Securities - Carrying Value of Held-to-Maturity Investment Securities (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Amortized Cost	$ 403,838	$ 481,340
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	2,441	1,424
Carrying value	$ 392,932	$ 469,417