v3.26.1
Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Segment Reporting [Line Items]        
Earned insurance premiums $ 384,600,000 $ 375,200,000   $ 770,200,000
Earned insurance premiums     $ 769,500,000  
Net investment income     142,300,000  
Net investment income 70,900,000 61,400,000   121,000,000.0
Net realized and unrealized investment gains (losses) 209,000,000.0 117,300,000 154,600,000 167,300,000
Interest income from BAM Surplus Notes 6,900,000 7,500,000 13,800,000 15,000,000.0
Commission and fee revenues 60,400,000 63,300,000   111,400,000
Commission and fee revenues     103,600,000  
Product and service revenues 107,500,000 56,300,000 164,000,000.0 69,900,000
Net gain on sale of the Bamboo Group     2,400,000 0
Other revenues 5,300,000 8,200,000 12,200,000 12,200,000
Total revenues 844,600,000 689,200,000 1,362,400,000 1,267,000,000
Loss and LAE     382,700,000  
Loss and LAE 175,700,000 166,500,000   428,300,000
Insurance acquisition expenses     208,300,000  
Insurance acquisition expenses 106,100,000 99,500,000   196,600,000
Cost of sales 80,100,000 42,400,000 122,800,000 49,900,000
Broker commission expenses 23,300,000 19,800,000 40,500,000 35,300,000
Amortization of other intangible assets 8,600,000 5,100,000 18,300,000 10,300,000
General and administrative expenses 143,300,000 132,400,000 278,500,000 223,200,000
Change in fair value of contingent consideration 31,700,000 28,400,000 41,700,000 38,100,000
Interest expense 19,800,000 18,600,000 39,100,000 36,400,000
Total expenses 588,600,000 512,700,000 1,131,900,000 1,018,100,000
Pre-tax income (loss) 256,000,000.0 176,500,000 230,500,000 248,900,000
Product and service revenues        
Segment Reporting [Line Items]        
Product and service revenues 107,500,000 56,300,000 164,000,000.0 69,900,000
Other Operations        
Segment Reporting [Line Items]        
Earned insurance premiums 700,000 2,300,000   16,200,000
Earned insurance premiums     4,100,000  
Net investment income     29,000,000.0  
Net investment income 15,500,000 8,600,000 29,000,000.0 18,300,000
Interest income from BAM Surplus Notes 0 0 0 0
Commission and fee revenues 3,800,000 4,200,000   8,100,000
Commission and fee revenues     7,400,000  
Net gain on sale of the Bamboo Group 0 0 2,400,000 0
Other revenues 0 500,000 (100,000) 500,000
Total revenues 165,500,000 91,400,000 142,800,000 98,700,000
Loss and LAE     400,000  
Loss and LAE 100,000 800,000   18,200,000
Insurance acquisition expenses     1,600,000  
Insurance acquisition expenses 300,000 900,000   6,000,000.0
Cost of sales 7,500,000 7,200,000 14,100,000 14,700,000
Broker commission expenses 0 0 0 0
Amortization of other intangible assets 900,000 1,000,000.0 1,900,000 2,100,000
General and administrative expenses 41,200,000 46,400,000 85,700,000 77,800,000
Change in fair value of contingent consideration 0 0 0 0
Interest expense 600,000 500,000 1,300,000 1,000,000.0
Total expenses 50,600,000 56,800,000 105,000,000.0 119,800,000
Pre-tax income (loss) 114,900,000 34,600,000 37,800,000 (21,100,000)
Other Operations | Product and service revenues        
Segment Reporting [Line Items]        
Product and service revenues 14,700,000 13,500,000 27,500,000 27,100,000
Investments, Excluding MediaAlpha        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) 150,600,000 86,800,000 161,400,000 173,400,000
Investments, Excluding MediaAlpha | Other Operations        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) 72,400,000 31,800,000 79,300,000 34,600,000
MediaAlpha        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) 58,400,000 30,500,000 (6,800,000) (6,100,000)
MediaAlpha | Other Operations        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) 58,400,000 30,500,000 (6,800,000) (6,100,000)
MediaAlpha | Other Operations | Product and service revenues        
Segment Reporting [Line Items]        
Product and service revenues 14,700,000 13,500,000 27,500,000 27,100,000
P&C Insurance and Reinsurance (Ark/WM Outrigger)        
Segment Reporting [Line Items]        
Loss and LAE 175,600,000 164,000,000.0 382,300,000 397,500,000
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments        
