Investment Securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Debt securities available for sale:
Due in one year or less	$ 9,830
Due after one year through five years	43,459
Due after five years through ten years	5,328
Due after ten years	170,594
Total available for sale (amortized cost)	229,211
Mortgage-backed securities available for sale	5,723,183
Total	5,952,394
Debt securities held to maturity:
Due in one year or less	17,944
Due after one year through five years	74,928
Due after five years through ten years	70,022
Due after ten years	8,656
Total available for held to maturity (amortized cost)	171,550
Mortgage-backed securities held to maturity	3,702,435
Total	3,873,985	3,966,130
Debt securities available for sale:
Due in one year or less	9,957
Due after one year through five years	44,086
Due after five years through ten years	5,442
Due after ten years	177,562
Total available for sale (fair value)	237,047
Mortgage-backed securities available for sale	5,801,787
Total	6,038,834
Debt securities held to maturity:
Due in one year or less	18,049
Due after one year through five years	76,540
Due after five years through ten years	71,391
Due after ten years	8,656
Total available for held to maturity (fair value)	174,636
Mortgage-backed securities held to maturity	3,640,130
Estimated fair value for held to maturity	$ 3,814,766	$ 3,860,127