Investment Securities - Expected Maturities of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Maturity, Within one year, Amortized Cost	$ 64,076
Available-for-Sale, Maturity, One to five years, Amortized Cost	420,949
Available-for-Sale, Maturity, Five to ten years, Amortized Cost	337,043
Available-for-Sale, Maturity, Beyond ten years, Amortized Cost	2,430,518
Available-for-Sale, Amortized Cost	3,252,586	$ 2,858,677
Held-to-Maturity, Maturity, Within one year, Amortized Cost	22,566
Held-to-Maturity, Maturity, One to five years, Amortized Cost	115,596
Held-to-Maturity, Maturity, Five to ten years, Amortized Cost	159,452
Held-to-Maturity, Maturity, Beyond ten years, Amortized Cost	386,449
Held-to-Maturity, Amortized Cost	684,063	745,090
Available-for-Sale, Maturity, Within one year, Fair Value	64,084
Available-for-Sale, Maturity, One to five years, Fair Value	417,941
Available-for-Sale, Maturity, Five to ten years, Fair Value	336,999
Available-for-Sale, Maturity, Beyond ten years, Fair Value	2,377,183
Available-for-Sale securities, Fair Value	3,196,207	2,797,174
Held-to-Maturity, Maturity, Within one year, Fair Value	22,692
Held-to-Maturity, Maturity, One to five years, Fair Value	120,904
Held-to-Maturity, Maturity, Five to ten years, Fair Value	167,547
Held-to-Maturity, Maturity, Beyond ten years, Fair Value	416,560
Held-to-Maturity, Fair Value	$ 727,703	$ 784,172
Available-for-Sale, Maturity, Within one year, Weighted Average Yield	2.13%
Available-for-Sale, Maturity, One to five years, Weighted Average Yield	2.02%
Available-for-Sale, Maturity, Five to ten years, Weighted Average Yield	2.72%
Available-for-Sale, Maturity, Beyond ten years, Weighted Average Yield	2.48%
Available-for-Sale, Weighted Average Yield	2.43%
Held-to-Maturity, Maturity, Within one year, Weighted Average Yield	6.23%
Held-to-Maturity, Maturity, One to five years, Weighted Average Yield	5.08%
Held-to-Maturity, Maturity, Five to ten years, Weighted Average Yield	4.87%
Held-to-Maturity, Maturity, Beyond ten years, Weighted Average Yield	5.75%
Held-to-Maturity, Weighted Average Yield	5.44%