Investment Securities - Expected Maturities of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Maturity, Within one year, Amortized Cost	$ 27,593
Available-for-Sale, Maturity, One to five years, Amortized Cost	355,595
Available-for-Sale, Maturity, Five to ten years, Amortized Cost	326,554
Available-for-Sale, Maturity, Beyond ten years, Amortized Cost	2,124,267
Available-for-Sale, Amortized Cost	2,834,009	$ 2,858,677
Held-to-Maturity, Maturity, Within one year, Amortized Cost	23,108
Held-to-Maturity, Maturity, One to five years, Amortized Cost	73,901
Held-to-Maturity, Maturity, Five to ten years, Amortized Cost	153,039
Held-to-Maturity, Maturity, Beyond ten years, Amortized Cost	445,091
Held-to-Maturity, Amortized Cost	695,139	745,090	$ 865,957
Available-for-Sale, Maturity, Within one year, Fair Value	27,663
Available-for-Sale, Maturity, One to five years, Fair Value	356,174
Available-for-Sale, Maturity, Five to ten years, Fair Value	330,878
Available-for-Sale, Maturity, Beyond ten years, Fair Value	2,099,623
Available-for-Sale securities, Fair Value	2,814,338	2,797,174	2,648,901
Held-to-Maturity, Maturity, Within one year, Fair Value	23,473
Held-to-Maturity, Maturity, One to five years, Fair Value	77,005
Held-to-Maturity, Maturity, Five to ten years, Fair Value	163,208
Held-to-Maturity, Maturity, Beyond ten years, Fair Value	485,677
Held-to-Maturity, Fair Value	$ 749,363	$ 784,172	$ 939,855
Available-for-Sale, Maturity, Within one year, Weighted Average Yield	2.04%
Available-for-Sale, Maturity, One to five years, Weighted Average Yield	2.14%
Available-for-Sale, Maturity, Five to ten years, Weighted Average Yield	2.85%
Available-for-Sale, Maturity, Beyond ten years, Weighted Average Yield	2.42%
Available-for-Sale, Weighted Average Yield	2.43%
Held-to-Maturity, Maturity, Within one year, Weighted Average Yield	6.12%
Held-to-Maturity, Maturity, One to five years, Weighted Average Yield	4.46%
Held-to-Maturity, Maturity, Five to ten years, Weighted Average Yield	4.85%
Held-to-Maturity, Maturity, Beyond ten years, Weighted Average Yield	5.74%
Held-to-Maturity, Weighted Average Yield	5.42%