Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Per Share data	Sep. 30, 2011	Dec. 31, 2010
Consolidated Balance Sheets [Abstract]
Fixed maturities, amortized cost	$ 5,966.1	$ 4,598.8
Equity securities, cost	$ 249.2	$ 120.7
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	20.0	20.0
Preferred stock, shares issued	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	300.0	300.0
Common stock, shares issued	60.5	60.5
Treasury stock, shares	15.9	15.6