Segment Reporting [Line Items]        
Earned insurance premiums 376,200,000 364,200,000 750,000,000.0 722,200,000
Net investment income 28,000,000.0 26,300,000 56,700,000 49,800,000
Net realized and unrealized investment gains (losses) 30,500,000 51,100,000 (2,400,000) 80,600,000
Loss and LAE 175,600,000 164,000,000.0 382,300,000 397,500,000
Insurance acquisition expenses 103,700,000 97,200,000 202,500,000 180,700,000
Change in fair value of contingent consideration 31,700,000 28,400,000 41,700,000 38,100,000
Interest expense 4,100,000 4,300,000 8,200,000 8,500,000
Total expenses 359,500,000 350,400,000 726,600,000 717,200,000
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Operating Segments        
Segment Reporting [Line Items]        
Earned insurance premiums 375,400,000 357,100,000   703,100,000
Earned insurance premiums     746,500,000  
Net investment income     54,300,000  
Net investment income 26,900,000 24,100,000   45,400,000
Interest income from BAM Surplus Notes 0 0 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 4,900,000 6,300,000 11,600,000 8,500,000
Total revenues 437,700,000 438,600,000 810,100,000 837,700,000
Loss and LAE     382,100,000  
Loss and LAE 175,700,000 162,300,000   375,600,000
Insurance acquisition expenses     201,300,000  
Insurance acquisition expenses 103,400,000 95,800,000   179,600,000
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 44,400,000 56,500,000 91,900,000 92,300,000
Change in fair value of contingent consideration 31,700,000 28,400,000 41,700,000 38,100,000
Interest expense 4,100,000 4,300,000 8,200,000 8,500,000
Total expenses 359,300,000 347,300,000 725,200,000 694,100,000
Pre-tax income (loss) 78,400,000 91,300,000 84,900,000 143,600,000
Other underwriting expenses 36,300,000 46,300,000 71,200,000 74,800,000
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) 30,500,000 51,100,000 (2,300,000) 80,700,000
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 7,100,000   19,100,000
Earned insurance premiums     3,500,000  
Net investment income     2,400,000  
Net investment income 1,100,000 2,200,000   4,400,000
Interest income from BAM Surplus Notes 0 0 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 1,900,000 9,300,000 5,800,000 23,400,000
Loss and LAE     200,000  
Loss and LAE (100,000) 1,700,000   21,900,000
Insurance acquisition expenses     1,200,000  
Insurance acquisition expenses 300,000 1,400,000   1,100,000
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 200,000 3,100,000 1,400,000 23,100,000
Pre-tax income (loss) 1,700,000 6,200,000 4,400,000 300,000
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     39,800,000  
Net investment income 19,000,000.0 19,300,000 39,800,000 38,700,000
Net realized and unrealized investment gains (losses) 49,700,000 800,000 91,700,000 44,800,000
Interest income from BAM Surplus Notes 0 0 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 200,000 300,000 400,000 700,000
Total revenues 68,900,000 20,400,000 131,900,000 84,200,000
Loss and LAE     0  
Loss and LAE 0 0   0
Insurance acquisition expenses     0  
Insurance acquisition expenses 0 0   0
Cost of sales 0 0 0 0
Broker commission expenses 0 0 0 0
Amortization of other intangible assets 0 100,000 100,000 200,000
General and administrative expenses 4,700,000 3,500,000 8,800,000 7,400,000
Change in fair value of contingent consideration 0 0 0 0
Interest expense 7,200,000 6,100,000 14,300,000 12,500,000
Total expenses 11,900,000 9,700,000 23,200,000 20,100,000
Pre-tax income (loss) 57,000,000.0 10,700,000 108,700,000 64,100,000
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) 49,700,000 800,000 91,700,000 44,800,000
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 7,700,000 7,100,000 15,400,000 15,300,000
Earned insurance premiums     15,400,000  
Net investment income     15,500,000  
Net investment income 7,800,000 6,500,000 15,500,000 12,800,000
Net realized and unrealized investment gains (losses) (2,000,000.0) 3,100,000 (7,200,000) 13,100,000
Interest income from BAM Surplus Notes 6,900,000 7,500,000 13,800,000 15,000,000.0
Commission and fee revenues 0 0   0
Commission and fee revenues     0  
Net gain on sale of the Bamboo Group     0  
