Investments - Summary of Amortized Cost and Fair Values of Fixed Maturity Securities (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Cost	$ 31.2	$ 31.0
Gross unrealized losses	(1.9)	(2.2)
Fair value	29.3	28.8
U.S. Treasury securities, obligations of U.S. Government agencies and municipalities
Debt Securities, Available-for-sale [Line Items]
Cost	26.1	22.8
Gross unrealized losses	(1.6)	(1.8)
Fair value	24.5	21.0
Corporate debt
Debt Securities, Available-for-sale [Line Items]
Cost	5.1	8.2
Gross unrealized losses	(0.3)	(0.4)
Fair value	$ 4.8	$ 7.8