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Composition of assets sold under agreements to repurchase (Detail) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2012
Dec. 31, 2011
Mar. 31, 2011
Assets Sold under Agreements to Repurchase [Line Items]      
Assets sold under agreements to repurchase $ 2,113,557 $ 2,141,097 $ 2,642,800