Other revenues 0 0 100,000 100,000
Total revenues 20,400,000 24,200,000 37,600,000 56,300,000
Loss and LAE     0  
Loss and LAE 0 0 0 0
Insurance acquisition expenses     4,200,000  
Insurance acquisition expenses 2,100,000 2,000,000.0 4,200,000 3,900,000
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 4,200,000 1,000,000.0 4,900,000 1,600,000
Change in fair value of contingent consideration 0 0 0 0
Interest expense 3,700,000 4,500,000 7,300,000 9,100,000
Total expenses 10,000,000.0 7,500,000 16,400,000 14,600,000
Pre-tax income (loss) 10,400,000 16,700,000 21,200,000 41,700,000
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) (2,000,000.0) 3,100,000 (7,200,000) 13,100,000
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 1,600,000 0 16,500,000
Net investment income   700,000   1,400,000
Interest income from BAM Surplus Notes   0   0
Commission and fee revenues 0 59,100,000 0 103,300,000
Other revenues   1,100,000   2,400,000
Total revenues   62,500,000   123,900,000
Loss and LAE 0 1,700,000 0 12,600,000
Insurance acquisition expenses 0 (600,000) 0 6,000,000.0
Cost of sales   0   0
Broker commission expenses 0 19,800,000 0 35,300,000
Amortization of other intangible assets   4,000,000.0   8,000,000.0
General and administrative expenses   18,600,000   34,600,000
Change in fair value of contingent consideration   0   0
Interest expense 0 2,900,000 0 5,000,000.0
Total expenses 0 46,400,000 0 101,500,000
Pre-tax income (loss)   16,100,000   22,400,000
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)   0   300,000
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      
Earned insurance premiums     0  
Net investment income     1,300,000  
Net investment income 600,000      
Interest income from BAM Surplus Notes 0   0  
Commission and fee revenues 56,600,000 0 96,200,000 0
Commission and fee revenues     96,200,000  
Net gain on sale of the Bamboo Group     0  
Total revenues 57,400,000   97,700,000  
Loss and LAE     0  
Loss and LAE 0      
Insurance acquisition expenses     0  
Insurance acquisition expenses 0      
Cost of sales 0   0  
Broker commission expenses 23,300,000 0 40,500,000 0
Amortization of other intangible assets 6,400,000   13,800,000  
General and administrative expenses 34,600,000   64,400,000  
Change in fair value of contingent consideration 0   0  
Interest expense 3,600,000 0 7,100,000 0
Total expenses 67,900,000 0 125,800,000 0
Pre-tax income (loss) (10,500,000)   (28,100,000)  
Distinguished | Operating Segments | Product and service revenues        
Segment Reporting [Line Items]        
Product and service revenues 0   0  
Distinguished | Investments, Excluding MediaAlpha | Operating Segments        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) 0   0  
Distinguished | MediaAlpha | Operating Segments        
Segment Reporting [Line Items]        
Net realized and unrealized investment gains (losses) 0   0  
Distinguished, net | Operating Segments        
Segment Reporting [Line Items]        
Other revenues 200,000   200,000  
WTM Partners | Operating Segments        
Segment Reporting [Line Items]        
Earned insurance premiums 0 0   0
Earned insurance premiums     0  
Net investment income     0  
Net investment income 0 0   0
Interest income from BAM Surplus Notes 0 0 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 92,800,000 42,800,000 136,500,000 42,800,000
Loss and LAE     0  
Loss and LAE 0 0   0
Insurance acquisition expenses     0  
Insurance acquisition expenses 0 0   0
Cost of sales 72,600,000 35,200,000 108,700,000 35,200,000
Broker commission expenses 0 0 0 0
Amortization of other intangible assets 1,300,000 0 2,500,000 0
General and administrative expenses 14,200,000 6,400,000 22,800,000 9,400,000
Change in fair value of contingent consideration 0 0 0 0
Interest expense 600,000 300,000 900,000 300,000
Total expenses 88,700,000 41,900,000 134,900,000 44,900,000
Pre-tax income (loss) 4,100,000 900,000 1,600,000 (2,100,000)
WTM Partners | Operating Segments | Product and service revenues        
Segment Reporting [Line Items]        
Product and service revenues 92,800,000 42,800,000 136,500,000 42,800,000
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