Investment Securities - Amortized Cost and Fair Value of Investment Securities by Contractual Maturities (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	$ 7,330
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Fair Value	7,361
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	17,226
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Fair Value	17,743
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	454,824
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Fair Value	496,762
Debt Securities, Held-to-maturity	479,380	$ 485,107
Debt Securities, Held-to-maturity, Fair Value	521,866	516,261
Securities available for sale, After one year through five years, Amortized Cost	6,705
Securities available for sale, After five years through ten years, Amortized Cost	173,884
Securities available for sale, After ten years, Amortized Cost	849,577
Securities available for sale, Mortgage backed securities, Amortized Cost	2,398,229
Securities available for sale Total, Amortized Cost	3,428,395	3,317,928
Securities available for sale, After one year through five years, Estimated Fair Value	6,715
Securities available for sale, After five years through ten years, Estimated Fair Value	161,998
Securities available for sale, After ten years, Estimated Fair Value	891,556
Securities available for sale, Mortgage backed securities, Estimated Fair Value	2,456,653
Securities available for sale Total, Estimated Fair Value	$ 3,516,922	$ 3,346,310