Investment securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012
Debt securities available for sale:
Due in one year or less	$ 4,918
Due after one year through five years	41,800
Due after five years through ten years	9,524
Due after ten years	182,386
Total available for sale (amortized cost)	238,628
Mortgage-backed securities available for sale	4,305,497
Total	4,544,125
Debt securities held to maturity:
Due in one year or less	26,716
Due after one year through five years	50,310
Due after five years through ten years	104,901
Due after ten years	10,631
Total available for held to maturity (amortized cost)	192,558
Mortgage-backed securities held to maturity	839,718
Total	1,032,276
Debt securities available for sale:
Due in one year or less	4,952
Due after one year through five years	43,139
Due after five years through ten years	10,217
Due after ten years	175,756
Total available for available for sale (fair value)	234,064
Mortgage-backed securities available for sale	4,394,927
Total	4,628,991
Debt securities held to maturity:
Due in one year or less	26,850
Due after one year through five years	52,526
Due after five years through ten years	110,167
Due after ten years	10,635
Total available for held to maturity (fair value)	200,178
Mortgage-backed securities held to maturity	776,705
Total	$ 976,883