Investment Securities - Summary of Amortized Cost and Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Due in one year or less, Amortized Cost, Held-to-maturity	$ 0
Due after one year through five years, Amortized Cost, Held-to-maturity	167,856
Due after five years through ten years, Amortized Cost, Held-to-maturity	240,842
Due after ten years, Amortized Cost, Held-to-maturity	315,659
Total Held-to-maturity, Debt Maturities, Amortized Cost Basis	724,357
Due in one year or less, Fair Value, Held-to-maturity	0
Due after one year through five years, Fair Value, Held-to-maturity	171,966
Due after five years through ten years, Fair Value, Held-to-maturity	245,295
Due after ten years, Fair Value, Held-to-maturity	326,220
Total, Held-to-maturity, Fair Value	743,481
Due in one year or less, Amortized Cost, Available-for-sale	12,202
Due after one year through five years, Amortized Cost, Available-for-sale	1,795,806
Due after five years through ten years, Amortized Cost, Available-for-sale	131,286
Due after ten years, Amortized Cost, Available-for-sale	242,184
Amortized Cost, Available-for-sale	2,181,478	$ 2,337,715
Due in one year or less, Fair Value, Available-for-sale	12,343
Due after one year through five years, Fair Value, Available-for-sale	1,851,485
Due after five years through ten years, Fair Value, Available-for-sale	134,700
Due after ten years, Fair Value, Available-for-sale	249,504
Total Available-for-sale Securities, Debt Maturities, Available-for-sale, Fair Value	$ 2,248,032	$ 2,368,646