Summary of Significant Accounting Policies - Deferred Policy Acquisition Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred Policy Acquisition Costs
Policy acquisition costs deferred	$ 10.1	$ 7.8	$ 7.4
Other underwriting and operating expenses
Deferred Policy Acquisition Costs
Amortization of deferred policy acquisition costs	$ 8.8	$ 8.1	$ 6.7