Operating Segments        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) 0 0 0 0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | Operating Segments        
Segment Reporting [Line Items]        
Earned insurance premiums (800,000) 43,600,000   62,700,000
Earned insurance premiums     2,700,000  
Net investment income 600,000 2,400,000   6,800,000
Net investment income     3,000,000.0  
Interest income from BAM Surplus Notes 0 0 0 0
Change in fair value of BAM Surplus Notes 0   0  
Commission and fee revenues 0 0   0
Commission and fee revenues     0  
Net gain on sale of the Bamboo Group     0  
Other revenues 0 0 0 0
Total revenues (200,000) 46,000,000.0 5,600,000 69,400,000
Loss and LAE 0 2,200,000   24,100,000
Loss and LAE     200,000  
Insurance acquisition expenses (300,000) 14,400,000   15,500,000
Insurance acquisition expenses     900,000  
Cost of sales 0 0 0 0
Broker commission expenses 0 0 0 0
Amortization of other intangible assets 0 0 0 0
General and administrative expenses 0 0 0 100,000
Change in fair value of contingent consideration 0 0 0 0
Interest expense 0 0 0 0
Total expenses (300,000) 16,600,000 1,100,000 39,700,000
Pre-tax income (loss) 100,000 29,400,000 4,500,000 29,700,000
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | Operating Segments | Product and service revenues        
Segment Reporting [Line Items]        
Product and service revenues 0 0 0 0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | Investments, Excluding MediaAlpha | Operating Segments        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) 0 0 (100,000) (100,000)
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | MediaAlpha | Operating Segments        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) 0 0 0 0
Asset Management (Kudu) | Operating Segments        
Segment Reporting [Line Items]        
Earned insurance premiums 0 0   0
Earned insurance premiums     0  
Net investment income 20,700,000 18,000,000.0 60,500,000 56,700,000
Net investment income     60,500,000  
Net realized and unrealized investment gains (losses) (25,700,000) 36,100,000 66,000,000.0 80,900,000
Interest income from BAM Surplus Notes 0 0 0 0
Change in fair value of BAM Surplus Notes 0   0  
Commission and fee revenues 0 0   0
Commission and fee revenues     0  
Net gain on sale of the Bamboo Group     0  
Other revenues 0 300,000 400,000 1,000,000.0
Total revenues (5,000,000.0) 54,400,000 126,900,000 138,600,000
Loss and LAE 0 0   0
Loss and LAE     0  
Insurance acquisition expenses 0 0   0
Insurance acquisition expenses     0  
Cost of sales 0 0 0 0
Broker commission expenses 0 0 0 0
Amortization of other intangible assets 0 0 100,000 200,000
General and administrative expenses 2,900,000 4,700,000 11,700,000 12,100,000
Change in fair value of contingent consideration 0 0 0 0
Interest expense 7,400,000 6,100,000 21,700,000 18,600,000
Total expenses 10,300,000 10,800,000 33,500,000 30,900,000
Pre-tax income (loss) (15,300,000) 43,600,000 93,400,000 107,700,000
Asset Management (Kudu) | Operating Segments | Product and service revenues        
Segment Reporting [Line Items]        
Product and service revenues 0 0 0 0
Asset Management (Kudu) | Investments, Excluding MediaAlpha | Operating Segments        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) (25,700,000) 36,100,000 66,000,000.0 80,900,000
Asset Management (Kudu) | MediaAlpha | Operating Segments        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) 0 0 0 0
HG Global | Operating Segments        
Segment Reporting [Line Items]        
Earned insurance premiums 8,600,000 7,500,000 24,000,000.0 22,800,000
Earned insurance premiums     24,000,000.0  
Net investment income 7,900,000 7,000,000.0 23,400,000 19,800,000
Net investment income     23,400,000  
Net realized and unrealized investment gains (losses) (28,800,000) 7,200,000 (36,000,000.0) 20,300,000
Interest income from BAM Surplus Notes 6,800,000 7,500,000 20,600,000 22,500,000
Change in fair value of BAM Surplus Notes (16,100,000) 0 (16,100,000) 0
Commission and fee revenues 0 0   0
Commission and fee revenues     0  
Net gain on sale of the Bamboo Group     0  
Other revenues 100,000 100,000 200,000 200,000
Total revenues (21,500,000) 29,300,000 16,100,000 85,600,000
Loss and LAE 0 0 0 0
Loss and LAE     0  
Insurance acquisition expenses 2,100,000 2,000,000.0 6,300,000 5,900,000
Insurance acquisition expenses     6,300,000  
Cost of sales 0 0 0 0
Broker commission expenses 0 0 0 0
Amortization of other intangible assets 0 0 0 0
General and administrative expenses 400,000 1,000,000.0 5,300,000 2,600,000
Change in fair value of contingent consideration 0 0 0 0
Interest expense 3,800,000 4,300,000 11,100,000 13,400,000
Total expenses 6,300,000 7,300,000 22,700,000 21,900,000
Pre-tax income (loss) (27,800,000) 22,000,000.0 (6,600,000) 63,700,000
HG Global | Operating Segments | Product and service revenues        
Segment Reporting [Line Items]        
