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Derivative instruments and hedging activities- Additional Information (Detail) (USD $)
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Derivative [Line Items]      
Derivative, Credit Risk By using derivative instruments, the Corporation exposes itself to credit and market risk. If a counterparty fails to fulfill its performance obligations under a derivative contract, the Corporation’s credit risk will equal the fair value of the derivative asset. Generally, when the fair value of a derivative contract is positive, this indicates that the counterparty owes the Corporation, thus creating a repayment risk for the Corporation. To manage the level of credit risk, the Corporation deals with counterparties of good credit standing, enters into master netting agreements whenever possible and, when appropriate, obtains collateral. On the other hand, when the fair value of a derivative contract is negative, the Corporation owes the counterparty and, therefore, the fair value of derivatives liabilities incorporates nonperformance risk or the risk that the obligation will not be fulfilled.    
Derivatives, Offsetting Fair Value Amounts, Policy [Policy Text Block]

Pursuant to the Corporation's accounting policy, the fair value of derivatives is not offset with the amounts for the right to reclaim cash collateral or the obligation to return cash collateral.

   
Derivative, Collateral, Right to Reclaim Cash $ 46,000,000 $ 72,000,000  
Derivative, Collateral, Obligation to Return Cash 1,000,000 2,000,000  
Derivative, Net Liability Position, Aggregate Fair Value 31,000,000 57,000,000  
Collateral Already Posted, Aggregate Fair Value 46,000,000 72,000,000  
Additional Collateral, Aggregate Fair Value 0 0  
Fair Value Hedges At Fair Value Net 0 0  
Gain (Loss) on Derivative Instruments Held for Trading Purposes, Net (4,837,000) (32,973,000) (9,414,000)
Derivative Credit Risk Valuation Adjustment, Derivative Assets (500,000) 1,100,000 (500,000)
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities $ 3,400,000 $ 1,700,000 $ 300,000
Cash Flow Hedging [Member]
      
Derivative [Line Items]      
Maximum Length of Time Hedged in Cash Flow Hedge 0 years 0 months 79 days