Consolidated Quarterly Results - Unaudited (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Net premiums, fees, and other income	$ 1,248.5	$ 1,295.5	$ 1,245.4	$ 1,186.0	$ 1,195.3	$ 1,204.0	$ 1,116.8	$ 1,069.1
Net investment income and realized gains (losses)	83.0	83.7	215.9	83.8	87.8	100.6	107.0	88.7
Total revenues	1,331.6	1,379.5	1,461.5	1,269.9	1,283.2	1,304.8	1,223.9	1,158.0	5,442.7	4,970.1	4,645.5
Benefits, claims, and expenses	1,189.3	1,229.7	1,165.2	1,185.4	1,319.7	1,335.9	1,282.2	1,160.7	4,769.7	5,098.7	4,882.2
Net income (loss)	$ 94.7	$ 102.9	$ 193.9	$ 56.2	$ (20.2)	$ (14.8)	$ (34.0)	$ 0.4	$ 447.8	$ (68.6)	$ (140.5)
Net income (loss) per share: basic (in dollars per share)	$ 0.37	$ 0.40	$ 0.76	$ 0.22	$ (0.08)	$ (0.06)	$ (0.13)	$ 0.00	$ 1.74	$ (0.27)	$ (0.55)
Net income (loss) per share: diluted (in dollars per share)	$ 0.33	$ 0.36	$ 0.67	$ 0.21	$ (0.08)	$ (0.06)	$ (0.13)	$ 0.00	$ 1.57	$ (0.27)	$ (0.55)
Average shares outstanding: Basic (in shares)	257,706,005	257,098,894	256,749,748	256,279,364	256,086,431	255,921,356	255,747,273	255,473,634	257,443,999	255,812,888	255,045,210
Average shares outstanding: Diluted (in shares)	294,396,055	293,444,269	292,842,386	292,081,785	256,086,431	255,921,356	255,747,273	255,779,449	293,684,035	255,812,888	255,045,210