Investment Securities - Expected Maturities of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Maturity, Within one year, Amortized Cost	$ 37,370
Available-for-Sale, Maturity, One to five years, Amortized Cost	320,815
Available-for-Sale, Maturity, Five to ten years, Amortized Cost	329,607
Available-for-Sale, Maturity, Beyond ten years, Amortized Cost	2,170,885
Available-for-Sale, Amortized Cost	2,858,677	$ 2,424,063
Held-to-Maturity, Maturity, Within one year, Amortized Cost	10,796
Held-to-Maturity, Maturity, One to five years, Amortized Cost	74,451
Held-to-Maturity, Maturity, Five to ten years, Amortized Cost	129,452
Held-to-Maturity, Maturity, Beyond ten years, Amortized Cost	530,391
Held-to-Maturity, Amortized Cost	745,090	872,111
Available-for-Sale, Maturity, Within one year, Fair Value	37,431
Available-for-Sale, Maturity, One to five years, Fair Value	318,852
Available-for-Sale, Maturity, Five to ten years, Fair Value	329,892
Available-for-Sale, Maturity, Beyond ten years, Fair Value	2,110,999
Available-for-Sale securities, Fair Value	2,797,174	2,418,221
Held-to-Maturity, Maturity, Within one year, Fair Value	10,906
Held-to-Maturity, Maturity, One to five years, Fair Value	77,228
Held-to-Maturity, Maturity, Five to ten years, Fair Value	135,004
Held-to-Maturity, Maturity, Beyond ten years, Fair Value	561,034
Held-to-Maturity, Fair Value	$ 784,172	$ 929,417
Available-for-Sale, Maturity, Within one year, Weighted Average Yield	1.72%
Available-for-Sale, Maturity, One to five years, Weighted Average Yield	2.18%
Available-for-Sale, Maturity, Five to ten years, Weighted Average Yield	2.85%
Available-for-Sale, Maturity, Beyond ten years, Weighted Average Yield	2.43%
Available-for-Sale, Weighted Average Yield	2.44%
Held-to-Maturity, Maturity, Within one year, Weighted Average Yield	4.02%
Held-to-Maturity, Maturity, One to five years, Weighted Average Yield	4.71%
Held-to-Maturity, Maturity, Five to ten years, Weighted Average Yield	4.86%
Held-to-Maturity, Maturity, Beyond ten years, Weighted Average Yield	5.58%
Held-to-Maturity, Weighted Average Yield	5.35%