Summary of Significant Accounting Policies - Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Deferred Policy Acquisition Costs
Policy acquisition costs deferred	$ 5.8	$ 10.1	$ 7.8
Other Underwriting and Operating Expenses
Deferred Policy Acquisition Costs
Amortization of deferred policy acquisition costs	$ 10.6	$ 8.8	$ 8.1