Derivative Instruments and Hedging Activities - Additional Information (Detail) (USD $)	9 Months Ended			9 Months Ended
	Sep. 30, 2011 Cash Flow Hedges Derivative	Sep. 30, 2010 Cash Flow Hedges	Apr. 30, 2011 Cash Flow Hedges Interest Rate Swaps	Sep. 30, 2011 Fair Value Hedges Derivative	Sep. 30, 2010 Fair Value Hedges	Sep. 30, 2011 Non-designated Hedges Derivative
Derivative [Line Items]
Notional balance			$ 430,000,000
Amount of increase (decrease) in comprehensive income to adjust hedging derivatives	$ (79,691,000)	$ (448,000)		$ (1,324,000)	$ 2,181,000
Number of derivative instruments held	7			2		4