Other Assets (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Ceding commission receivable	$ 14.5	$ 5.4
Prepaid expenses	10.6	7.5
Investment income due and accrued	3.4	0.9
Other	2.8	0.7
Total other assets	53.2	14.5
Other Assets
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Line Items]
Right-of-use asset	$ 21.9	$ 0.0