Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Amortized cost of fixed maturities	$ 6,372.7	$ 6,284.5
Cost of equity securities	818.8	720.8
Debt	$ 820.2	$ 911.2
Common stock, par value (in dollars per share)	$ 0.10	$ 0.1
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares issued (in shares)	66,665,888	64,756,833
Common stock, shares outstanding (in shares)	66,665,888	64,756,833