Other Comprehensive Income (Loss) - Reclassifications Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net realized gains (losses) on investments	$ 11	$ 501	$ 292	$ 5,353
Income taxes	(17,602)	(15,811)	(49,811)	(38,662)
NET INCOME	30,298	21,341	77,332	52,016
Accumulated Net Unrealized Investment Gain (Loss) [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net realized gains (losses) on investments	11	501	292	5,353
Income taxes	(4)	(193)	(113)	(2,065)
NET INCOME	$ 7	$ 308	$ 179	$ 3,288