Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt securities available for sale:
Due in one year or less	$ 1,141,640
Due after one year through five years	9,470
Due after five years through ten years	102,101
Due after ten years	30,000
Total available for sale (amortized cost)	1,283,211
Mortgage-backed securities available for sale	7,115,475
Investment securities available for sale, amortized cost	8,398,686	$ 8,869,423
Debt securities held to maturity:
Due in one year or less	947,176
Due after one year through five years	3,864
Due after ten years	3,582
Total available for held to maturity (amortized cost)	954,622
Mortgage-backed securities held to maturity	2,760,077
Amortized cost for held to maturity	3,714,699	3,316,640
Debt securities available for sale:
Due in one year or less	1,136,864
Due after one year through five years	9,355
Due after five years through ten years	99,642
Due after ten years	25,800
Total available for sale (fair value)	1,271,661
Mortgage-backed securities available for sale	7,053,923
Total	8,325,584	8,682,509
Debt securities held to maturity:
Due in one year or less	947,905
Due after one year through five years	3,882
Due after ten years	3,582
Total available for held to maturity (fair value)	955,369
Mortgage-backed securities held to maturity	2,744,227
Total	$ 3,699,596	$ 3,255,483