Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Debt securities available for sale:
Due in one year or less	$ 9,059
Due after one year through five years	163,114
Due after five years through ten years	3,272
Due after ten years	162,001
Total available for sale (amortized cost)	337,446
Mortgage-backed securities available for sale	10,008,287
Total	10,345,733
Debt securities held to maturity:
Due in one year or less	27,663
Due after one year through five years	87,320
Due after five years through ten years	33,715
Due after ten years	7,185
Total available for held to maturity (amortized cost)	155,883
Mortgage-backed securities held to maturity	3,204,929
Amortized cost for held to maturity	3,360,812	3,507,868
Debt securities available for sale:
Due in one year or less	9,117
Due after one year through five years	165,027
Due after five years through ten years	3,314
Due after ten years	162,177
Total available for sale (fair value)	339,635
Mortgage-backed securities available for sale	10,265,316
Total	10,604,951
Debt securities held to maturity:
Due in one year or less	27,865
Due after one year through five years	88,357
Due after five years through ten years	34,304
Due after ten years	7,185
Total available for held to maturity (fair value)	157,711
Mortgage-backed securities held to maturity	3,254,123
Total	$ 3,411,834	$ 3,538,282