Revision of Previously Issued Consolidated Financial Statements - Balance Sheets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Ceded unearned premiums	$ 145,858			$ 143,446
Reinsurance recoverable	515,088			497,846
Federal income tax recoverable	17,347			24,691
Total assets	1,520,208			1,496,717
Reinsurance payable	71,183			61,100
Total liabilities	1,342,428			1,329,011
Retained earnings	77,270			65,699
Total stockholders' equity	177,780			167,706	$ 175,251
Liabilities and Equity	$ 1,520,208			1,496,717
Incorrect Accounting For Reinsurance Treaties [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Ceded unearned premiums		$ 143,034	$ 138,010	143,446	146,176	$ 167,998
Reinsurance recoverable		504,742	503,854	497,846	508,725	317,137
Federal income tax recoverable		19,274	23,003	24,691	19,995	9,134
Total assets		1,519,637	1,529,534	1,496,717	1,504,108	1,500,861
Reinsurance payable		68,832	67,255	61,100	74,875	65,381
Total liabilities		1,344,721	1,353,373	1,329,011	1,328,857	1,238,099
Retained earnings		73,825	74,668	65,699	74,029	160,049
Total stockholders' equity		174,916	176,160	167,706	175,251	262,761
Liabilities and Equity		1,519,637	1,529,533	1,496,717	1,504,108	1,500,860
Incorrect Accounting For Reinsurance Treaties [Member] | As Previously Reported [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Ceded unearned premiums		139,609	134,206	138,926	144,075	164,221
Reinsurance recoverable		494,473	494,815	490,231	504,472	315,466
Federal income tax recoverable		20,025	23,855	25,642	19,748	8,995
Total assets		1,506,694	1,517,543	1,485,533	1,497,507	1,495,274
Reinsurance payable		51,921	51,673	46,700	67,346	59,274
Total liabilities		1,327,810	1,337,791	1,314,611	1,321,328	1,231,992
Retained earnings		77,793	78,260	68,915	74,957	160,570
Total stockholders' equity		178,884	179,752	170,922	176,179	263,282
Liabilities and Equity		1,506,694	1,517,543	1,485,533	1,497,507	1,495,274
Incorrect Accounting For Reinsurance Treaties [Member] | Revisions [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Ceded unearned premiums		3,425	3,804	4,520	2,101	3,777
Reinsurance recoverable		10,269	9,039	7,615	4,253	1,671
Federal income tax recoverable		(751)	(852)	(951)	247	139
Total assets		12,943	11,991	11,184	6,601	5,587
Reinsurance payable		16,911	15,582	14,400	7,529	6,107
Total liabilities		16,911	15,582	14,400	7,529	6,107
Retained earnings		(3,968)	(3,592)	(3,216)	(928)	(521)
Total stockholders' equity		(3,968)	(3,592)	(3,216)	(928)	(521)
Liabilities and Equity		$ 12,943	$ 11,990	$ 11,184	$ 6,601	$ 5,586