Investment Securities - Contractual Maturities of Available for Sale (Detail) $ in Thousands	Mar. 31, 2018 USD ($)
Amortized Cost
Due in one year	$ 19,047
Due after one year through five years	123,697
Due after five years through ten years	83,785
Due after ten years	148,944
Securities without a single maturity date	1,514,862
Total investment securities available for sale, Amortized Cost	1,890,335
Fair Value
Due in one year	19,133
Due after one year through five years	120,633
Due after five years through ten years	83,376
Due after ten years	146,297
Securities without a single maturity date	1,474,075
Total investment securities available for sale, Fair Value	$ 1,843,514