Investment Securities - Contractual Maturities of Debt Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized Cost
Due in one year	$ 32,026
Due after one year through five years	223,337
Due after five years through ten years	322,061
Due after ten years	133,322
Residential mortgage-backed securities	1,131,945
Amortized Cost	1,842,691	$ 1,925,572
Fair Value
Due in one year	32,320
Due after one year through five years	228,738
Due after five years through ten years	335,740
Due after ten years	122,778
Residential mortgage-backed securities	1,118,044
Total investment securities held to maturity	$ 1,837,620	$ 1,924,597