Investments - Additional Information (Details)	12 Months Ended
	Dec. 31, 2021 USD ($) security	Dec. 31, 2020 USD ($)
Investment Holdings [Line Items]
Accrued investment income	$ 17,505,000	$ 16,126,000
ASU 2016-13
Investment Holdings [Line Items]
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]	true
Change in Accounting Principle, Accounting Standards Update, Adoption Date	Jan. 01, 2020
Debt Securities
Investment Holdings [Line Items]
Accrued investment income	$ 16,400	14,900
Number of debt securities for which there is an allowance for credit losses | security	7
Net realized gains (losses)	$ 0	(600,000)
Number of debt securities in an unrealized loss position for which an allowance for credit losses has not been recorded | security	443
Unrealized loss	$ 16,891,000	2,554,000
Unrealized losses as percentage of fixed income portfolio cost basis	0.70%
Unrealized losses relative to total invested assets (as a percent)	0.50%
Number of unrealized loss positions | security	443
Number of securities in unrealized loss positions for 12 months or longer | security	76
ABS/CMBS/MBS
Investment Holdings [Line Items]
Percentage of securities of portfolio rated with highest credit rating by by one or more major rating agency	83.00%
Municipal
Investment Holdings [Line Items]
General obligations to state and local governments Percentage	48.00%
Revenue based obligations percentage	52.00%
Percentage of securities of portfolio rated as AA or better	90.00%
Percentage of securities of portfolio rated as A or better	99.00%
Unrealized loss	$ 4,281,000	$ 218,000