Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Statement Of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 6,785.1	$ 6,815.2
Equity securities, cost	$ 446.4	$ 366.5
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock, issued	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	60,500,000	60,500,000
Treasury stock, shares	16,600,000	16,800,000