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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	$ 17,800	$ 23,556	$ 22,151	$ 18,733	$ 27,784	$ 26,027	$ 24,615	$ 34,395	$ 82,240	$ 112,821	$ 100,967
Income tax expense									(36,009)	(34,956)	(30,505)
Net income	$ 649	$ 16,577	$ 15,481	$ 13,524	$ 19,073	$ 17,896	$ 16,639	$ 24,257	46,231	77,865	70,462
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									1,106	(234)	(494)
Other-than-temporary impairment											(1,600)
Income before income taxes									1,106	(234)	(2,094)
Income tax expense									(387)	82	733
Net income									$ 719	$ (152)	$ (1,361)