Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains, pre-tax	$ 1.1	$ 256.8
Net unrealized investment gains (losses), pre-tax	50.2	(227.3)
Net realized and unrealized investment gains, pre-tax	51.3	29.5
Income tax expense attributable to net realized and unrealized investment gains	(7.2)	(8.5)
Net realized and unrealized investment gains, after tax	44.1	21.0
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains, pre-tax	(2.1)	(1.1)
Net unrealized investment gains (losses), pre-tax	19.2	21.7
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains, pre-tax	1.3	257.6
Net unrealized investment gains (losses), pre-tax	29.6	(247.4)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains, pre-tax	1.9	0.3
Net unrealized investment gains (losses), pre-tax	$ 4.4	$ (1.6)

The sum of the realized and unrealized net gain (loss) on trading securities during the period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27476-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Trading Securities
-URI http://asc.fasb.org/extlink&oid=6526789

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