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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, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Items [Line Items]
Trading Securities	$ 1,649.3	$ 1,265.7
Fixed income securities — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	1,634.9	1,264.8
Gross Unrealized Gains	21.5	14.0
Gross Unrealized Losses	(7.1)	(13.1)
Trading Securities	1,649.3	1,265.7
Short-term Investments
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	73.0	185.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Trading Securities	73.0	185.4
Total equity securities — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	414.8	554.3
Gross Unrealized Gains	83.5	55.4
Gross Unrealized Losses	(7.3)	(25.0)
Trading Securities	491.0	584.7
Total catastrophe bonds — Trading
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	33.5	42.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(1.1)	0.0
Trading Securities	32.4	42.5
Total
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	2,156.2	2,047.0
Gross Unrealized Gains	105.0	69.4
Gross Unrealized Losses	(15.5)	(38.1)
Trading Securities	2,245.7	2,078.3
U.S. government
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	162.3	82.8
Gross Unrealized Gains	0.4	0.4
Gross Unrealized Losses	(0.8)	(0.8)
Trading Securities	161.9	82.4
Municipal
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	32.4	15.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.2)	(0.2)
Trading Securities	32.2	15.5
Corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	1,036.5	817.8
Gross Unrealized Gains	14.0	9.9
Gross Unrealized Losses	(4.2)	(7.1)
Trading Securities	1,046.3	820.6
Non-U.S. government-backed corporate
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	1.0
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Trading Securities	1.0
Non-U.S government
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	196.1	203.4
Gross Unrealized Gains	6.9	3.5
Gross Unrealized Losses	(0.5)	(4.1)
Trading Securities	202.5	202.8
Asset-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	9.9	14.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Trading Securities	9.9	14.5
Agency mortgage-backed
Schedule of Trading Securities and Other Trading Items [Line Items]
Cost or Amortized Cost	196.7	130.6
Gross Unrealized Gains	0.2	0.2
Gross Unrealized Losses	(1.4)	(0.9)
Trading Securities	$ 195.5	$ 129.9

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-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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-Topic 320
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-Section 45
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