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Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities and Short-Term Investments (Details)
$ in Millions
Dec. 31, 2022
USD ($)
security
Security
Dec. 31, 2021
USD ($)
security
Security
Schedule of Trading Securities and Other Trading Items [Line Items]    
Fair market value $ 3,788.6 $ 4,881.6
Gross unrealized gains 3.6 95.3
Gross unrealized losses (338.6) (34.5)
Debt Securities, Available-for-Sale, Allowance for Credit Loss (7.7) (2.7)
Fixed income securities, available for sale amortized cost 4,131.3 4,823.5
0-12 months, Fair Market Value 2,321.4 2,014.6
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 145.4 30.9
Over 12 months, Fair Market Value 1,323.7 72.0
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 3,645.1 2,086.6
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 339.0 $ 34.5
Number of Securities | security 1,166 644
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 193.6 $ 3.6
Total short-term investments — Available for sale    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Gross unrealized gains 0.0 0.0
Gross unrealized losses (0.4) 0.0
Debt Securities, Available-for-Sale, Allowance for Credit Loss 0.0 0.0
Total fixed income securities — Available for sale    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Fair market value 3,840.6 4,891.7
Gross unrealized gains 3.6 95.3
Gross unrealized losses (339.0) (34.5)
Debt Securities, Available-for-Sale, Allowance for Credit Loss (7.7) (2.7)
Fixed income securities, available for sale amortized cost 4,183.7 4,833.6
Fixed income securities — Available for sale    
Schedule of Trading Securities and Other Trading Items [Line Items]    
0-12 months, Fair Market Value 2,269.5 2,010.2
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 145.0 30.9
Over 12 months, Fair Market Value 1,323.7 72.0
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 3,593.2 2,082.2
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 338.6 $ 34.5
Number of Securities | Security 1,165 637
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 193.6 $ 3.6
Short-term investments    
Schedule of Trading Securities and Other Trading Items [Line Items]    
0-12 months, Fair Market Value 51.9 4.4
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 0.4 0.0
Over 12 months, Fair Market Value 0.0 0.0
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 51.9 4.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 0.4 $ 0.0
Number of Securities 1 7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 0.0 $ 0.0
U.S. government    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Fair market value 953.0 1,084.3
Gross unrealized gains 0.0 24.0
Gross unrealized losses (50.7) (5.9)
Debt Securities, Available-for-Sale, Allowance for Credit Loss 0.0 0.0
Fixed income securities, available for sale amortized cost 1,003.7 1,066.2
0-12 months, Fair Market Value 741.1 309.1
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 30.6 4.0
Over 12 months, Fair Market Value 203.4 29.3
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 944.5 338.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 50.7 $ 5.9
Number of Securities | security 118 51
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 20.1 $ 1.9
U.S. agency    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Fair market value 8.8 21.5
Gross unrealized gains 0.0 0.8
Gross unrealized losses (0.4) 0.0
Debt Securities, Available-for-Sale, Allowance for Credit Loss 0.0 0.0
Fixed income securities, available for sale amortized cost 9.2 20.7
0-12 months, Fair Market Value 7.1 6.1
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 0.4 0.0
Over 12 months, Fair Market Value 1.7 0.0
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 8.8 6.1
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 0.4 $ 0.0
Number of Securities | security 5 5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 0.0 $ 0.0
Municipal    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Fair market value 149.5 82.6
Gross unrealized gains 0.0 2.0
Gross unrealized losses (9.4) (0.5)
Debt Securities, Available-for-Sale, Allowance for Credit Loss (1.0) (0.1)
Fixed income securities, available for sale amortized cost 159.9 81.2
0-12 months, Fair Market Value 133.1 29.7
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 7.1 0.5
Over 12 months, Fair Market Value 16.4 0.0
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 149.5 29.7
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 9.4 $ 0.5
Number of Securities | security 59 20
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 2.3 $ 0.0
Corporate    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Fair market value 1,845.0 2,208.3
Gross unrealized gains 3.5 47.5
Gross unrealized losses (169.1) (13.0)
Debt Securities, Available-for-Sale, Allowance for Credit Loss (6.3) (2.3)
Fixed income securities, available for sale amortized cost 2,016.9 2,176.1
0-12 months, Fair Market Value 1,104.7 804.6
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 77.6 11.7
Over 12 months, Fair Market Value 568.2 23.6
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 1,672.9 828.2
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 169.1 $ 13.0
Number of Securities | security 701 400
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 91.5 $ 1.3
Non-U.S. government-backed corporate    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Fair market value 110.4 136.8
Gross unrealized gains 0.0 0.2
Gross unrealized losses (8.8) (1.9)
Debt Securities, Available-for-Sale, Allowance for Credit Loss (0.2) 0.0
Fixed income securities, available for sale amortized cost 119.4 138.5
0-12 months, Fair Market Value 11.3 114.4
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 0.5 1.9
Over 12 months, Fair Market Value 99.0 0.0
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 110.3 114.4
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 8.8 $ 1.9
Number of Securities | security 16 10
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 8.3 $ 0.0
Non-U.S government    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Fair market value 213.6 250.7
Gross unrealized gains 0.1 1.4
Gross unrealized losses (11.5) (1.2)
Debt Securities, Available-for-Sale, Allowance for Credit Loss (0.2) (0.3)
Fixed income securities, available for sale amortized cost 225.2 250.8
0-12 months, Fair Market Value 63.8 181.8
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 2.9 1.2
Over 12 months, Fair Market Value 135.9 3.1
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 199.7 184.9
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 11.5 $ 1.2
Number of Securities | security 45 35
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 8.6 $ 0.0
Asset-backed    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Fair market value   0.5
Gross unrealized gains   0.0
Gross unrealized losses   0.0
Debt Securities, Available-for-Sale, Allowance for Credit Loss   0.0
Fixed income securities, available for sale amortized cost   0.5
Non-agency commercial mortgage-backed securities    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Fair market value 5.6 6.9
Gross unrealized gains 0.0 0.3
Gross unrealized losses (1.0) 0.0
Debt Securities, Available-for-Sale, Allowance for Credit Loss 0.0 0.0
Fixed income securities, available for sale amortized cost 6.6 6.6
0-12 months, Fair Market Value 5.6  
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 1.0  
Over 12 months, Fair Market Value 0.0  
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 5.6  
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 1.0  
Number of Securities | security 1  
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 0.0  
Agency mortgage-backed    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Fair market value 502.7 1,090.0
Gross unrealized gains 0.0 19.1
Gross unrealized losses (87.7) (12.0)
Debt Securities, Available-for-Sale, Allowance for Credit Loss 0.0 0.0
Fixed income securities, available for sale amortized cost 590.4 1,082.9
0-12 months, Fair Market Value 202.8 564.5
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 24.9 11.6
Over 12 months, Fair Market Value 299.1 16.0
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 501.9 580.5
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 87.7 $ 12.0
Number of Securities | security 220 116
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 62.8 $ 0.4
Total short-term investments — Available for sale    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Fair market value 52.0 10.1
Fixed income securities, available for sale amortized cost $ 52.4 $ 10.1