Investment Securities - Contractual Maturities of Available for Sale (Detail) $ in Thousands	Mar. 31, 2017 USD ($)
Amortized Cost
Due in one year	$ 24,953
Due after one year through five years	69,116
Due after five years through ten years	113,108
Due after ten years	80,735
Total investment securities available for sale, Amortized Cost	1,470,374
Fair Value
Due in one year	24,844
Due after one year through five years	69,513
Due after five years through ten years	111,725
Due after ten years	79,245
Total investment securities available for sale, Fair Value	1,454,331
Residential mortgage-backed securities
Amortized Cost
Securities without a single maturity date	1,171,958
Fair Value
Securities without a single maturity date	1,158,064
Equity securities
Amortized Cost
Securities without a single maturity date	10,504
Fair Value
Securities without a single maturity date	$ 10,940