Investment securities - Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Due in one year or less	$ 33,460
Due after one year through five years	42,405
Due after five years through ten years	8,611
Due after ten years	183,926
Total available for sale (amortized cost)	268,402
Mortgage-backed securities available for sale	4,711,970
Total	4,980,372
Due in one year or less	36,459
Due after one year through five years	45,054
Due after five years through ten years	110,731
Due after ten years	11,471
Total available for held to maturity (amortized cost)	203,715
Mortgage-backed securities held to maturity	917,610
Total	1,121,325
Due in one year or less	33,496
Due after one year through five years	43,791
Due after five years through ten years	9,409
Due after ten years	170,235
Total available for available for sale (fair value)	256,931
Mortgage-backed securities available for sale	4,813,888
Total	5,070,819
Due in one year or less	36,660
Due after one year through five years	47,410
Due after five years through ten years	116,979
Due after ten years	11,485
Total available for held to maturity (fair value)	212,534
Mortgage-backed securities held to maturity	862,237
Total	$ 1,074,771