Revision of Previously Issued Consolidated Financial Statements - Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income (loss)	$ 3,445			$ (28,388)		$ 11,571	$ (86,541)
Change in reinsurance payable						10,083	15,601
Change in reinsurance recoverable						(17,242)	(193,259)
Change in ceded unearned premiums						(2,412)	18,045
Change in federal income tax payable (recoverable)						7,344	(11,000)
Net cash provided by (used in) operating activities						$ 47,348	(62,895)
Incorrect Accounting For Reinsurance Treaties [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income (loss)		$ (845)	$ 8,971	(28,388)	$ 8,126		(86,541)	$ (94,351)	$ (1,132)
Change in reinsurance payable			6,155		7,731		15,601	(4,281)	(2,400)
Change in reinsurance recoverable			(6,008)		(6,895)		(193,259)	(180,709)	(65,056)
Change in ceded unearned premiums			5,436		411		18,045	24,552	(34,583)
Change in federal income tax payable (recoverable)			1,687		5,417		(11,000)	(15,557)	(9,134)
Net cash provided by (used in) operating activities			29,476		28,899		(62,895)	(69,327)	27,670
Incorrect Accounting For Reinsurance Treaties [Member] | As Previously Reported [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income (loss)		(467)	9,345	(28,004)	8,878		(85,613)	(91,655)	(625)
Change in reinsurance payable			4,973		5,221		8,072	(12,574)	(8,054)
Change in reinsurance recoverable			(4,584)		(4,242)		(189,006)	(174,765)	(63,437)
Change in ceded unearned premiums			4,720		(683)		20,146	25,295	(31,190)
Change in federal income tax payable (recoverable)			1,787		5,617		(10,753)	(16,647)	(8,999)
Net cash provided by (used in) operating activities			29,476		28,899		(62,895)	(69,327)	27,670
Incorrect Accounting For Reinsurance Treaties [Member] | Revisions [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Net income (loss)		$ (378)	(374)	$ (384)	(752)		(928)	(2,696)	(507)
Change in reinsurance payable			1,182		2,510		7,529	8,293	5,654
Change in reinsurance recoverable			(1,424)		(2,653)		(4,253)	(5,944)	(1,619)
Change in ceded unearned premiums			716		1,094		(2,101)	(743)	(3,393)
Change in federal income tax payable (recoverable)			$ (100)		$ (200)		$ (247)	$ 1,090	$ (135)