Condensed Consolidated Balance Sheets (Parenthetical) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Fixed maturities, at amortized cost	$ 7,222,500,000	$ 6,692,700,000
Fixed maturities, allowance for credit losses	(4,400,000)	(3,300,000)
Equity securities, at cost	725,100,000	684,100,000
Premium receivables, allowance for credit losses	12,700,000	20,900,000
Long-term Debt	$ 1,166,100,000	$ 1,247,800,000
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)	63,651,526	65,436,207