Condensed Consolidated Balance Sheets Parenthetical - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Statement of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 20,256.8	$ 19,085.0
Equity securities, cost	117.4	107.4
Mortgage loans on real estate valuation allowance	1.2	1.1
Securities pledged, amortized costs	$ 261.8	$ 224.4
Common stock, par value	$ 50	$ 50
Common stock, shares authorized	100,000	100,000
Common stock, shares issued	55,000	55,000
Common stock, shares outstanding	55,000	55,000