Comprehensive Income and Accumulated Other Comprehensive Income (Loss) - Reclassification Out of Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	$ 23,936	$ 19,348	$ 17,061	$ 13,404	$ 25,030	$ 12,447	$ 10,647	$ 11,080	$ 73,749	$ 59,204	$ 37,143
Income tax expense									(20,083)	(15,567)	(7,790)
Net income	16,865	13,479	12,773	10,549	17,443	9,699	7,644	8,851	53,666	43,637	29,353
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									(825)	30	1,211
Other-than-temporary impairment									(16)	(416)
Income before income taxes									(841)	(386)	1,211
Income tax expense									294	135	(423)
Net income									$ (547)	$ (251)	$ 788