Investment Securities - Summary of Amortized Cost and Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
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	315,036
Due after five years through ten years, Amortized Cost, Held-to-maturity	172,281
Due after ten years, Amortized Cost, Held-to-maturity	271,712
Total Held-to-maturity, Debt Maturities, Amortized Cost Basis	759,029
Due in one year or less, Fair Value, Held-to-maturity	0
Due after one year through five years, Fair Value, Held-to-maturity	298,729
Due after five years through ten years, Fair Value, Held-to-maturity	167,824
Due after ten years, Fair Value, Held-to-maturity	260,202
Total, Held-to-maturity, Fair Value	726,755
Due in one year or less, Amortized Cost, Available-for-sale	19,806
Due after one year through five years, Amortized Cost, Available-for-sale	1,676,348
Due after five years through ten years, Amortized Cost, Available-for-sale	113,539
Due after ten years, Amortized Cost, Available-for-sale	41,030
Amortized Cost, Available-for-sale	1,850,723	$ 2,078,131
Due in one year or less, Fair Value, Available-for-sale	19,964
Due after one year through five years, Fair Value, Available-for-sale	1,637,053
Due after five years through ten years, Fair Value, Available-for-sale	109,910
Due after ten years, Fair Value, Available-for-sale	39,304
Total Available-for-sale Securities, Debt Maturities, Available-for-sale, Fair Value	$ 1,806,231	$ 2,080,985