Investments - Cost, Gross Unrealized Gains and Losses, and Estimated Fair Value of Trading Investments in Fixed Income Maturities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Statement [Line Items]
Cost or Amortized Cost	$ 1,273.1	$ 1,205.0
Fair Market Value	1,289.3	1,187.8
U.S. government
Statement [Line Items]
Cost or Amortized Cost	217.4	146.6
Gross Unrealized Gains	1.6	1.6
Gross Unrealized Losses	(0.2)	(0.5)
Fair Market Value	218.8	147.7
Municipal
Statement [Line Items]
Cost or Amortized Cost	3.0	2.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	(0.1)
Fair Market Value	3.0	2.7
Corporate
Statement [Line Items]
Cost or Amortized Cost	754.9	734.2
Gross Unrealized Gains	12.4	2.6
Gross Unrealized Losses	(3.4)	(16.6)
Fair Market Value	763.9	720.2
Non-U.S. government
Statement [Line Items]
Cost or Amortized Cost	246.4	268.7
Gross Unrealized Gains	6.8	1.9
Gross Unrealized Losses	(0.7)	(5.2)
Fair Market Value	252.5	265.4
Asset-backed
Statement [Line Items]
Cost or Amortized Cost	2.3	2.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Market Value	2.3	2.4
Agency mortgage-backed
Statement [Line Items]
Cost or Amortized Cost	49.1	50.3
Gross Unrealized Gains	0.3	0.2
Gross Unrealized Losses	(0.6)	(1.1)
Fair Market Value	48.8	49.4
Total fixed income securities — Trading
Statement [Line Items]
Cost or Amortized Cost	1,273.1	1,205.0
Gross Unrealized Gains	21.1	6.3
Gross Unrealized Losses	(4.9)	(23.5)
Fair Market Value	1,289.3	1,187.8
Total short-term investments — Trading
Statement [Line Items]
Cost or Amortized Cost	81.0	9.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Market Value	81.0	9.5
Total catastrophe bonds — Trading
Statement [Line Items]
Cost or Amortized Cost	39.5	37.9
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	(1.4)	(1.8)
Fair Market Value	38.1	36.2
Total
Statement [Line Items]
Cost or Amortized Cost	1,393.6	1,252.4
Gross Unrealized Gains	21.1	6.4
Gross Unrealized Losses	(6.3)	(25.3)
Fair Market Value	$ 1,408.4	$ 1,233.5

Trading Securities, Accumulated Gross Unrealized Gain, before Tax

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Trading Securities, Accumulated Gross Unrealized Loss, before Tax

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- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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