Investment Securities Investment Securities (Net Unrealized Investment Gains (Losses)) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	$ (267.3)	$ 152.9	$ (79.3)
Foreign Currency Transaction Gain (Loss), Unrealized	(2.7)	(4.8)	(9.0)
Net unrealized investment (losses) gains, pre-tax	(270.0)	148.1	(88.3)
Trading Securities, Tax on Unrealized Holding Gain (Loss) on Investments	40.7	(12.3)	9.9
Trading Securities, Unrealized Foreign Currency Transaction Gain (Loss) Tax	0.0	(0.1)	0.5
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Tax	(40.7)	(12.4)	10.4
Trading Securities Unrealized Holding Gain (Loss) on Investments after Tax	(226.6)	140.6	(69.4)
Trading Securities, Unrealized Foreign Currency Gain (Loss) Net of Tax	(2.7)	(4.9)	(8.5)
Net Unrealized Gain (Loss) and Foreign Currency Gain (Loss) on Trading Securities Net of Tax	(229.3)	135.7	(77.9)
Fixed maturity investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	(12.1)	(15.6)	11.3
Foreign Currency Transaction Gain (Loss), Unrealized	2.1	0.0	0.0
Net unrealized investment (losses) gains, pre-tax	(10.0)	(15.6)	11.3
Common equity securities
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	(254.6)	207.6	(83.0)
Foreign Currency Transaction Gain (Loss), Unrealized	(3.3)	(3.7)	(7.7)
Net unrealized investment (losses) gains, pre-tax	(257.9)	203.9	(90.7)
Other long-term investments
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	(0.6)	(39.1)	(7.6)
Foreign Currency Transaction Gain (Loss), Unrealized	(0.3)	(1.1)	(1.3)
Net unrealized investment (losses) gains, pre-tax	(0.9)	$ (40.2)	$ (8.9)
Forward Contracts [Member]
Investment securities
Trading Securities, Change in Unrealized Holding Gain (Loss)	0.0
Foreign Currency Transaction Gain (Loss), Unrealized	(1.2)
Net unrealized investment (losses) gains, pre-tax	$ (1.2)

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
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 20
-Section 45
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Trading Securities
-URI http://asc.fasb.org/extlink&oid=6526789

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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