Investment Securities - Expected Maturities of Investment Securities Portfolio (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale, Maturity, Within one year, Amortized Cost	$ 36,566
Available-for-Sale, Maturity, One to five years, Amortized Cost	400,424
Available-for-Sale, Maturity, Five to ten years, Amortized Cost	339,108
Available-for-Sale, Maturity, Beyond ten years, Amortized Cost	1,851,079
Available-for-Sale, Amortized Cost	2,627,177	$ 2,424,063
Held-to-Maturity, Maturity, Within one year, Amortized Cost	15,011
Held-to-Maturity, Maturity, One to five years, Amortized Cost	49,934
Held-to-Maturity, Maturity, Five to ten years, Amortized Cost	220,589
Held-to-Maturity, Maturity, Beyond ten years, Amortized Cost	580,423
Held-to-Maturity, Amortized Cost	865,957	872,111	$ 823,255
Available-for-Sale, Maturity, Within one year, Fair Value	36,649
Available-for-Sale, Maturity, One to five years, Fair Value	405,665
Available-for-Sale, Maturity, Five to ten years, Fair Value	344,653
Available-for-Sale, Maturity, Beyond ten years, Fair Value	1,861,934
Available-for-Sale securities, Fair Value	2,648,901	2,418,221	2,572,521
Held-to-Maturity, Maturity, Within one year, Fair Value	15,075
Held-to-Maturity, Maturity, One to five years, Fair Value	52,935
Held-to-Maturity, Maturity, Five to ten years, Fair Value	231,585
Held-to-Maturity, Maturity, Beyond ten years, Fair Value	640,260
Held-to-Maturity, Fair Value	$ 939,855	$ 929,417	$ 867,345
Available-for-Sale, Maturity, Within one year, Weighted Average Yield	1.54%
Available-for-Sale, Maturity, One to five years, Weighted Average Yield	2.00%
Available-for-Sale, Maturity, Five to ten years, Weighted Average Yield	2.68%
Available-for-Sale, Maturity, Beyond ten years, Weighted Average Yield	2.34%
Available-for-Sale, Weighted Average Yield	2.32%
Held-to-Maturity, Maturity, Within one year, Weighted Average Yield	5.57%
Held-to-Maturity, Maturity, One to five years, Weighted Average Yield	4.53%
Held-to-Maturity, Maturity, Five to ten years, Weighted Average Yield	3.79%
Held-to-Maturity, Maturity, Beyond ten years, Weighted Average Yield	5.49%
Held-to-Maturity, Weighted Average Yield	5.00%