Investment securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Debt securities available for sale:
Due in one year or less	$ 24,886
Due after one year through five years	21,212
Due after five years through ten years	8,282
Due after ten years	181,013
Total available for sale (amortized cost)	235,393
Mortgage-backed securities available for sale	4,217,684
Total	4,453,077
Debt securities held to maturity:
Due in one year or less	18,617
Due after one year through five years	65,392
Due after five years through ten years	86,361
Due after ten years	9,290
Total available for held to maturity (amortized cost)	179,660
Mortgage-backed securities held to maturity	3,139,454
Total	3,319,114	1,032,276
Debt securities available for sale:
Due in one year or less	24,971
Due after one year through five years	22,016
Due after five years through ten years	8,553
Due after ten years	179,017
Total available for available for sale (fair value)	234,557
Mortgage-backed securities available for sale	4,334,215
Total	4,568,772
Debt securities held to maturity:
Due in one year or less	18,751
Due after one year through five years	67,400
Due after five years through ten years	88,410
Due after ten years	9,290
Total available for held to maturity (fair value)	183,851
Mortgage-backed securities held to maturity	3,093,535
Estimated fair value for held to maturity	$ 3,277,386	$ 976,883