Summary of Business Segment Information (Detail) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Revenues:
Gross written premiums		$ 174,303	$ 143,749	$ 513,097	$ 464,022
Net written premiums		162,299	130,611	470,635	416,987
Net earned premiums		157,565	140,302	450,673	426,617
Other income (loss)		414	597	1,334	1,509
Total revenues		157,979	140,899	452,007	428,126
Losses and Expenses:
Net losses and loss adjustment expenses		109,195	97,148	290,916	242,092
Acquisition costs and other underwriting expenses		59,282	53,268	171,259	163,258
Income (loss) from segments		(10,498)	(9,517)	(10,168)	22,776
Unallocated Items:
Net investment income (loss)		9,344	11,746	29,813	19,516
Net realized investment gains (losses)		(310)	7,323	7,342	(22,332)
Other income (loss)		(25)	(55)	(47)	(36)
Corporate and other operating expenses		(5,387)	(21,196)	(15,992)	(34,037)
Interest expense		(2,596)	(3,620)	(7,887)	(13,197)
Loss on extinguishment of debt		0	(3,060)	0	(3,060)
Income (loss) before income taxes		(9,472)	(18,379)	3,061	(30,370)
Income tax benefit		1,759	3,209	1,118	8,173
Net income (loss)		(7,713)	(15,170)	4,179	(22,197)
Total assets		1,950,621	1,939,757	1,950,621	1,939,757	$ 1,904,908
Segment Assets
Unallocated Items:
Total assets		1,545,600	1,367,301	1,545,600	1,367,301
Corporate Assets
Unallocated Items:
Total assets		405,021	572,456	405,021	572,456
Commercial Specialty
Revenues:
Gross written premiums		95,734	74,971	279,746	243,099
Net written premiums		89,160	69,074	256,007	219,437
Net earned premiums		84,209	73,887	240,505	211,329
Other income (loss)		0	0	0	0
Total revenues		84,209	73,887	240,505	211,329
Losses and Expenses:
Net losses and loss adjustment expenses		62,545	42,879	167,598	109,191
Acquisition costs and other underwriting expenses		30,257	26,943	88,067	79,452
Income (loss) from segments		(8,593)	4,065	(15,160)	22,686
Unallocated Items:
Net investment income (loss)		0	0	0	0
Net realized investment gains (losses)		0	0	0	0
Other income (loss)		0	0	0	0
Corporate and other operating expenses		0	0	0	0
Interest expense		0	0	0	0
Loss on extinguishment of debt			0		0
Income (loss) before income taxes		0	0	0	0
Income tax benefit		0	0	0	0
Net income (loss)		0	0	0	0
Total assets		0	0	0	0
Commercial Specialty | Segment Assets
Unallocated Items:
Total assets		904,050	752,002	904,050	752,002
Commercial Specialty | Corporate Assets
Unallocated Items:
Total assets		0	0	0	0
Specialty Property
Revenues:
Gross written premiums		30,504	34,730	96,875	107,951
Net written premiums		27,204	29,971	86,652	93,053
Net earned premiums		29,343	31,388	89,826	99,147
Other income (loss)		435	450	1,323	1,306
Total revenues		29,778	31,838	91,149	100,453
Losses and Expenses:
Net losses and loss adjustment expenses		20,516	34,430	50,296	65,619
Acquisition costs and other underwriting expenses		12,127	13,364	37,745	41,357
Income (loss) from segments		(2,865)	(15,956)	3,108	(6,523)
Unallocated Items:
Net investment income (loss)		0	0	0	0
Net realized investment gains (losses)		0	0	0	0
Other income (loss)		0	0	0	0
Corporate and other operating expenses		0	0	0	0
Interest expense		0	0	0	0
Loss on extinguishment of debt			0		0
Income (loss) before income taxes		0	0	0	0
Income tax benefit		0	0	0	0
Net income (loss)		0	0	0	0
Total assets		0	0	0	0
Specialty Property | Segment Assets
Unallocated Items:
Total assets		201,589	207,831	201,589	207,831
Specialty Property | Corporate Assets
Unallocated Items:
Total assets		0	0	0	0
Farm, Ranch & Stable
Revenues:
Gross written premiums		18,500	19,443	60,353	64,798
Net written premiums		16,370	16,961	51,853	56,323
Net earned premiums		17,936	19,978	54,037	57,691
Other income (loss)		37	35	111	107
Total revenues		17,973	20,013	54,148	57,798
Losses and Expenses:
Net losses and loss adjustment expenses		10,678	14,649	33,640	37,698
Acquisition costs and other underwriting expenses		7,267	7,443	21,440	22,687
Income (loss) from segments		28	(2,079)	(932)	(2,587)
Unallocated Items:
Net investment income (loss)		0	0	0	0
Net realized investment gains (losses)		0	0	0	0
Other income (loss)		0	0	0	0
Corporate and other operating expenses		0	0	0	0
Interest expense		0	0	0	0
Loss on extinguishment of debt			0		0
Income (loss) before income taxes		0	0	0	0
Income tax benefit		0	0	0	0
Net income (loss)		0	0	0	0
Total assets		0	0	0	0
Farm, Ranch & Stable | Segment Assets
Unallocated Items:
Total assets		134,854	137,697	134,854	137,697
Farm, Ranch & Stable | Corporate Assets
Unallocated Items:
Total assets		0	0	0	0
Reinsurance Operations
Revenues:
Gross written premiums	[1]	29,565	14,605	76,123	48,174
Net written premiums	[1]	29,565	14,605	76,123	48,174
Net earned premiums	[1]	26,077	15,049	66,305	58,450
Other income (loss)	[1]	(58)	112	(100)	96
Total revenues	[1]	26,019	15,161	66,205	58,546
Losses and Expenses:
Net losses and loss adjustment expenses	[1]	15,456	5,190	39,382	29,584
Acquisition costs and other underwriting expenses	[1]	9,631	5,518	24,007	19,762
Income (loss) from segments	[1]	932	4,453	2,816	9,200
Unallocated Items:
Net investment income (loss)	[1]	0	0	0	0
Net realized investment gains (losses)	[1]	0	0	0	0
Other income (loss)	[1]	0	0	0	0
Corporate and other operating expenses	[1]	0	0	0	0
Interest expense	[1]	0	0	0	0
Loss on extinguishment of debt	[1]		0		0
Income (loss) before income taxes	[1]	0	0	0	0
Income tax benefit	[1]	0	0	0	0
Net income (loss)	[1]	0	0	0	0
Total assets	[1]	0	0	0	0
Reinsurance Operations | Segment Assets
Unallocated Items:
Total assets	[1]	305,107	269,771	305,107	269,771
Reinsurance Operations | Corporate Assets
Unallocated Items:
Total assets	[1]	$ 0	$ 0	$ 0	$ 0

[1] External business only, excluding business assumed from affiliates.