Derivative instruments and hedging activities (Details) (USD $)	12 Months Ended
	Dec. 31, 2010
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Liabilities	$ 0
Derivative [Line Items]
Gain (Loss) on Interest Rate Cash Flow Hedge Ineffectiveness	16,800,000
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, Net of Tax	$ 10,600,000