Supplementary Insurance Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Supplementary Insurance Information, by Segment [Line Items]
Premium Revenue	$ 525,262	$ 467,775	$ 438,034
Benefits, Claims, Losses And Settlement Expenses	275,402	334,625	269,212
Amortization of Deferred Policy Acquisition Costs	132,329	117,988	108,964
Net Written Premium	562,089	472,547	450,180
Commercial Specialty
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	27,415	23,059	21,222
Future Policy Benefits, Losses, Claims And Loss Expenses	390,148	417,175	419,042
Unearned Premiums	134,433	110,704	102,191
Other Policy and Benefits Payable	0	0	0
Premium Revenue	237,758	218,357	178,798
Benefits, Claims, Losses And Settlement Expenses	108,911	114,476	62,834
Amortization of Deferred Policy Acquisition Costs	56,339	49,715	42,008
Net Written Premium	258,719	226,827	186,448
Specialty Property
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	18,249	18,161	18,668
Future Policy Benefits, Losses, Claims And Loss Expenses	50,334	82,722	68,900
Unearned Premiums	81,922	88,809	99,631
Other Policy and Benefits Payable	0	0	0
Premium Revenue	140,232	128,768	149,786
Benefits, Claims, Losses And Settlement Expenses	75,426	122,709	112,055
Amortization of Deferred Policy Acquisition Costs	37,811	37,854	38,893
Net Written Premium	140,670	127,470	144,271
Farm, Ranch & Stable
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	9,612	8,897	8,895
Future Policy Benefits, Losses, Claims And Loss Expenses	45,601	50,923	51,355
Unearned Premiums	44,048	40,265	38,073
Other Policy and Benefits Payable	0	0	0
Premium Revenue	71,312	69,248	66,197
Benefits, Claims, Losses And Settlement Expenses	42,700	41,180	53,743
Amortization of Deferred Policy Acquisition Costs	18,307	17,536	17,723
Net Written Premium	74,416	70,217	65,528
Reinsurance Operations
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Costs	15,401	11,559	12,862
Future Policy Benefits, Losses, Claims And Loss Expenses	144,098	129,211	95,367
Unearned Premiums	54,458	42,134	45,502
Other Policy and Benefits Payable	0	0	0
Premium Revenue	75,960	51,402	43,253
Benefits, Claims, Losses And Settlement Expenses	48,365	56,260	40,580
Amortization of Deferred Policy Acquisition Costs	19,872	12,883	10,340
Net Written Premium	$ 88,284	$ 48,033	$ 53,933