Investment Securities - Expected Maturities of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Maturity, Within one year, Amortized Cost	$ 38,790
Available-for-Sale, Maturity, One to five years, Amortized Cost	311,574
Available-for-Sale, Maturity, Five to ten years, Amortized Cost	386,471
Available-for-Sale, Maturity, Beyond ten years, Amortized Cost	2,127,331
Available-for-Sale, Amortized Cost	2,864,166	$ 2,858,677
Held-to-Maturity, Maturity, Within one year, Amortized Cost	22,670
Held-to-Maturity, Maturity, One to five years, Amortized Cost	68,261
Held-to-Maturity, Maturity, Five to ten years, Amortized Cost	147,381
Held-to-Maturity, Maturity, Beyond ten years, Amortized Cost	503,136
Held-to-Maturity, Amortized Cost	741,448	745,090	$ 869,012
Available-for-Sale, Maturity, Within one year, Fair Value	38,870
Available-for-Sale, Maturity, One to five years, Fair Value	311,107
Available-for-Sale, Maturity, Five to ten years, Fair Value	388,129
Available-for-Sale, Maturity, Beyond ten years, Fair Value	2,078,837
Available-for-Sale securities, Fair Value	2,816,943	2,797,174	2,348,443
Held-to-Maturity, Maturity, Within one year, Fair Value	23,140
Held-to-Maturity, Maturity, One to five years, Fair Value	70,943
Held-to-Maturity, Maturity, Five to ten years, Fair Value	154,829
Held-to-Maturity, Maturity, Beyond ten years, Fair Value	535,994
Held-to-Maturity, Fair Value	$ 784,906	$ 784,172	$ 932,590
Available-for-Sale, Maturity, Within one year, Weighted Average Yield	1.78%
Available-for-Sale, Maturity, One to five years, Weighted Average Yield	2.19%
Available-for-Sale, Maturity, Five to ten years, Weighted Average Yield	2.78%
Available-for-Sale, Maturity, Beyond ten years, Weighted Average Yield	2.45%
Available-for-Sale, Weighted Average Yield	2.46%
Held-to-Maturity, Maturity, Within one year, Weighted Average Yield	5.87%
Held-to-Maturity, Maturity, One to five years, Weighted Average Yield	4.37%
Held-to-Maturity, Maturity, Five to ten years, Weighted Average Yield	4.83%
Held-to-Maturity, Maturity, Beyond ten years, Weighted Average Yield	5.61%
Held-to-Maturity, Weighted Average Yield	5.35%