Derivative Financial Instruments (Details 4) (Derivatives in Cash Flow Hedging Relationship, USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Gains and losses from derivatives
Amount of Gain (Loss) Recognized in Other Comprehensive Income on Derivatives	$ 2,591,000	$ (4,039,000)	$ (6,371,000)
Interest Rate collar
Gains and losses from derivatives
Amount of Gain (Loss) Recognized in Other Comprehensive Income on Derivatives	1,949,000	2,225,000	1,005,000
Reclassification of gain (loss) from other comprehensive income to earnings
Amount of gain (loss) reclassified from other comprehensive income into earnings - effective portion	2,500,000	3,800,000	5,900,000
Interest rate swap
Gains and losses from derivatives
Amount of Gain (Loss) Recognized in Other Comprehensive Income on Derivatives	913,000	(5,151,000)	(7,376,000)
Interest rate cap
Gains and losses from derivatives
Amount of Gain (Loss) Recognized in Other Comprehensive Income on Derivatives	$ (271,000)	$ (1,113,000)