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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	Sep. 30, 2018	Dec. 31, 2017
Statement [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	$ 5,393.8	$ 5,291.2
Gross Unrealized Gains	18.1	63.6
Gross Unrealized Losses	(115.1)	(33.9)
Fair Market Value	5,296.8	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	5,273.4	5,201.2
Gross Unrealized Gains	18.1	63.6
Gross Unrealized Losses	(115.1)	(33.8)
Fair Market Value	5,176.4	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	120.4	90.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	(0.1)
Fair Market Value	120.4	89.9
U.S. government
Statement [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	1,420.1	1,166.5
Gross Unrealized Gains	0.8	4.5
Gross Unrealized Losses	(33.1)	(11.6)
Fair Market Value	1,387.8	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	(0.8)	(0.2)
Fair Market Value	46.4	52.1
Municipal
Statement [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	51.9	53.0
Gross Unrealized Gains	1.7	2.1
Gross Unrealized Losses	(1.5)	(0.2)
Fair Market Value	52.1	54.9
Corporate
Statement [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	2,305.9	2,391.4
Gross Unrealized Gains	8.0	36.1
Gross Unrealized Losses	(47.9)	(11.8)
Fair Market Value	2,266.0	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	91.4	91.5
Gross Unrealized Gains	0.1	0.3
Gross Unrealized Losses	(0.7)	(0.5)
Fair Market Value	90.8	91.3
Non-U.S. government
Statement [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	404.1	479.7
Gross Unrealized Gains	3.1	6.4
Gross Unrealized Losses	(1.8)	(1.2)
Fair Market Value	405.4	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	(0.2)	(0.1)
Fair Market Value	18.5	26.2
Agency mortgage-backed
Statement [Line Items]
Total, Available for sale investments in fixed income maturities, Cost or Amortized Cost	934.2	941.0
Gross Unrealized Gains	4.3	13.7
Gross Unrealized Losses	(29.1)	(8.2)
Fair Market Value	$ 909.4	$ 946.5

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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