Schedule III - Supplementary Insurance Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Net Premiums Earned	$ 756,078	$ 741,291	$ 702,515
Net Investment Income	29,515	26,908	23,527
Net Losses and Loss Expenses	506,388	576,458	487,268
Amortization of Deferred Policy Acquisition Costs	122,443	120,964	115,065
Other Underwriting Expenses	114,562	113,270	116,538
Net Premiums Written	752,601	744,016	729,139
Deferred Policy Acquisition Costs	59,285	60,615
Liability For Losses and Loss Expenses	869,674	814,665
Unearned Premiums	510,147	506,529
Commercial Lines [Member]
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Net Premiums Earned	385,465	337,924	318,391
Net Losses and Loss Expenses	242,685	246,048	197,344
Amortization of Deferred Policy Acquisition Costs	62,424	55,143	52,149
Other Underwriting Expenses	61,631	51,635	52,817
Net Premiums Written	404,879	349,895	329,116
Deferred Policy Acquisition Costs	30,947	27,762
Liability For Losses and Loss Expenses	582,682	518,127
Unearned Premiums	266,297	231,990
Personal Lines [Member]
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Net Premiums Earned	370,613	403,367	384,124
Net Losses and Loss Expenses	263,703	330,410	289,924
Amortization of Deferred Policy Acquisition Costs	60,019	65,821	62,916
Other Underwriting Expenses	52,931	61,635	63,721
Net Premiums Written	347,722	394,121	400,023
Deferred Policy Acquisition Costs	28,338	32,853
Liability For Losses and Loss Expenses	286,992	296,538
Unearned Premiums	243,850	274,539
Investments [Member]
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items]
Net Investment Income	$ 29,515	$ 26,908	$ 23,527