Derivative Instruments (Details 4) (Cash Flow Hedging, USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Effects of derivative instruments
Amount of Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	$ 472,000	$ 202,000
Amount of Loss Reclassified from Accumulated OCI into Income (Effective Portion)	(366,000)	(2,448,000)
Significant ineffectiveness on hedges	0	0
Interest rate contracts
Effects of derivative instruments
Amounts of other comprehensive income reclassified to the income statement	500,000	2,500,000
Interest rate contracts | Interest expense
Effects of derivative instruments
Amount of Gain (Loss) Recognized in OCI on Derivatives (Effective Portion)	472,000	202,000
Amount of Loss Reclassified from Accumulated OCI into Income (Effective Portion)	$ (366,000)	$ (2,448,000)