v3.26.1
Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities (Details) - USD ($)
$ in Millions
Dec. 31, 2025
Dec. 31, 2024
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost $ 1,158.9 $ 1,497.7
Gross Unrealized Gains 13.5 7.7
Gross Unrealized Losses (10.4) (16.7)
Fair Market Value 1,162.0 1,488.7
Fixed income investments    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 1,037.8 1,207.7
Gross Unrealized Gains 9.4 6.2
Gross Unrealized Losses (7.0) (14.0)
Fair Market Value 1,040.2 1,199.9
U.S. government    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 259.2 262.9
Gross Unrealized Gains 5.1 0.6
Gross Unrealized Losses (0.4) (2.2)
Fair Market Value 263.9 261.3
Municipal    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 4.7 1.6
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses 0.0 0.0
Fair Market Value 4.7 1.6
Corporate    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 141.4 155.5
Gross Unrealized Gains 1.8 0.5
Gross Unrealized Losses (2.0) (4.9)
Fair Market Value 141.2 151.1
High yield loans    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 99.6 101.7
Gross Unrealized Gains 0.6 0.9
Gross Unrealized Losses (0.2) (0.2)
Fair Market Value 100.0 102.4
Non-U.S. government-backed corporate    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost   2.8
Gross Unrealized Gains   0.0
Gross Unrealized Losses   0.0
Fair Market Value   2.8
Non-U.S. government    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 37.6 24.4
Gross Unrealized Gains 0.2 0.1
Gross Unrealized Losses (0.2) (0.1)
Fair Market Value 37.6 24.4
Asset-backed    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 461.8 624.6
Gross Unrealized Gains 1.4 4.1
Gross Unrealized Losses (2.5) (3.5)
Fair Market Value 460.7 625.2
Agency mortgage-backed    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 33.5 34.2
Gross Unrealized Gains 0.3 0.0
Gross Unrealized Losses (1.7) (3.1)
Fair Market Value 32.1 31.1
Short-term investments — Available for sale    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost   1.0
Gross Unrealized Gains   0.0
Gross Unrealized Losses   0.0
Fair Market Value   1.0
Catastrophe bonds, trading    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost   1.0
Gross Unrealized Gains   0.0
Gross Unrealized Losses   0.0
Fair Market Value   1.0
Privately-held investments    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 121.1 288.0
Gross Unrealized Gains 4.1 1.5
Gross Unrealized Losses (3.4) (2.7)
Fair Market Value 121.8 286.8
Commercial mortgage loans    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 25.3 80.9
Gross Unrealized Gains 0.1 0.5
Gross Unrealized Losses 0.0 (1.7)
Fair Market Value 25.4 79.7
Middle market loans and other private debt    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 32.0 61.6
Gross Unrealized Gains 0.1 0.1
Gross Unrealized Losses (3.2) (0.7)
Fair Market Value 28.9 61.0
Asset-backed securities    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost 63.8 126.7
Gross Unrealized Gains 3.9 0.9
Gross Unrealized Losses (0.2) 0.0
Fair Market Value 67.5 127.6
Global corporate securities    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Cost or Amortized Cost   18.8
Gross Unrealized Gains   0.0
Gross Unrealized Losses   (0.3)
Fair Market Value $ 34.2 $ 18.5