Comprehensive Income and Accumulated Other Comprehensive Income - Reclassification 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
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	$ 25,890	$ 19,348	$ 67,199	$ 49,813
Income tax expense	(7,950)	(5,869)	(19,810)	(13,012)
Net income	17,940	13,479	47,389	36,801
Changes in Net Unrealized Gains (Losses) on Investment Securities [Member] | Reclassification Out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Unrealized gains/(losses) on available-for-sale securities	142	1	464	(270)
Other-than-temporary impairment		5	(1,600)	(8)
Income before income taxes	142	6	(1,136)	(278)
Income tax expense	(50)	(2)	398	97
Net income	$ 92	$ 4	$ (738)	$ (181)