Condensed Consolidated Balance Sheets (Parenthetical) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Fixed maturities, at amortized cost	$ 6,667,300,000	$ 6,372,700,000
Fixed maturities, allowance for credit losses	4,600,000	0
Premium receivables, allowance for credit losses	23,100,000	22,300,000
Equity securities, at cost	698,000,000	818,800,000
Debt, at fair value	$ 779,700,000	$ 820,200,000
Common stock, par value (in dollars per share)	$ 0.1	$ 0.1
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares issued (in shares)	65,365,417	66,665,888
Common stock, shares outstanding (in shares)	65,365,417	66,665,888