Investment Securities (Net Realized and Unrealized Investment Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Equity in earnings of unconsolidated affiliates, net of tax	$ 0.0	$ 7.3
Net realized investment gains, pre-tax	256.8	14.8
Net unrealized investment (losses) gains, pre-tax	(227.3)	10.2
Net realized and unrealized investment gains, pre-tax	29.5	25.0
Income tax expense attributable to net realized and unrealized investment gains	(8.5)	(7.1)
Net realized and unrealized investment gains, after tax	21.0	17.9
Fixed maturity investments.
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains, pre-tax	(1.1)	0.5
Net unrealized investment (losses) gains, pre-tax	21.7	11.4
Short-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains, pre-tax	0.0	0.0
Net unrealized investment (losses) gains, pre-tax	0.0	0.0
Common equity securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains, pre-tax	257.6	22.5
Net unrealized investment (losses) gains, pre-tax	(247.4)	(6.8)
Other long-term investments
Schedule of Trading Securities and Other Trading Assets [Line Items]
Net realized investment gains, pre-tax	0.3	(8.2)
Net unrealized investment (losses) gains, pre-tax	$ (1.6)	$ 5.6

This item represents the entity's proportionate share for the period of the net income (loss) of its investee (such as unconsolidated subsidiaries and joint ventures) to which the equity method of accounting is applied. This item includes income or expense related to stock-based compensation based on the investor's grant of stock to employees of an equity method investee.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=35755396&loc=d3e33749-111570

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.12)
-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 9
-Article 5

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 04
-Paragraph 11
-Article 7

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+ References

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=27724398&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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