Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt securities available for sale:
Due in one year or less	$ 429,891
Due after one year through five years	8,048
Due after five years through ten years	102,181
Due after ten years	30,000
Total available for sale (amortized cost)	570,120
Mortgage-backed securities available for sale	6,789,325
Investment securities available for sale, amortized cost	7,359,445	$ 8,869,423
Debt securities held to maturity:
Due in one year or less	948,757
Due after one year through five years	3,440
Due after ten years	3,538
Total available for held to maturity (amortized cost)	955,735
Mortgage-backed securities held to maturity	2,648,498
Amortized cost for held to maturity	3,604,233	3,316,640
Debt securities available for sale:
Due in one year or less	428,384
Due after one year through five years	8,048
Due after five years through ten years	99,357
Due after ten years	26,202
Total available for sale (fair value)	561,991
Mortgage-backed securities available for sale	6,818,349
Total	7,380,340	8,682,509
Debt securities held to maturity:
Due in one year or less	950,308
Due after one year through five years	3,468
Due after ten years	3,538
Total available for held to maturity (fair value)	957,314
Mortgage-backed securities held to maturity	2,672,844
Total	$ 3,630,158	$ 3,255,483