Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Debt securities available for sale:
Due in one year or less	$ 7,412
Due after one year through five years	164,412
Due after five years through ten years	3,524
Due after ten years	162,400
Total available for sale (amortized cost)	337,748
Mortgage-backed securities available for sale	8,507,675
Total	8,845,423
Debt securities held to maturity:
Due in one year or less	23,107
Due after one year through five years	84,112
Due after five years through ten years	41,742
Due after ten years	7,854
Total available for held to maturity (amortized cost)	156,815
Mortgage-backed securities held to maturity	3,351,053
Amortized cost for held to maturity	3,507,868	3,966,130
Debt securities available for sale:
Due in one year or less	7,468
Due after one year through five years	165,340
Due after five years through ten years	3,577
Due after ten years	165,564
Total available for sale (fair value)	341,949
Mortgage-backed securities available for sale	8,731,226
Total	9,073,175
Debt securities held to maturity:
Due in one year or less	23,254
Due after one year through five years	85,457
Due after five years through ten years	42,612
Due after ten years	7,854
Total available for held to maturity (fair value)	159,177
Mortgage-backed securities held to maturity	3,379,105
Total	$ 3,538,282	$ 3,860,127