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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, 2020 | Dec. 31, 2019
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Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Short term investment, available for sale(amortized cost - $117.6 and $117.66) | $ 4,348.1 | $ 4,958.2
Gross unrealized gains | 222.4 | 116.8
Gross unrealized losses | (0.8) | (3.8)
Allowance for credit loss | (0.2) |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 4,126.7 | 4,845.2
Total short-term investments — Available for sale |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Short term investment, available for sale(amortized cost - $117.6 and $117.66) | 87.8 | 117.6
Gross unrealized gains | 0.0 | 0.0
Gross unrealized losses | (0.2) | 0.0
Allowance for credit loss | 0.0 |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 88.0 | 117.6
Total fixed income securities — Available for sale |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Short term investment, available for sale(amortized cost - $117.6 and $117.66) | 4,435.9 | 5,075.8
Gross unrealized gains | 222.4 | 116.8
Gross unrealized losses | (1.0) | (3.8)
Allowance for credit loss | (0.2) |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 4,214.7 | 4,962.8
U.S. government |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Short term investment, available for sale(amortized cost - $117.6 and $117.66) | 1,101.2 | 1,413.1
Gross unrealized gains | 60.5 | 31.3
Gross unrealized losses | (0.6) | (1.4)
Allowance for credit loss | 0.0 |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 1,041.3 | 1,383.2
U.S. agency |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Short term investment, available for sale(amortized cost - $117.6 and $117.66) | 34.3 | 39.6
Gross unrealized gains | 1.5 | 0.9
Gross unrealized losses | 0.0 | 0.0
Allowance for credit loss | 0.0 |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 32.8 | 38.7
Municipal |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Short term investment, available for sale(amortized cost - $117.6 and $117.66) | 67.1 | 50.7
Gross unrealized gains | 5.5 | 2.9
Gross unrealized losses | 0.0 | 0.0
Allowance for credit loss | 0.0 |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 61.6 | 47.8
Corporate |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Short term investment, available for sale(amortized cost - $117.6 and $117.66) | 1,826.2 | 1,959.8
Gross unrealized gains | 112.0 | 54.8
Gross unrealized losses | (0.1) | (0.6)
Allowance for credit loss | (0.2) |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 1,714.5 | 1,905.6
Non-U.S. government-backed corporate |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Short term investment, available for sale(amortized cost - $117.6 and $117.66) | 63.4 | 86.5
Gross unrealized gains | 0.7 | 0.5
Gross unrealized losses | 0.0 | (0.1)
Allowance for credit loss | 0.0 |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 62.7 | 86.1
Non-U.S government |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Short term investment, available for sale(amortized cost - $117.6 and $117.66) | 273.1 | 328.8
Gross unrealized gains | 4.3 | 4.5
Gross unrealized losses | 0.0 | (0.4)
Allowance for credit loss | 0.0 |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 268.8 | 324.7
Asset-backed |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Short term investment, available for sale(amortized cost - $117.6 and $117.66) | 2.3 | 0.2
Gross unrealized gains | 0.0 | 0.0
Gross unrealized losses | 0.0 | 0.0
Allowance for credit loss | 0.0 |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 2.3 | 0.2
Non-agency commercial mortgage-backed securities |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Short term investment, available for sale(amortized cost - $117.6 and $117.66) | 7.4 | 6.5
Gross unrealized gains | 0.7 | 0.0
Gross unrealized losses | 0.0 | (0.2)
Allowance for credit loss | 0.0 |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 6.7 | 6.7
Agency mortgage-backed |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Short term investment, available for sale(amortized cost - $117.6 and $117.66) | 973.1 | 1,073.0
Gross unrealized gains | 37.2 | 21.9
Gross unrealized losses | (0.1) | (1.1)
Allowance for credit loss | 0.0 |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | $ 936.0 | $ 1,052.2

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