Investment Securities - Contractual Maturities of Held to Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Amortized Cost
Due in one year	$ 53,859
Due after one year through five years	210,615
Due after five years through ten years	325,933
Due after ten years	161,719
Residential mortgage-backed securities	1,070,137
Amortized Cost	1,822,263	$ 1,925,572
Fair Value
Due in one year	54,621
Due after one year through five years	218,428
Due after five years through ten years	343,717
Due after ten years	149,958
Residential mortgage-backed securities	1,062,008
Total investment securities held to maturity	$ 1,828,732	$ 1,924,597