Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Debt securities available for sale:
Due in one year or less	$ 156,211
Due after one year through five years	1,756,628
Due after five years through ten years	56,297
Due after ten years	75,266
Total available for sale (amortized cost)	2,044,402
Mortgage-backed securities available for sale	10,525,643
Total	12,570,045
Debt securities held to maturity:
Due in one year or less	21,613
Due after one year through five years	23,326
Due after five years through ten years	1,897
Due after ten years	5,281
Total available for held to maturity (amortized cost)	52,117
Mortgage-backed securities held to maturity	2,824,002
Amortized cost for held to maturity	2,876,119	$ 2,457,278
Debt securities available for sale:
Due in one year or less	156,378
Due after one year through five years	1,747,311
Due after five years through ten years	54,094
Due after ten years	63,358
Total available for sale (fair value)	2,021,141
Mortgage-backed securities available for sale	10,504,355
Total	12,525,496
Debt securities held to maturity:
Due in one year or less	21,711
Due after one year through five years	23,261
Due after five years through ten years	1,941
Due after ten years	5,281
Total available for held to maturity (fair value)	52,194
Mortgage-backed securities held to maturity	2,813,691
Total	$ 2,865,885	$ 2,451,222