Derivative Instruments (Details 4) (Cash Flow Hedging, USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Effects of derivative instruments
Amount of Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 508,000	$ 1,933,000	$ (6,380,000)
Amount of Loss (Gain) Reclassified from Accumulated OCI into Income (Effective Portion)	6,427,000	11,349,000	18,633,000
Interest expense
Effects of derivative instruments
Amount of Loss (Gain) Reclassified from Accumulated OCI into Income (Effective Portion)	6,427,000	11,349,000	18,633,000
Interest rate contracts
Effects of derivative instruments
Amount of Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	508,000	1,933,000	(6,380,000)
Amount of interest payments reclassified to income statement	$ 6,700,000	$ 10,900,000	$ 15,700,000