Summary of Significant Accounting Policies (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Assets and (Liabilities) at Fair Value
Liabilities under derivative instruments	$ (1,218)	$ (1,690)
Ineffectiveness on hedges recorded in earnings	0	0
Recurring | Total
Assets and (Liabilities) at Fair Value
Liabilities under derivative instruments	(1,218)	(1,690)
Total	(1,218)	(1,690)
Recurring | Level 2
Assets and (Liabilities) at Fair Value
Liabilities under derivative instruments	(1,218)	(1,690)
Total	$ (1,218)	$ (1,690)