Investment securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013
Amortized Cost And Fair Value Debt Securities [Abstract]
Due in one year or less	$ 25,043
Due after one year through five years	21,020
Due after five years through ten years	8,772
Due after ten years	181,277
Total available for sale (amortized cost)	236,112
Mortgage-backed securities available for sale	2,617,237
Total	2,853,349
Due in one year or less	24,360
Due after one year through five years	61,785
Due after five years through ten years	91,679
Due after ten years	9,673
Total available for held to maturity (amortized cost)	187,497
Mortgage-backed securities held to maturity	1,632,194
Total	1,819,691
Due in one year or less	25,170
Due after one year through five years	21,951
Due after five years through ten years	9,189
Due after ten years	178,896
Total available for available for sale (fair value)	235,206
Mortgage-backed securities available for sale	2,700,165
Total	2,935,371
Due in one year or less	24,490
Due after one year through five years	63,641
Due after five years through ten years	93,975
Due after ten years	9,673
Total available for held to maturity (fair value)	191,779
Mortgage-backed securities held to maturity	1,545,799
Total	$ 1,737,578