Segment Information - Summary of Financial Data by Segment (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Premiums earned:
Premiums earned	$ 192,488,665	$ 184,373,768	$ 379,740,265	$ 371,626,546
Net investment income	7,652,180	7,172,144	15,162,758	14,548,428
Investment gains (losses)	4,241,031	6,485,896	6,710,085	(4,208,947)
Total revenues	205,145,943	198,899,772	403,115,841	383,810,978
Underwriting (loss) income:
Net investment income	7,652,180	7,172,144	15,162,758	14,548,428
Net investment gains (losses)	4,241,031	6,485,896	6,710,085	(4,208,947)
Income before income tax expense	19,696,825	27,988,049	32,447,510	30,491,220
Operating Segments [Member]
Premiums earned:
Premiums earned	192,489,000	184,374,000	379,740,000	371,627,000
Net investment income	7,652,000	7,172,000	15,163,000	14,548,000
Investment gains (losses)	4,241,000	6,486,000	6,710,000	(4,209,000)
Other	764,000	868,000	1,503,000	1,845,000
Total revenues	205,146,000	198,900,000	403,116,000	383,811,000
Underwriting (loss) income:
SAP underwriting (loss) income	2,314,000	12,762,000	(891,000)	17,007,000
GAAP adjustments	5,256,000	1,378,000	11,238,000	2,762,000
GAAP underwriting income	7,570,000	14,140,000	10,347,000	19,769,000
Net investment income	7,652,000	7,172,000	15,163,000	14,548,000
Net investment gains (losses)	4,241,000	6,486,000	6,710,000	(4,209,000)
Other	234,000	190,000	228,000	383,000
Income before income tax expense	19,697,000	27,988,000	32,448,000	30,491,000
Commercial Lines [Member] | Operating Segments [Member]
Premiums earned:
Premiums earned	115,300,000	101,870,000	224,525,000	203,645,000
Underwriting (loss) income:
SAP underwriting (loss) income	1,767,000	4,472,000	(6,475,000)	3,906,000
Personal Lines [Member] | Operating Segments [Member]
Premiums earned:
Premiums earned	77,189,000	82,504,000	155,215,000	167,982,000
Underwriting (loss) income:
SAP underwriting (loss) income	$ 547,000	$ 8,290,000	$ 5,584,000	$ 13,101,000