Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Statement of Financial Position [Abstract]
Available-for-sale fixed maturity securities investments, current, amortized cost	$ 17,120.4	$ 16,826.7
Available-for-sale equity securities investments, current, cost	1,303.7	1,168.5
Available-for-sale fixed maturity securities investments, long-term, amortized cost	500.7	455.9
Available-for-sale equity securities investments, long-term, cost	$ 27.0	$ 27.4
Preferred stock, par value	$ 0	$ 0
Preferred stock, shares authorized	100,000,000	100,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	900,000,000	900,000,000
Common stock, shares issued	268,109,932	293,273,830
Common stock, shares outstanding	268,109,932	293,273,830