Derivative Instruments (Details) (Interest rate swaps [Member], Cash Flow Hedge [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Effects of changes in the fair values of derivatives designated as cash flow hedges on the accompanying Consolidated Statements of Comprehensive Income
Loss recognized in OCI (effective portion)	$ 0	$ (882)	$ (4,633)
Interest Expense [Member]
Effects of changes in the fair values of derivatives designated as cash flow hedges on the accompanying Consolidated Statements of Comprehensive Income
Loss reclassified from AOCI (effective portion)	$ (11,657)	$ (19,619)	$ (16,776)