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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

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