CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Statement of Financial Position [Abstract]
Fixed maturities available-for-sale, amortized cost	$ 645.4	$ 673.5
Short -term investments, cost	73.4	99.9
Premium receivable, allowance for doubtful accounts	$ 2.5	$ 2.7
Common stock, par value (usd per share)	$ 0.00001	$ 0.00001
Common stock, authorized (in shares)	200,000,000	200,000,000
Common stock, issued (in shares)	69,909,356	69,275,030
Common stock, outstanding (in shares)	69,909,356	69,275,030