Comprehensive Income - Components of Comprehensive Income, Net of Related Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Components Of Comprehensive Income [Abstract]
Net income	$ 52,168	$ 40,053	$ 119,600	$ 120,527
Other comprehensive income (loss):
Unrealized (losses) gains arising during the period, net of tax (benefit) expense of $(1,535), $3,650, $9,818 and $4,302	(2,851)	6,778	18,234	7,989
Reclassification adjustment for gains included in net income, net of tax expense of $105, $110, $209 and $112	(195)	(204)	(388)	(207)
Unrealized gains arising during the period, net of tax expense of $284, $1,669, $2,811 and $2,313	528	3,099	5,221	4,295
Reclassification adjustment for gains included in net income, net of tax expense of $138, $246, $517 and $814	(256)	(457)	(960)	(1,511)
Unrealized gains arising during the period, net of tax expense of $205, $183, $632 and $560	380	340	1,173	1,040
Other comprehensive (loss) income	(2,394)	9,556	23,280	11,606
Comprehensive income	$ 49,774	$ 49,609	$ 142,880	$ 132,133