Investment Securities - Contractual Maturities of Debt Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Amortized Cost
Due in one year	$ 85,384
Due after one year through five years	181,107
Due after five years through ten years	369,454
Due after ten years	179,041
Residential mortgage-backed securities	1,030,165
Held to Maturity, Amortized Cost	1,845,151	$ 1,596,385
Fair Value
Due in one year	85,388
Due after one year through five years	192,968
Due after five years through ten years	398,494
Due after ten years	168,745
Residential mortgage-backed securities	1,046,387
Total investment securities held to maturity	$ 1,891,982	$ 1,621,039