Significant Accounting Policies - Home Office and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Significant Accounting Policies [Line Items]
Accumulated depreciation of home office and equipment	$ 55.4	$ 51.2	$ 43.0
Depreciation expense of home office and equipment	$ 5.6	$ 6.3	$ 6.5
Minimum
Significant Accounting Policies [Line Items]
Estimated useful life	3 years
Maximum
Significant Accounting Policies [Line Items]
Estimated useful life	45 years