Investment Securities - Contractual Maturities of Investment Securities Available for Sale (Detail) $ in Thousands	Mar. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Due in one year, Amortized Cost	$ 361,613
Due after one year through five years, Amortized Cost	91,097
Due after five years through ten years, Amortized Cost	118,340
Due after ten years, Amortized Cost	78,866
Total investment securities available for sale, Amortized Cost	1,449,213
Due in one year, Fair Value	361,604
Due after one year through five years, Fair Value	92,129
Due after five years through ten years, Fair Value	119,037
Due after ten years, Fair Value	78,176
Total investment securities available for sale, Fair Value	1,452,489
Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Securities without a single maturity date, Amortized Cost	778,775
Securities without a single maturity date, Fair Value	782,648
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Securities without a single maturity date, Amortized Cost	20,522
Securities without a single maturity date, Fair Value	$ 18,895