Securities - Debt Securities Available for Sale by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Available For Sale Securities Debt Maturities [Abstract]
Due after five years through ten years, amortized cost	$ 198,973
Due after ten years, amortized cost	27,294
Mortgage-backed GSE securities: residential, amortized cost	140,845
Total amortized cost	367,112	$ 506,429
Due after five years through ten years, fair value	198,338
Due after ten years, fair value	26,813
Mortgage-backed GSE securities: residential, fair value	141,614
Total fair value	$ 366,765	$ 499,790