Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 6,945.6	$ 6,452.2
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	20,000,000.0	20,000,000.0
Preferred stock, issued	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	300,000,000.0	300,000,000.0
Common stock, shares issued	60,500,000	60,500,000
Treasury stock, shares	24,100,000	22,100,000