Summary of Condensed Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Gross Written Premiums	$ 464,558	$ 455,943	$ 497,236	$ 495,224	$ 408,743	$ 402,038	$ 452,179	$ 450,305	$ 1,912,961	$ 1,713,265	$ 1,568,911
Revenues:
Net Written Premiums	346,119	352,412	379,292	393,262	304,869	296,016	333,282	337,163	1,471,085	1,271,330	1,186,224
Change in Net Unearned Premiums	16,424	(5,374)	(48,277)	(70,635)	230	5,339	(39,447)	(51,032)	(107,862)	(84,910)	(85,879)
Net Earned Premiums	362,543	347,038	331,015	322,627	305,099	301,355	293,835	286,131	1,363,223	1,186,420	1,100,345
Net Investment Income	29,159	25,651	24,601	23,702	22,982	22,598	22,265	21,448	103,113	89,293	79,451
Net Realized and Unrealized Gains (Losses):
Total Other-Than-Temporary Impairment Losses	(48)	(15)	(18)	(37)	3	(957)	29	(1,077)
Portion of Loss Recognized in Other Comprehensive Income	48	15	18	37	(3)	(15)	(29)	(16)	118	(62)	77
Net Other-Than-Temporary Impairment Losses Recognized in Earnings						(972)		(1,093)	0	(2,064)	(150)
Net Realized Gains on Investments Sold	(220)	253	1,787	1,169					2,989	45,073	9,186
Net Unrealized Gains (Losses) on Equity Securities	(33,795)	2,310	1,329	(3,181)					(33,337)	0	0
Total Net Realized and Unrealized Gains (Losses)	(34,015)	2,563	3,116	(2,012)					(30,348)	43,009	9,036
Net Realized Gains					37,141	5,190	1,694	1,049
Other Income (Loss)	2,934	1,724	2,628	(117)	(3,201)	(1,699)	(411)	1,068	7,169	(4,243)	8,701
Total Revenues	360,621	376,976	361,360	344,200	362,021	326,472	317,383	308,603	1,443,157	1,314,479	1,197,533
Expenses:
Net Losses and LAE	270,401	240,900	196,333	186,145	183,384	276,171	177,110	169,600	893,779	806,265	665,448
Commission Expenses	54,800	57,999	53,193	54,152	43,205	45,509	48,173	47,844	220,144	184,731	165,045
Other Operating Expenses	62,532	67,584	68,182	62,926	68,153	45,773	60,766	58,538	261,224	233,230	234,096
Merger Transaction Costs	1,511	2,439							3,950	0	0
Interest Expense	3,866	3,891	3,864	3,864	3,863	3,862	3,861	3,861	15,485	15,447	15,435
Total Expenses	393,110	372,813	321,572	307,087	298,605	371,315	289,910	279,843	1,394,582	1,239,673	1,080,024
Income Before Income Taxes	(32,489)	4,163	39,788	37,113	63,416	(44,843)	27,473	28,760	48,575	74,806	117,509
Income Tax Expense	879	(462)	7,684	6,235	36,555	(16,864)	6,971	7,650	14,336	34,312	34,783
Net Income	(33,368)	4,625	32,104	30,878	26,861	(27,979)	20,502	21,110	34,239	40,494	82,726
Comprehensive Income (Loss)	$ (41,960)	$ (2,681)	$ 15,373	$ (2,962)	$ (1,812)	$ (18,311)	$ 35,699	$ 35,911	$ (32,230)	$ 51,487	$ 67,997
Combined Ratio	106.60%	105.30%	96.00%	94.00%	96.60%	121.90%	97.30%	96.40%	100.70%	103.20%	96.70%
Basic	$ (1.12)	$ 0.16	$ 1.08	$ 1.04	$ 0.91	$ (0.95)	$ 0.70	$ 0.72	$ 1.15	$ 1.38	$ 2.85
Diluted	$ (1.12)	$ 0.15	$ 1.07	$ 1.02	$ 0.89	$ (0.95)	$ 0.69	$ 0.70	$ 1.13	$ 1.35	$ 2.75