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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, 2018	Dec. 31, 2017
Schedule of Trading Securities and Other Trading Items [Line Items]
Fair Market Value	$ 1,187.8	$ 1,649.3
Debt Securities, Trading, Amortized Cost	1,205.0	1,634.9
Fixed income securities - trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Gross Unrealized Gains	6.3	21.5
Gross Unrealized Losses	(23.5)	(7.1)
Fair Market Value	1,187.8	1,649.3
Debt Securities, Trading, Amortized Cost	1,205.0	1,634.9
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	9.5	73.0
Debt Securities, Trading, Amortized Cost	9.5	73.0
Equity securities
Schedule of Trading Securities and Other Trading Items [Line Items]
Gross Unrealized Gains		83.5
Gross Unrealized Losses		(7.3)
Fair Market Value		491.0
Debt Securities, Trading, Amortized Cost		414.8
Catastrophe Bonds
Schedule of Trading Securities and Other Trading Items [Line Items]
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	(1.8)	(1.1)
Fair Market Value	36.2	32.4
Debt Securities, Trading, Amortized Cost	37.9	33.5
Total - Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Gross Unrealized Gains	6.4	105.0
Gross Unrealized Losses	(25.3)	(15.5)
Fair Market Value	1,233.5	2,245.7
Debt Securities, Trading, Amortized Cost	1,252.4	2,156.2
U.S. government
Schedule of Trading Securities and Other Trading Items [Line Items]
Gross Unrealized Gains	1.6	0.4
Gross Unrealized Losses	(0.5)	(0.8)
Fair Market Value	147.7	161.9
Debt Securities, Trading, Amortized Cost	146.6	162.3
Municipal
Schedule of Trading Securities and Other Trading Items [Line Items]
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.1)	(0.2)
Fair Market Value	2.7	32.2
Debt Securities, Trading, Amortized Cost	2.8	32.4
Corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Gross Unrealized Gains	2.6	14.0
Gross Unrealized Losses	(16.6)	(4.2)
Fair Market Value	720.2	1,046.3
Debt Securities, Trading, Amortized Cost	734.2	1,036.5
Non-U.S. government-backed corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Gross Unrealized Gains		6.9
Gross Unrealized Losses		(0.5)
Fair Market Value		202.5
Debt Securities, Trading, Amortized Cost		196.1
Non-U.S government
Schedule of Trading Securities and Other Trading Items [Line Items]
Gross Unrealized Gains	1.9
Gross Unrealized Losses	(5.2)
Fair Market Value	265.4
Debt Securities, Trading, Amortized Cost	268.7
Asset-backed
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	2.4	9.9
Debt Securities, Trading, Amortized Cost	2.4	9.9
Agency mortgage-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Gross Unrealized Gains	0.2	0.2
Gross Unrealized Losses	(1.1)	(1.4)
Fair Market Value	49.4	195.5
Debt Securities, Trading, Amortized Cost	$ 50.3	$ 196.7

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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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