Investment Securities (Investment Gains (Losses) For Level 3) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investment securities
Realized gains	$ 61.5	$ 283.7	$ 48.7
Realized losses	37.4	13.7	18.6
Unrealized investment gains	97.5	(294.7)	235.5
Level 3 Inputs
Investment securities
Unrealized investment gains	(15.4)	(14.2)	(8.1)
Fixed maturity investments
Investment securities
Unrealized investment gains	13.8	(14.6)	0.1
Fixed maturity investments \| Level 3 Inputs
Investment securities
Unrealized investment gains	0.0	0.1	0.0
Common equity securities
Investment securities
Unrealized investment gains	99.3	(257.4)	240.8
Common equity securities \| Level 3 Inputs
Investment securities
Unrealized investment gains	0.0	0.0	(9.0)
Other long-term investments
Investment securities
Unrealized investment gains	(15.6)	(22.7)	(5.4)
Other long-term investments \| Level 3 Inputs
Investment securities
Unrealized investment gains	$ (15.4)	$ (14.3)	$ 0.9

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This item represents the gain realized during the period from the sale of trading securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7(c))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7(c))
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 35
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82895926&loc=d3e24584-111560

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-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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