Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Debt securities available for sale:
Due in one year or less	$ 393,523
Due after one year through five years	1,730,289
Due after five years through ten years	72,374
Due after ten years	59,287
Total available for sale (amortized cost)	2,255,473
Mortgage-backed securities available for sale	9,601,846
Total	11,857,319
Debt securities held to maturity:
Due in one year or less	21,284
Due after one year through five years	16,110
Due after five years through ten years	1,564
Due after ten years	5,213
Total available for held to maturity (amortized cost)	44,171
Mortgage-backed securities held to maturity	3,344,097
Amortized cost for held to maturity	3,388,268	$ 2,457,278
Debt securities available for sale:
Due in one year or less	392,835
Due after one year through five years	1,720,332
Due after five years through ten years	72,728
Due after ten years	50,707
Total available for sale (fair value)	2,236,602
Mortgage-backed securities available for sale	9,608,217
Total	11,844,819
Debt securities held to maturity:
Due in one year or less	21,363
Due after one year through five years	16,068
Due after five years through ten years	1,613
Due after ten years	5,213
Total available for held to maturity (fair value)	44,257
Mortgage-backed securities held to maturity	3,336,275
Total	$ 3,380,532	$ 2,451,222