Summary of Significant Accounting Policies - Reinsurance - Additional Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Amount of allowance for uncollectible amounts on paid recoverables	$ 15,900,000		$ 15,700,000
Reinsurance balances recoverable on unpaid losses and settlement expenses, allowances for uncollectible amounts	8,097,000		$ 9,402,000
Financing Receivable, Allowance for Credit Loss, Writeoff	0
Financing Receivable, Allowance for Credit Loss, Recovery	$ 0
Accounting Standards Update 2016-13
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Adoption impact of ASU 2016-13 on reinsurance balances recoverable		$ 1,345,000
Accounting Standards Update 2016-13 | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Adoption impact of ASU 2016-13 on reinsurance balances recoverable		$ 1,300,000