Derivative Instruments - Statements of Income (Details) (Cash Flow Hedging [Member], Designated as Hedging Instrument [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Effects of derivative instruments
Amount of (Gain) Loss Recognized on Derivatives in the Statements of Income	$ (122)	$ 390	$ (371)	$ 1,149
Interest Rate Contract [Member] | Interest Expense [Member]
Effects of derivative instruments
Amount of (Gain) Loss Recognized on Derivatives in the Statements of Income	$ (122)	$ 390	$ (371)	$ 1,149