Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	$ 26.1	$ 9.9	$ 82.1	$ (240.7)
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	(3.3)	0.1	(5.9)	41.1
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	22.8	10.0	76.2	(199.6)
Foreign Currency Transaction Gain (Loss), Unrealized	(0.4)	0.3	(0.4)	2.9
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0.0	0.0	0.0	0.0
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(0.4)	0.3	(0.4)	2.9
Net unrealized investment gains (losses), pre-tax	25.7	10.2	81.7	(237.8)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(3.3)	0.1	(5.9)	41.1
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	22.4	10.3	75.8	(196.7)
Fixed maturity investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	2.0	(2.0)	19.4	13.3
Foreign Currency Transaction Gain (Loss), Unrealized	5.1	0.0	12.6	0.0
Net unrealized investment gains (losses), pre-tax	7.1	(2.0)	32.0	13.3
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	26.6	8.5	53.5	(256.1)
Foreign Currency Transaction Gain (Loss), Unrealized	(3.1)	0.2	0.0	2.6
Net unrealized investment gains (losses), pre-tax	23.5	8.7	53.5	(253.5)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Unrealized gains (losses) on investments	(2.5)	3.4	9.2	2.1
Foreign Currency Transaction Gain (Loss), Unrealized	(2.4)	0.1	(13.0)	0.3
Net unrealized investment gains (losses), pre-tax	$ (4.9)	$ 3.5	$ (3.8)	$ 2.4

Amount before tax of foreign currency transaction unrealized gain (loss) recognized in the income statement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-URI http://asc.fasb.org/extlink&oid=6526789

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