Business Segment Information (Details 1) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Schedule of Reconciliation of Segment Profits and Operating Income
Segment profits	$ 128,504	$ 111,538	$ 262,073	$ 201,506
General and administrative expenses	(45,540)	(34,138)	(86,898)	(65,479)
Depreciation and amortization	(45,536)	(34,784)	(89,520)	(67,764)
(Gain) loss on disposal of assets	(980)	(942)	(2,699)	763
Operating income	$ 36,448	$ 41,674	$ 82,956	$ 69,026