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Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Available for Sale Investments in Fixed Income Maturities, Short-Term Investments and Equities (Details) - USD ($) $ in Millions | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | $ 5,228.1 | $ 5,291.2
Gross Unrealized Gains | 23.5 | 63.6
Gross Unrealized Losses | (99.4) | (33.9)
Fair Market Value | 5,152.2 | 5,320.9
Total fixed income securities — Available for sale |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 5,155.9 | 5,201.2
Gross Unrealized Gains | 23.5 | 63.6
Gross Unrealized Losses | (99.4) | (33.8)
Fair Market Value | 5,080.0 | 5,231.0
Total short-term investments — Available for sale |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 72.2 | 90.0
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | 0.0 | (0.1)
Fair Market Value | 72.2 | 89.9
U.S. government |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 1,253.5 | 1,166.5
Gross Unrealized Gains | 1.6 | 4.5
Gross Unrealized Losses | (25.5) | (11.6)
Fair Market Value | 1,229.6 | 1,159.4
U.S. agency |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 47.1 | 51.8
Gross Unrealized Gains | 0.1 | 0.5
Gross Unrealized Losses | (0.6) | (0.2)
Fair Market Value | 46.6 | 52.1
Municipal |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 105.1 | 53.0
Gross Unrealized Gains | 1.7 | 2.1
Gross Unrealized Losses | (1.1) | (0.2)
Fair Market Value | 105.7 | 54.9
Corporate |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 2,304.2 | 2,391.4
Gross Unrealized Gains | 10.2 | 36.1
Gross Unrealized Losses | (47.3) | (11.8)
Fair Market Value | 2,267.1 | 2,415.7
Non-U.S. government-backed corporate |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 92.2 | 91.5
Gross Unrealized Gains | 0.1 | 0.3
Gross Unrealized Losses | (0.5) | (0.5)
Fair Market Value | 91.8 | 91.3
Non-U.S. government |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 427.5 | 479.7
Gross Unrealized Gains | 4.0 | 6.4
Gross Unrealized Losses | (1.3) | (1.2)
Fair Market Value | 430.2 | 484.9
Asset-backed |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 18.7 | 26.3
Gross Unrealized Gains | 0.0 | 0.0
Gross Unrealized Losses | (0.3) | (0.1)
Fair Market Value | 18.4 | 26.2
Agency mortgage-backed |  |
Statement [Line Items] |  |
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost | 907.6 | 941.0
Gross Unrealized Gains | 5.8 | 13.7
Gross Unrealized Losses | (22.8) | (8.2)
Fair Market Value | $ 890.6 | $ 946.5

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