Comprehensive Income and Accumulated Other Comprehensive Income - Reclassification Out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	$ 20,112	$ 17,061	$ 41,309	$ 30,465
Income tax expense	(5,793)	(4,288)	(11,860)	(7,143)
Net income	14,319	12,773	29,449	23,322
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	1,273	195	261	(209)
Other-than-temporary impairment	(2,126)		(1,600)
Income before income taxes	(853)	195	(1,339)	(209)
Income tax expense	299	(68)	469	73
Net income	$ (554)	$ 127	$ (870)	$ (136)