Comprehensive Income and Accumulated Other Comprehensive Income - Reclassification Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	$ 33,768	$ 25,890	$ 20,112	$ 21,197	$ 23,936	$ 19,348	$ 17,061	$ 13,404	$ 100,967	$ 73,749	$ 59,204
Income tax expense									(30,505)	(20,083)	(15,567)
Net income	$ 23,073	$ 17,940	$ 14,319	$ 15,130	$ 16,865	$ 13,479	$ 12,773	$ 10,549	70,462	53,666	43,637
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									(494)	(825)	30
Other-than-temporary impairment									(1,600)	(16)	(416)
Income before income taxes									(2,094)	(841)	(386)
Income tax expense									733	294	135
Net income									$ (1,361)	$ (547)	$ (251)