Condensed Consolidated Balance Sheets (Parenthetical) - USD ($)	Mar. 31, 2022	Dec. 31, 2021
Statement of Financial Position [Abstract]
Fixed maturities, at amortized cost	$ 7,810,400,000	$ 7,358,200,000
Fixed maturities, allowance for credit losses	(9,100,000)	(7,500,000)
Equity securities, at cost	466,000,000.0	618,700,000
Premium receivables, allowance for credit losses	13,000,000.0	13,600,000
Long-term Debt	$ 1,343,900,000	$ 1,152,100,000
Common stock, par value (in dollars per share)	$ 0.10	$ 0.10
Common stock, shares authorized (in shares)		100,000,000
Common stock, shares outstanding (in shares)	63,800,041	63,684,628