Derivative Instruments (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Interest rate contracts
Effects of derivative instruments
Amount of Loss (Gain) Recognized on Derivatives in the Statements of Income	$ 500	$ (1,500)	$ (6,400)
Cash Flow Hedging
Effects of derivative instruments
Amount of Loss (Gain) Recognized on Derivatives in the Statements of Income	(499)	1,485	16,570
Cash Flow Hedging | Loss on debt extinguishment and derivatives termination
Effects of derivative instruments
Reclassification adjustment for losses included in termination of derivative instruments and recognized on derivatives in the statements of income			10,143
Cash Flow Hedging | Interest rate contracts | Interest expense Member
Effects of derivative instruments
Amount of Loss (Gain) Recognized on Derivatives in the Statements of Income	$ (499)	$ 1,485	$ 6,427