Comprehensive Income and Accumulated Other Comprehensive Income - Reclassification Out of Accumulated Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	$ 21,197	$ 13,404
Income tax expense	(6,067)	(2,855)
Net income	15,130	10,549
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	100	(163)
Income before income taxes	100	(163)
Income tax expense	(35)	57
Net income	$ 65	$ (106)