Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Available-for-sale fixed maturity securities investments, current, amortized cost	$ 18,269.0	$ 16,991.8
Available-for-sale equity securities investments, current, cost	1,033.7	1,076.1
Available-for-sale fixed maturity securities investments, long-term, amortized cost	524.7	524.6
Available-for-sale equity securities investments, long-term, cost	$ 27.3	$ 27.2
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	257,404,755	263,747,395
Common stock, shares outstanding	257,404,755	263,747,395