Assets sold under agreements to repurchase - Additional Information (Detail) (USD $)	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Assets Sold under Agreements to Repurchase [Line Items]
Available-for-sale Securities Pledged as Collateral	$ 939,286,000	$ 1,737,868,000
Trading Securities Pledged as Collateral	356,624,000	402,591,000
Pledged Assets Separately Reported Other Assets Pledged As Collateral At Fair Value	73,000,000	68,000,000	37,000,000
Federal Funds Sold And Securities Purchased Under Agreements To Resell Pledged As Collateral	249,000,000	274,000,000	264,000,000
Investment Securities Available For Sale [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Available-for-sale Securities Pledged as Collateral	$ 1,000,000,000	$ 1,800,000,000	$ 1,900,000,000