Federal funds purchased and assets sold under agreements to repurchase - Additional Information (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
Assets Sold under Agreements to Repurchase [Line Items]
Available-for-sale Securities Pledged as Collateral	$ 1,757,178,000	$ 1,286,839,000
Trading Securities Pledged as Collateral	330,680,000	308,978,000
Pledged Assets Separately Reported Other Assets Pledged As Collateral At Fair Value	59,000,000	70,000,000
Federal Funds Sold And Securities Purchased Under Agreements To Resell Pledged As Collateral	187,000,000	189,000,000
Investment Securities Available For Sale [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Available-for-sale Securities Pledged as Collateral	$ 1,800,000,000	$ 1,300,000,000