Federal funds purchased and assets sold under agreements to repurchase - Additional Information (Detail) (USD $)			3 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Repos Refinancing Transaction Of September 30, 2014 [Member]	Sep. 30, 2014 Investment Securities Available For Sale [Member]	Dec. 31, 2013 Investment Securities Available For Sale [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Available-for-sale Securities Pledged as Collateral	$ 1,515,639,000	$ 1,286,839,000		$ 1,500,000,000	$ 1,300,000,000
Trading Securities Pledged as Collateral	122,483,000	308,978,000
Pledged Assets Separately Reported Other Assets Pledged As Collateral At Fair Value	18,000,000	70,000,000
Federal Funds Sold And Securities Purchased Under Agreements To Resell Pledged As Collateral	142,000,000	189,000,000
Securities Sold under Agreements to Repurchase			638,000,000
Interest Expense Securities			20,700,000
Securities Sold Under Agreements to Repurchase Yield			4.41%
Payments of Debt Restructuring			$ 39,700,000