Financial Instruments (Details) (Recurring [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Carrying value and estimated fair value of financial instruments
Interest rate swap derivative assets	$ 4,599	$ 6,212
Restricted investments:
Guaranteed Investment Contract	5,742	5,742
Rabbi Trust	8,163	7,718
Fixed income securities	1,996	2,152
Liabilities:
Interest rate swap derivative liabilities	490	708
Quoted Prices in Active Markets (Level 1)
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	8,163	7,718
Fixed income securities	0	0
Liabilities:
Interest rate swap derivative liabilities	0	0
Significant Other Observable Inputs (Level 2)
Carrying value and estimated fair value of financial instruments
Interest rate swap derivative assets	4,599	6,212
Restricted investments:
Guaranteed Investment Contract	5,742	5,742
Rabbi Trust	0	0
Fixed income securities	1,996	2,152
Liabilities:
Interest rate swap derivative liabilities	490	708
Significant Unobservable Inputs (Level 3)
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
Liabilities:
Interest rate swap derivative liabilities	$ 0	$ 0