Investment Securities - Summary of Amortized Cost and Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
Due in one year or less, Amortized Cost, Held-to-maturity	$ 885
Due after one year through five years, Amortized Cost, Held-to-maturity	251,948
Due after five years through ten years, Amortized Cost, Held-to-maturity	237,282
Due after ten years, Amortized Cost, Held-to-maturity	358,267
Total Held-to-maturity, Debt Maturities, Amortized Cost Basis	848,382
Due in one year or less, Fair Value, Held-to-maturity	887
Due after one year through five years, Fair Value, Held-to-maturity	249,473
Due after five years through ten years, Fair Value, Held-to-maturity	235,069
Due after ten years, Fair Value, Held-to-maturity	356,621
Total, Held-to-maturity, Fair Value	842,050
Due in one year or less, Amortized Cost, Available-for-sale	18,711
Due after one year through five years, Amortized Cost, Available-for-sale	1,985,884
Due after five years through ten years, Amortized Cost, Available-for-sale	116,933
Due after ten years, Amortized Cost, Available-for-sale	33,802
Amortized Cost, Available-for-sale	2,155,330	$ 2,255,874
Due in one year or less, Fair Value, Available-for-sale	18,895
Due after one year through five years, Fair Value, Available-for-sale	2,005,750
Due after five years through ten years, Fair Value, Available-for-sale	116,800
Due after ten years, Fair Value, Available-for-sale	34,203
Total Available-for-sale Securities, Debt Maturities, Available-for-sale, Fair Value	$ 2,175,648	$ 2,270,466