Investment Securities - Expected Maturities of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Maturity, Within one year, Amortized Cost	$ 34,563
Available-for-Sale, Maturity, One to five years, Amortized Cost	338,424
Available-for-Sale, Maturity, Five to ten years, Amortized Cost	302,515
Available-for-Sale, Maturity, Beyond ten years, Amortized Cost	2,126,933
Available-for-Sale, Amortized Cost	2,802,435	$ 2,858,677
Held-to-Maturity, Maturity, Within one year, Amortized Cost	23,751
Held-to-Maturity, Maturity, One to five years, Amortized Cost	89,739
Held-to-Maturity, Maturity, Five to ten years, Amortized Cost	172,648
Held-to-Maturity, Maturity, Beyond ten years, Amortized Cost	402,813
Held-to-Maturity, Amortized Cost	688,951	745,090	$ 850,803
Available-for-Sale, Maturity, Within one year, Fair Value	34,686
Available-for-Sale, Maturity, One to five years, Fair Value	339,100
Available-for-Sale, Maturity, Five to ten years, Fair Value	305,146
Available-for-Sale, Maturity, Beyond ten years, Fair Value	2,096,905
Available-for-Sale securities, Fair Value	2,775,837	2,797,174	2,758,678
Held-to-Maturity, Maturity, Within one year, Fair Value	24,064
Held-to-Maturity, Maturity, One to five years, Fair Value	94,617
Held-to-Maturity, Maturity, Five to ten years, Fair Value	183,505
Held-to-Maturity, Maturity, Beyond ten years, Fair Value	438,378
Held-to-Maturity, Fair Value	$ 740,564	$ 784,172	$ 922,311
Available-for-Sale, Maturity, Within one year, Weighted Average Yield	2.36%
Available-for-Sale, Maturity, One to five years, Weighted Average Yield	2.11%
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	6.15%
Held-to-Maturity, Maturity, One to five years, Weighted Average Yield	4.88%
Held-to-Maturity, Maturity, Five to ten years, Weighted Average Yield	4.88%
Held-to-Maturity, Maturity, Beyond ten years, Weighted Average Yield	5.75%
Held-to-Maturity, Weighted Average Yield	5.43%