Investment Securities - Contractual Maturities of Held to Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Amortized Cost
Due in one year	$ 54,778
Due after one year through five years	221,351
Due after five years through ten years	313,923
Due after ten years	145,181
Residential mortgage-backed securities	1,088,389
Amortized Cost	1,823,622	$ 1,925,572
Fair Value
Due in one year	55,334
Due after one year through five years	229,067
Due after five years through ten years	330,775
Due after ten years	134,266
Residential mortgage-backed securities	1,081,703
Total investment securities held to maturity	$ 1,831,145	$ 1,924,597