Financial Instruments (Details) (Recurring [Member], USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Carrying value and estimated fair value of financial instruments
Interest rate swap derivative assets	$ 5,455	$ 6,212
Restricted investments:
Guaranteed Investment Contract	5,742	5,742
Rabbi Trust	8,111	7,718
Fixed income securities	2,128	2,152
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Derivative Liabilities	534	708
Quoted Prices in Active Markets (Level 1) [Member]
Carrying value and estimated fair value of financial instruments
Interest rate swap derivative assets	0	0
Restricted investments:
Guaranteed Investment Contract	0	0
Rabbi Trust	8,111	7,718
Fixed income securities	0	0
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Derivative Liabilities	0	0
Significant Other Observable Inputs (Level 2) [Member]
Carrying value and estimated fair value of financial instruments
Interest rate swap derivative assets	5,455	6,212
Restricted investments:
Guaranteed Investment Contract	5,742	5,742
Rabbi Trust	0	0
Fixed income securities	2,128	2,152
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Derivative Liabilities	534	708
Significant Unobservable Inputs (Level 3) [Member]
Carrying value and estimated fair value of financial instruments
Interest rate swap derivative assets	0	0
Restricted investments:
Guaranteed Investment Contract	0	0
Rabbi Trust	0	0
Fixed income securities	0	0
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Derivative Liabilities	$ 0	$ 0