Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Fixed maturity securities available for sale, amortized cost	$ 8,888.9	$ 8,870.8
Equity securities available for sale, cost	348.7	381.8
Other assets (includes assets of special purpose entities of $349.7 in 2017)	349.7	0.0
Accounts payable and other liabilities (includes liabilities of special purpose entities of $279.7 in 2017)	$ 279.7	$ 0.0
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	800,000,000	800,000,000
Common stock, shares outstanding (in shares)	53,796,348	55,941,480
Treasury stock, at cost (in shares)	96,577,040	94,041,583