Summary of Significant Accounting Policies - Allowance for Credit Losses on Premium Receivables (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Beginning balance	$ 1,445	$ 1,458	$ 1,258
Provision during the period for expected credit losses	10,294	6,162	6,463
Write-off amounts during the period	(4,406)	(7,000)	(6,908)
Recoveries during the period of amounts previously written off	812	825	645
Ending balance	10,000	1,445	1,458
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Beginning balance	1,855	0	0
Ending balance		1,855	0
Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Beginning balance	$ 3,300	1,458	1,258
Ending balance		$ 3,300	$ 1,458