Comprehensive Income - Components of Comprehensive Income, Net of Related Tax (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Components Of Comprehensive Income [Abstract]
Net income	$ 41,300	$ 40,131	$ 67,432	$ 80,474
Other comprehensive income:
Unrealized gains (losses) arising during the period, net of tax (benefit) expense of $3,634, $(3,870), $11,353 and $652	6,749	(7,188)	21,085	1,212
Reclassification adjustment for (gains) losses included in net income, net of tax (benefit) expense of $79, $5, $104 and $2	(147)	(9)	(193)	(3)
Unrealized gains (losses) arising during the period, net of tax expense (benefit) of $833, $(860), $2,527 and $645	1,548	(1,597)	4,693	1,196
Reclassification adjustment for gains included in net income, net of tax expense of $191, $285, $379 and $568	(355)	(529)	(704)	(1,055)
Unrealized gains arising during the period, net of tax expense of $214, $189, $427 and $377	397	350	793	700
Other comprehensive income (loss)	8,192	(8,973)	25,674	2,050
Comprehensive income	$ 49,492	$ 31,158	$ 93,106	$ 82,524