Summarized Activity in Liability for Unpaid Losses and Loss Adjustment Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Balance at beginning of period	$ 615,763	$ 683,850	$ 651,042	$ 680,047
Less: Ceded reinsurance receivables	104,245	111,579	130,439	108,130
Net balance at beginning of period	511,518	572,271	520,603	571,917
Purchased reserves, gross	9,063	1,410	18,024	1,410
Purchased reserves ceded	63	641	573	641
Purchased reserves, net of third party reinsurance	9,126	2,051	18,597	2,051
Incurred losses and loss adjustment expenses related to:
Current year	91,766	81,579	237,460	239,991
Prior years	(9,371)	(9,417)	(34,804)	(24,934)
Total incurred losses and loss adjustment expenses	82,395	72,162	202,656	215,057
Paid losses and loss adjustment expenses related to:
Current year	45,193	49,704	115,927	113,090
Prior years	25,190	35,147	93,273	114,302
Total paid losses and loss adjustment expenses	70,383	84,851	209,200	227,392
Net balance at end of period	532,656	561,633	532,656	561,633
Plus: Ceded reinsurance receivables	117,070	102,749	117,070	102,749
Balance at end of period	$ 649,726	$ 664,382	$ 649,726	$ 664,382