Product and service revenues 0 0 0 0
HG Global | Investments, Excluding MediaAlpha | Operating Segments        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) (28,800,000) 7,200,000 (36,000,000.0) 20,300,000
HG Global | MediaAlpha | Operating Segments        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) 0 0 0 0
P&C Insurance Distribution (Bamboo) | Operating Segments        
Segment Reporting [Line Items]        
Earned insurance premiums 0 5,200,000 0 21,700,000
Net investment income   600,000   2,000,000.0
Interest income from BAM Surplus Notes   0   0
Commission and fee revenues 0 63,900,000 0 167,200,000
Other revenues   1,500,000   3,900,000
Total revenues   71,400,000   195,300,000
Loss and LAE 0 3,300,000 0 15,900,000
Insurance acquisition expenses 0 2,000,000.0 0 8,000,000.0
Cost of sales   0   0
Broker commission expenses 0 21,500,000 0 56,800,000
Amortization of other intangible assets   4,000,000.0   12,000,000.0
General and administrative expenses   22,800,000   57,400,000
Change in fair value of contingent consideration   0   0
Interest expense 0 2,800,000 0 7,800,000
Total expenses 0 56,400,000 0 157,900,000
Pre-tax income (loss)   15,000,000.0   37,400,000
P&C Insurance Distribution (Bamboo) | Operating Segments | Product and service revenues        
Segment Reporting [Line Items]        
Product and service revenues   0   0
P&C Insurance Distribution (Bamboo) | Investments, Excluding MediaAlpha | Operating Segments        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses)   200,000   500,000
P&C Insurance Distribution (Bamboo) | MediaAlpha | Operating Segments        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses)   0   0
Distinguished | Operating Segments        
Segment Reporting [Line Items]        
Earned insurance premiums 0 0    
Earned insurance premiums     0 0
Net investment income 700,000 200,000    
Net investment income     2,000,000.0 200,000
Interest income from BAM Surplus Notes 0 0 0 0
Change in fair value of BAM Surplus Notes 0   0  
Commission and fee revenues 55,200,000 14,100,000 151,400,000 14,100,000
Commission and fee revenues     151,400,000 14,100,000
Net gain on sale of the Bamboo Group     0  
Total revenues 56,000,000.0 14,300,000 153,700,000 14,300,000
Loss and LAE 0 0    
Loss and LAE     0 0
Insurance acquisition expenses 0 0    
Insurance acquisition expenses     0 0
Cost of sales 0 0 0 0
Broker commission expenses 22,000,000.0 5,500,000 62,500,000 5,500,000
Amortization of other intangible assets 6,400,000 0 20,200,000 0
General and administrative expenses 36,900,000 10,200,000 101,300,000 10,200,000
Change in fair value of contingent consideration 0 0 0 0
Interest expense 3,400,000 1,300,000 10,500,000 1,300,000
Total expenses 68,700,000 17,000,000.0 194,500,000 17,000,000.0
Pre-tax income (loss) (12,700,000) (2,700,000) (40,800,000) (2,700,000)
Distinguished | Operating Segments | Product and service revenues        
Segment Reporting [Line Items]        
Product and service revenues 0 0 0 0
Distinguished | Investments, Excluding MediaAlpha | Operating Segments        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) 0 0 0 0
Distinguished | MediaAlpha | Operating Segments        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) 0 0 0 0
Distinguished, net | Operating Segments        
Segment Reporting [Line Items]        
Other revenues 100,000 0 300,000 0
WTM Partners | Operating Segments        
Segment Reporting [Line Items]        
Earned insurance premiums 0 0   0
Earned insurance premiums     0  
Net investment income 0 0   0
Net investment income     0  
Interest income from BAM Surplus Notes 0 0 0 0
Change in fair value of BAM Surplus Notes 0   0  
Commission and fee revenues 0 0   0
Commission and fee revenues     0  
Net gain on sale of the Bamboo Group     0  
Other revenues 0 0 0 0
Total revenues 106,800,000 54,000,000.0 243,300,000 96,800,000
Loss and LAE 0 0   0
Loss and LAE     0  
Insurance acquisition expenses 0 0   0
Insurance acquisition expenses     0  
Cost of sales 84,100,000 47,100,000 192,800,000 82,300,000
Broker commission expenses 0 0 0 0
Amortization of other intangible assets 5,100,000 2,500,000 7,600,000 2,500,000
General and administrative expenses 15,000,000.0 5,300,000 37,800,000 14,700,000
Change in fair value of contingent consideration 0 0 0 0
Interest expense 1,200,000 300,000 2,100,000 600,000
Total expenses 105,400,000 55,200,000 240,300,000 100,100,000
Pre-tax income (loss) 1,400,000 (1,200,000) 3,000,000.0 (3,300,000)
WTM Partners | Operating Segments | Product and service revenues        
Segment Reporting [Line Items]        
Product and service revenues 106,800,000 54,000,000.0 243,300,000 96,800,000
WTM Partners | Investments, Excluding MediaAlpha | Operating Segments        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) 0 0 0 0
WTM Partners | MediaAlpha | Operating Segments        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) $ 0 $ 0 $ 0 $ 0