Summarized Activity in Liability for Unpaid Losses and Loss Adjustment Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Insurance [Abstract]
Balance at beginning of period	$ 613,670	$ 615,763	$ 634,664	$ 651,042
Less: Ceded reinsurance receivables	91,397	104,245	97,243	130,439
Net balance at beginning of period	522,273	511,518	537,421	520,603
Purchased reserves, gross	0	9,063	0	18,024
Purchased reserves ceded	0	63	0	573
Purchased reserves, net of third party reinsurance	0	9,126	0	18,597
Incurred losses and loss adjustment expenses related to:
Current year	92,469	91,766	222,916	237,460
Prior years	(11,976)	(9,371)	(27,490)	(34,804)
Total incurred losses and loss adjustment expenses	80,493	82,395	195,426	202,656
Paid losses and loss adjustment expenses related to:
Current year	53,121	45,193	103,695	115,927
Prior years	27,312	25,190	106,819	93,273
Total paid losses and loss adjustment expenses	80,433	70,383	210,514	209,200
Net balance at end of period	522,333	532,656	522,333	532,656
Plus: Ceded reinsurance receivables	86,274	117,070	86,274	117,070
Balance at end of period	$ 608,607	$ 649,726	$ 608,607	$ 649,726