Investment Securities - Contractual Maturities of Available for Sale (Detail) $ in Thousands	Sep. 30, 2017 USD ($)
Amortized Cost
Due in one year	$ 25,341
Due after one year through five years	65,472
Due after five years through ten years	108,222
Due after ten years	77,522
Total investment securities available for sale, Amortized Cost	1,458,572
Fair Value
Due in one year	25,295
Due after one year through five years	65,631
Due after five years through ten years	107,858
Due after ten years	76,816
Total investment securities available for sale, Fair Value	1,447,737
Residential mortgage-backed securities
Amortized Cost
Securities without a single maturity date	1,171,510
Fair Value
Securities without a single maturity date	1,161,157
Equity securities
Amortized Cost
Securities without a single maturity date	10,505
Fair Value
Securities without a single maturity date	$ 10,980