Securities - Carrying Value of Held-to-Maturity Investment Securities (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Amortized Cost	$ 480,136	$ 405,361
Net loss on date of transfer from available-for-sale	(13,347)	(13,347)
Accretion of net unrealized loss on securities reclassified as held-to-maturity	3,853	2,799
Carrying value	$ 470,642	$ 394,813