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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	$ 27,784	$ 26,027	$ 24,615	$ 34,395	$ 33,768	$ 25,890	$ 20,112	$ 21,197	$ 112,821	$ 100,967	$ 73,749
Income tax expense									(34,956)	(30,505)	(20,083)
Net income	$ 19,073	$ 17,896	$ 16,639	$ 24,257	$ 23,073	$ 17,940	$ 14,319	$ 15,130	77,865	70,462	53,666
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 losses on available-for-sale securities									(234)	(494)	(825)
Other-than-temporary impairment										(1,600)	(16)
Income before income taxes									(234)	(2,094)	(841)
Income tax expense									82	733	294
Net income									$ (152)	$ (1,361)	$ (547)