Accounting for Interest Rate Swap Agreements (Details) (Designated as a hedging instrument, Interest rate hedges, USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Accounting for Interest Rate Swap Agreements
Notional amount of interest rate swaps	$ 250,000,000
Weighted average effective fixed interest rate on interest rate swaps (as a percent)	1.80%
Amount of ineffectiveness related to interest rate swaps	0	0
Deferred losses to be reclassified during next 12 months	(3,500,000)
Derivatives liability designated as hedging instruments, Fair value	(8,727,000)		(9,916,000)
Current portion of interest rate swaps
Accounting for Interest Rate Swap Agreements
Derivatives liability designated as hedging instruments, Fair value	(3,497,000)		(3,873,000)
Interest rate swaps
Accounting for Interest Rate Swap Agreements
Derivatives liability designated as hedging instruments, Fair value	$ (5,230,000)		$ (6,043,000)