Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt securities available for sale:
Due in one year or less	$ 355,210
Due after one year through five years	11,470
Due after five years through ten years	102,283
Due after ten years	30,000
Total available for sale (amortized cost)	498,963
Mortgage-backed securities available for sale	6,435,746
Investment securities available for sale, amortized cost	6,934,709	$ 8,869,423
Debt securities held to maturity:
Due in one year or less	500,476
Due after one year through five years	3,075
Due after ten years	3,353
Total available for held to maturity (amortized cost)	506,904
Mortgage-backed securities held to maturity	2,524,395
Amortized cost for held to maturity	3,031,299	3,316,640
Debt securities available for sale:
Due in one year or less	354,825
Due after one year through five years	11,585
Due after five years through ten years	100,091
Due after ten years	24,000
Total available for sale (fair value)	490,501
Mortgage-backed securities available for sale	6,494,916
Total	6,985,417	8,682,509
Debt securities held to maturity:
Due in one year or less	501,438
Due after one year through five years	3,090
Due after ten years	3,353
Total available for held to maturity (fair value)	507,881
Mortgage-backed securities held to maturity	2,567,423
Total	$ 3,075,304	$ 3,255,483