Accounting for Derivatives (Details) (USD $)	3 Months Ended		9 Months Ended				3 Months Ended	9 Months Ended										3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Terminated hedge, May 2013 Interest rate hedges	May 31, 2013 Terminated hedge, May 2013 Interest rate hedges	Sep. 30, 2013 Terminated hedge, May 2013 Interest rate hedges Losses from terminated interest rate swaps Reclassification out of accumulated other comprehensive income	Sep. 30, 2013 Terminated hedge, May 2013 Interest rate hedges Losses from terminated interest rate swaps Reclassification out of accumulated other comprehensive income	Sep. 30, 2013 Designated as a hedging instrument Interest rate hedges	Sep. 30, 2012 Designated as a hedging instrument Interest rate hedges	Dec. 31, 2012 Designated as a hedging instrument Interest rate hedges	Sep. 30, 2013 Designated as a hedging instrument Interest rate hedges Current portion of interest rate swaps	Dec. 31, 2012 Designated as a hedging instrument Interest rate hedges Current portion of interest rate swaps	Sep. 30, 2013 Designated as a hedging instrument Interest rate hedges Interest rate swaps	Dec. 31, 2012 Designated as a hedging instrument Interest rate hedges Interest rate swaps	Sep. 30, 2013 Amended designated as a hedging instrument Interest rate hedges	May 31, 2013 Amended designated as a hedging instrument Interest rate hedges	Sep. 30, 2013 Amended designated as a hedging instrument Interest rate hedges Losses from terminated interest rate swaps Reclassification out of accumulated other comprehensive income	Sep. 30, 2013 Amended designated as a hedging instrument Interest rate hedges Losses from terminated interest rate swaps Reclassification out of accumulated other comprehensive income
Accounting for Interest Rate Swap Agreements
Notional amount of interest rate swaps						$ 250,000,000			$ 250,000,000
Weighted average effective fixed interest rate on interest rate swaps (as a percent)									1.70%
Interest income recorded due to ineffectiveness related to interest rate swaps									0	0
Inception gain recorded in accumulated other comprehensive income (loss)																	9,200,000
Deferred gain (loss) to be reclassified during next 12 months					(3,800,000)				(3,400,000)							3,500,000
Gains (losses) reclassified into interest expense	9,735,000	6,465,000	27,458,000	18,746,000			1,000,000	1,600,000										(900,000)	(1,300,000)
Derivatives liability designated as hedging instruments, Fair value						(8,800,000)						(3,374,000)	(3,873,000)	(1,230,000)	(6,043,000)
Total derivatives, Fair value									$ (4,604,000)		$ (9,916,000)