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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) - USD ($) $ in Millions | Dec. 31, 2021 | Dec. 31, 2020
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Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fair market value | $ 4,881.6 | $ 4,348.1
Gross unrealized gains | 95.3 | 222.4
Gross unrealized losses | (34.5) | (0.8)
Allowance for credit loss | (2.7) | 0.2
Fixed income securities, available for sale amortized cost | 4,823.5 | 4,126.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 3.6 | 0.2
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | 34.5 | 1.0
0-12 months, Fair Market Value | 2,014.6 | 147.8
Total short-term investments — Available for sale |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fair market value | 10.1 | 87.8
Gross unrealized gains | 0.0 | 0.0
Gross unrealized losses | 0.0 | (0.2)
Allowance for credit loss | 0.0 | 0.0
Fixed income securities, available for sale amortized cost | 10.1 | 88.0
Total fixed income securities — Available for sale |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fair market value | 4,891.7 | 4,435.9
Gross unrealized gains | 95.3 | 222.4
Gross unrealized losses | (34.5) | (1.0)
Allowance for credit loss | (2.7) | 0.2
Fixed income securities, available for sale amortized cost | 4,833.6 | 4,214.7
Fixed income securities — Available for sale |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 3.6 | 0.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | 34.5 | 0.8
0-12 months, Fair Market Value | 2,010.2 | 139.1
Short-term investments |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.0 | 0.2
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | 0.0 | 0.2
0-12 months, Fair Market Value | 4.4 | 8.7
U.S. government |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fair market value | 1,084.3 | 1,101.2
Gross unrealized gains | 24.0 | 60.5
Gross unrealized losses | (5.9) | (0.6)
Allowance for credit loss | 0.0 | 0.0
Fixed income securities, available for sale amortized cost | 1,066.2 | 1,041.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 1.9 | 0.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | 5.9 | 0.6
0-12 months, Fair Market Value | 309.1 | 58.7
U.S. agency |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fair market value | 21.5 | 34.3
Gross unrealized gains | 0.8 | 1.5
Gross unrealized losses | 0.0 | 0.0
Allowance for credit loss | 0.0 | 0.0
Fixed income securities, available for sale amortized cost | 20.7 | 32.8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.0 |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | 0.0 |
0-12 months, Fair Market Value | 6.1 |
Municipal |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fair market value | 82.6 | 67.1
Gross unrealized gains | 2.0 | 5.5
Gross unrealized losses | (0.5) | 0.0
Allowance for credit loss | (0.1) | 0.0
Fixed income securities, available for sale amortized cost | 81.2 | 61.6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.0 |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | 0.5 |
0-12 months, Fair Market Value | 29.7 |
Corporate |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fair market value | 2,208.3 | 1,826.2
Gross unrealized gains | 47.5 | 112.0
Gross unrealized losses | (13.0) | (0.1)
Allowance for credit loss | (2.3) | 0.2
Fixed income securities, available for sale amortized cost | 2,176.1 | 1,714.5
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 1.3 | 0.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | 13.0 | 0.1
0-12 months, Fair Market Value | 804.6 | 39.6
Non-U.S. government-backed corporate |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fair market value | 136.8 | 63.4
Gross unrealized gains | 0.2 | 0.7
Gross unrealized losses | (1.9) | 0.0
Allowance for credit loss | 0.0 | 0.0
Fixed income securities, available for sale amortized cost | 138.5 | 62.7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.0 |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | 1.9 |
0-12 months, Fair Market Value | 114.4 |
Non-U.S government |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fair market value | 250.7 | 273.1
Gross unrealized gains | 1.4 | 4.3
Gross unrealized losses | (1.2) | 0.0
Allowance for credit loss | (0.3) | 0.0
Fixed income securities, available for sale amortized cost | 250.8 | 268.8
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.0 | 0.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | 1.2 | 0.0
0-12 months, Fair Market Value | 181.8 | 7.5
Asset-backed |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fair market value | 0.5 | 2.3
Gross unrealized gains | 0.0 | 0.0
Gross unrealized losses | 0.0 | 0.0
Allowance for credit loss | 0.0 | 0.0
Fixed income securities, available for sale amortized cost | 0.5 | 2.3
Non-agency commercial mortgage-backed securities |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fair market value | 6.9 | 7.4
Gross unrealized gains | 0.3 | 0.7
Gross unrealized losses | 0.0 | 0.0
Allowance for credit loss | 0.0 | 0.0
Fixed income securities, available for sale amortized cost | 6.6 | 6.7
Agency mortgage-backed |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Fair market value | 1,090.0 | 973.1
Gross unrealized gains | 19.1 | 37.2
Gross unrealized losses | (12.0) | (0.1)
Allowance for credit loss | 0.0 | 0.0
Fixed income securities, available for sale amortized cost | 1,082.9 | 936.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0.4 | 0.0
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss | 12.0 | 0.1
0-12 months, Fair Market Value | $ 564.5 | $ 33.3

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