Investment Securities - Amortized Cost and Fair Value of Investment Securities by Contractual Maturities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	$ 15,363
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount	37,058
Investment securities - HTM, at amortized cost	52,421	$ 0
Held-to-maturity Securities, Debt Maturities, Year Six Through Ten, Fair Value	15,508
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	40,209
Held-to-maturity Securities, Fair Value	55,717	0
Securities available for sale, Due in one year or less, Amortized Cost	40,295
Securities available for sale, After one year through five years, Amortized Cost	48,744
Securities available for sale, After five years through ten years, Amortized Cost	150,034
Securities available for sale, After ten years, Amortized Cost	408,877
Securities available for sale, Mortgage backed securities, Amortized Cost	1,969,078
Securities available for sale Total, Amortized Cost	2,617,028	1,966,034
Securities available for sale, Due in one year or less, Estimated Fair Value	40,695
Securities available for sale, After one year through five years, Estimated Fair Value	50,645
Securities available for sale, After five years through ten years, Estimated Fair Value	155,411
Securities available for sale, After ten years, Estimated Fair Value	427,060
Securities available for sale, Mortgage backed securities, Estimated Fair Value	1,985,371
Securities available for sale Total, Estimated Fair Value	$ 2,659,182	$ 1,982,523