Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt securities available for sale:
Due in one year or less	$ 4,950
Due after one year through five years	12,051
Due after five years through ten years	98,604
Due after ten years	30,000
Total available for sale (amortized cost)	145,605
Mortgage-backed securities available for sale	4,475,422
Investment securities available for sale, amortized cost	4,621,027	$ 6,258,276
Debt securities held to maturity:
Due in one year or less	4,355
Due after one year through five years	175
Due after ten years	2,861
Total available for held to maturity (amortized cost)	7,391
Mortgage-backed securities held to maturity	1,741,598
Amortized cost for held to maturity	1,748,989	2,656,917
Debt securities available for sale:
Due in one year or less	4,945
Due after one year through five years	12,585
Due after five years through ten years	97,301
Due after ten years	24,321
Total available for sale (fair value)	139,152
Mortgage-backed securities available for sale	4,683,454
Total	4,822,606	6,318,776
Debt securities held to maturity:
Due in one year or less	4,361
Due after one year through five years	178
Due after ten years	2,861
Total available for held to maturity (fair value)	7,400
Mortgage-backed securities held to maturity	1,834,881
Total	$ 1,842,281	$ 2,699,206