Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Statement of Financial Position [Abstract]
Fixed maturities, available-for-sale, amortized cost	$ 66,036.9	$ 65,546.3
Equity securities, available-for-sale, cost	247.8	300.4
Mortgage loans on real estate, valuation allowance	3.0	3.2
Securities pledged amortized cost	$ 1,867.4	$ 1,082.1
Common stock, shares authorized	900,000,000	900,000,000
Common stock, shares issued	268,040,988	265,327,196
Common stock, shares outstanding	194,605,971	209,095,793
Treasury stock, shares	73,435,017	56,231,403
Common stock, par value (usd per share)	$ 0.01	$ 0.01