CONSOLIDATED BALANCE SHEETS (UNAUDITED) (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Statement of Financial Position [Abstract]
Fixed maturities, available for sale, amortized cost	$ 5,875.0	$ 5,652.9
Equity Securities, FV-NI, Cost	$ 102.9	$ 86.2
Preferred stock, par value (in usd per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	1,000,000	1,000,000
Preferred stock, shares issued (in shares)	0	0
Common stock, par value (in usd per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	75,000,000	75,000,000
Common stock, shares issued (in shares)	66,894,309	66,747,821
Treasury stock (in shares)	26,167,246	25,911,087
Reinsurance balances receivable	$ 438.6	$ 480.5
Other assets	$ 397.4	$ 357.6