Fair value of financial instruments (Details) (USD $)	3 Months Ended			3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011	Mar. 31, 2012 Credit valuation adjustments	Dec. 31, 2011 Credit valuation adjustments	Mar. 31, 2012 Credit valuation adjustments Foreign currency forward contracts	Dec. 31, 2011 Credit valuation adjustments Foreign currency forward contracts	Mar. 31, 2012 Recurring Level 1	Dec. 31, 2011 Recurring Level 1	Mar. 31, 2012 Recurring Level 2	Dec. 31, 2011 Recurring Level 2	Mar. 31, 2012 Recurring Total	Dec. 31, 2011 Recurring Total
Assets:
Cash and cash equivalents								$ 106,609,000	$ 60,651,000			$ 106,609,000	$ 60,651,000
Restricted cash	27,761,000		21,412,000					27,761,000	21,412,000			27,761,000	21,412,000
Derivative instruments asset										27,199,000	32,414,000	27,199,000	32,414,000
Total								134,370,000	82,063,000	27,199,000	32,414,000	161,569,000	114,477,000
Liabilities:
Derivative instruments liability										159,903,000	53,762,000	159,903,000	53,762,000
Total										159,903,000	53,762,000	159,903,000	53,762,000
Net increase in fair value				27,100,000	5,800,000
Pre-tax gain in other comprehensive income				600,000	900,000
Gain in change in fair value of derivatives				26,600,000	5,100,000
Foreign exchange loss (gain)	$ 986,000	$ (658,000)				$ 100,000	$ 200,000