Investment Securities - Summary of Amortized Cost and Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 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	149,171
Due after five years through ten years, Amortized Cost, Held-to-maturity	366,305
Due after ten years, Amortized Cost, Held-to-maturity	297,417
Total Held-to-maturity, Debt Maturities, Amortized Cost Basis	812,893
Due in one year or less, Fair Value, Held-to-maturity	0
Due after one year through five years, Fair Value, Held-to-maturity	150,627
Due after five years through ten years, Fair Value, Held-to-maturity	370,388
Due after ten years, Fair Value, Held-to-maturity	304,913
Total, Held-to-maturity, Fair Value	825,928
Due in one year or less, Amortized Cost, Available-for-sale	15,931
Due after one year through five years, Amortized Cost, Available-for-sale	1,789,155
Due after five years through ten years, Amortized Cost, Available-for-sale	145,732
Due after ten years, Amortized Cost, Available-for-sale	284,866
Amortized Cost, Available-for-sale	2,235,684	$ 2,337,715
Due in one year or less, Fair Value, Available-for-sale	16,125
Due after one year through five years, Fair Value, Available-for-sale	1,837,957
Due after five years through ten years, Fair Value, Available-for-sale	148,847
Due after ten years, Fair Value, Available-for-sale	291,730
Total Available-for-sale Securities, Debt Maturities, Available-for-sale, Fair Value	$ 2,294,659	$ 2,368,646