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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
Jun. 30, 2023
USD ($)
security
Security
Dec. 31, 2022
USD ($)
security
Security
Schedule of Trading Securities and Other Trading Items [Line Items]    
Available for sale investments in fixed income maturities, Fair Market Value $ 3,908.2 $ 3,788.6
Gross unrealized gains 2.4 3.6
Gross unrealized losses (321.4) (338.6)
Allowance for credit losses (4.0) (7.7)
Fixed income securities, available for sale amortized cost 4,231.2 4,131.3
0-12 months, Fair Market Value 1,175.6 2,321.4
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 32.5 145.4
Over 12 months, Fair Market Value 2,582.7 1,323.7
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 3,758.3 3,645.1
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 321.4 $ 339.0
Number of Securities | security 1,193 1,166
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ (288.9) $ (193.6)
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 0.0 (0.4)
Allowance for credit losses 0.0 0.0
Total fixed income securities — Available for sale    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Available for sale investments in fixed income maturities, Fair Market Value 4,077.5 3,840.6
Gross unrealized gains 2.4 3.6
Gross unrealized losses (321.4) (339.0)
Allowance for credit losses (4.0) (7.7)
Fixed income securities, available for sale amortized cost 4,400.5 4,183.7
Fixed income securities — Available for sale    
Schedule of Trading Securities and Other Trading Items [Line Items]    
0-12 months, Fair Market Value 1,163.8 2,269.5
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 32.5 145.0
Over 12 months, Fair Market Value 2,582.7 1,323.7
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 3,746.5 3,593.2
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 321.4 $ 338.6
Number of Securities | Security 1,188 1,165
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ (288.9) $ (193.6)
Short-term investments    
Schedule of Trading Securities and Other Trading Items [Line Items]    
0-12 months, Fair Market Value 11.8 51.9
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 0.0 0.4
Over 12 months, Fair Market Value 0.0 0.0
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 11.8 51.9
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 0.0 $ 0.4
Number of Securities 5 1
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]    
Available for sale investments in fixed income maturities, Fair Market Value 1,013.8 953.0
Gross unrealized gains 0.0 0.0
Gross unrealized losses (51.1) (50.7)
Allowance for credit losses 0.0 0.0
Fixed income securities, available for sale amortized cost 1,064.9 1,003.7
0-12 months, Fair Market Value 494.8 741.1
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 12.8 30.6
Over 12 months, Fair Market Value 519.1 203.4
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 1,013.9 944.5
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 51.1 $ 50.7
Number of Securities | security 105 118
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ (38.3) $ (20.1)
U.S. agency    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Available for sale investments in fixed income maturities, Fair Market Value 8.2 8.8
Gross unrealized gains 0.0 0.0
Gross unrealized losses (0.4) (0.4)
Allowance for credit losses 0.0 0.0
Fixed income securities, available for sale amortized cost 8.6 9.2
0-12 months, Fair Market Value 7.0 7.1
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 0.4 0.4
Over 12 months, Fair Market Value 1.1 1.7
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 8.1 8.8
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 0.4 $ 0.4
Number of Securities | security 4 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]    
Available for sale investments in fixed income maturities, Fair Market Value 149.5 149.5
Gross unrealized gains 0.0 0.0
Gross unrealized losses (8.8) (9.4)
Allowance for credit losses (0.7) (1.0)
Fixed income securities, available for sale amortized cost 159.0 159.9
0-12 months, Fair Market Value 90.9 133.1
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 3.7 7.1
Over 12 months, Fair Market Value 58.7 16.4
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 149.6 149.5
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 8.8 $ 9.4
Number of Securities | security 60 59
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ (5.1) $ (2.3)
Corporate    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Available for sale investments in fixed income maturities, Fair Market Value 1,875.9 1,845.0
Gross unrealized gains 2.4 3.5
Gross unrealized losses (155.2) (169.1)
Allowance for credit losses (3.2) (6.3)
Fixed income securities, available for sale amortized cost 2,031.9 2,016.9
0-12 months, Fair Market Value 467.4 1,104.7
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 13.1 77.6
Over 12 months, Fair Market Value 1,254.7 568.2
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 1,722.1 1,672.9
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 155.2 $ 169.1
Number of Securities | security 727 701
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ (142.1) $ (91.5)
Non-U.S. government-backed corporate    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Available for sale investments in fixed income maturities, Fair Market Value 110.9 110.4
Gross unrealized gains 0.0 0.0
Gross unrealized losses (8.7) (8.8)
Allowance for credit losses 0.0 (0.2)
Fixed income securities, available for sale amortized cost 119.6 119.4
0-12 months, Fair Market Value 5.2 11.3
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 0.0 0.5
Over 12 months, Fair Market Value 105.7 99.0
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 110.9 110.3
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 8.7 $ 8.8
Number of Securities | security 16 16
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ (8.7) $ (8.3)
Non-U.S government    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Available for sale investments in fixed income maturities, Fair Market Value 269.2 213.6
Gross unrealized gains 0.0 0.1
Gross unrealized losses (12.0) (11.5)
Allowance for credit losses (0.1) (0.2)
Fixed income securities, available for sale amortized cost 281.3 225.2
0-12 months, Fair Market Value 80.1 63.8
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 1.2 2.9
Over 12 months, Fair Market Value 182.2 135.9
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 262.3 199.7
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 12.0 $ 11.5
Number of Securities | security 61 45
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ (10.8) $ (8.6)
Non-agency commercial mortgage-backed securities    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Available for sale investments in fixed income maturities, Fair Market Value 5.7 5.6
Gross unrealized gains 0.0 0.0
Gross unrealized losses (0.9) (1.0)
Allowance for credit losses 0.0 0.0
Fixed income securities, available for sale amortized cost 6.6 6.6
0-12 months, Fair Market Value 0.0 5.6
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 0.0 1.0
Over 12 months, Fair Market Value 5.6 0.0
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 5.6 5.6
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 0.9 $ 1.0
Number of Securities | security 1 1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ (0.9) $ 0.0
Agency mortgage-backed    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Available for sale investments in fixed income maturities, Fair Market Value 475.0 502.7
Gross unrealized gains 0.0 0.0
Gross unrealized losses (84.3) (87.7)
Allowance for credit losses 0.0 0.0
Fixed income securities, available for sale amortized cost 559.3 590.4
0-12 months, Fair Market Value 18.4 202.8
Debt Securities, Available-for-Sale, Continuous Unrealized Loss Position, Less than 12 Months 1.3 24.9
Over 12 months, Fair Market Value 455.6 299.1
Debt Securities, Available for Sale, Continuous Loss Position, Fair Market Value 474.0 501.9
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss $ 84.3 $ 87.7
Number of Securities | security 214 220
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ (83.0) $ (62.8)
Total short-term investments — Available for sale    
Schedule of Trading Securities and Other Trading Items [Line Items]    
Available for sale investments in fixed income maturities, Fair Market Value 169.3 52.0
Fixed income securities, available for sale amortized cost $ 169.3 $ 52.4