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Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Debt Securities, Trading, Amortized Cost | $ 844.8 | $ 1,106.6
High yield loans |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Gross Unrealized Gains | 0.0 |
Gross Unrealized Losses | (0.2) |
Fair market value, trading | 9.8 |
Debt Securities, Trading, Amortized Cost | 10.0 |
Fixed income securities - trading |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Gross Unrealized Gains | 16.8 | 24.2
Gross Unrealized Losses | (6.0) | (2.0)
Fair market value, trading | 855.6 | 1,128.8
Debt Securities, Trading, Amortized Cost | 844.8 | 1,106.6
Short-term investments |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | 0.0
Fair market value, trading | 35.4 | 79.2
Debt Securities, Trading, Amortized Cost | 35.4 | 79.2
Equity securities — Trading |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Gross Unrealized Gains | 0.0 | 0.2
Gross Unrealized Losses | 0.0 | 0.0
Fair market value, trading | 5.0 | 2.7
Debt Securities, Trading, Amortized Cost | 5.0 | 2.5
Catastrophe Bonds |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | (0.8)
Fair market value, trading | 18.8 | 28.6
Debt Securities, Trading, Amortized Cost | 18.8 | 29.4
Commercial mortgage loans |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Gross Unrealized Gains | 0.1 | 0.3
Gross Unrealized Losses | (15.1) | 0.0
Fair market value, trading | 163.6 | 156.6
Debt Securities, Trading, Amortized Cost | 178.6 | 156.3
Middle market loans |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Gross Unrealized Gains | 0.2 | 0.2
Gross Unrealized Losses | (5.5) | (0.2)
Fair market value, trading | 112.1 | 111.7
Debt Securities, Trading, Amortized Cost | 117.4 | 111.7
Securities, asset-backed |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (0.1) | 0.0
Fair market value, trading | 18.6 | 8.7
Debt Securities, Trading, Amortized Cost | 18.7 | 8.7
Privately-held investments |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Gross Unrealized Gains | 0.3 | 0.7
Gross Unrealized Losses | (20.7) | (0.2)
Fair market value, trading | 299.3 | 279.7
Debt Securities, Trading, Amortized Cost | 319.7 | 279.2
Total - Trading |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Gross Unrealized Gains | 17.1 | 24.9
Gross Unrealized Losses | (26.7) | (3.0)
Fair market value, trading | 1,209.1 | 1,516.3
Debt Securities, Trading, Amortized Cost | 1,218.7 | 1,494.4
U.S. government |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Gross Unrealized Gains | 2.7 | 1.8
Gross Unrealized Losses | (0.1) | (0.1)
Fair market value, trading | 120.5 | 185.0
Debt Securities, Trading, Amortized Cost | 117.9 | 183.3
Municipal |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Gross Unrealized Gains | 0.3 | 0.1
Gross Unrealized Losses | 0.0 | 0.0
Fair market value, trading | 3.5 | 3.2
Debt Securities, Trading, Amortized Cost | 3.2 | 3.1
Corporate |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Gross Unrealized Gains | 9.1 | 11.6
Gross Unrealized Losses | 0.0 | (0.1)
Fair market value, trading | 103.5 | 243.2
Debt Securities, Trading, Amortized Cost | 94.4 | 231.7
Non-U.S government |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Gross Unrealized Gains | 0.1 | 7.4
Gross Unrealized Losses | 0.0 | (0.1)
Fair market value, trading | 41.6 | 151.2
Debt Securities, Trading, Amortized Cost | 41.5 | 143.9
Asset-backed |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Gross Unrealized Gains | 3.1 | 2.4
Gross Unrealized Losses | (5.7) | (1.7)
Fair market value, trading | 539.1 | 492.4
Debt Securities, Trading, Amortized Cost | 541.7 | 491.7
Agency mortgage-backed |  |
Schedule of Trading Securities and Other Trading Items [Line Items] |  |
Gross Unrealized Gains | 1.5 | 0.9
Gross Unrealized Losses | 0.0 | 0.0
Fair market value, trading | 37.6 | 53.8
Debt Securities, Trading, Amortized Cost | $ 36.1 | $ 52.9

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