Segment Information (Reconciliation of Segment Profit to Operating Income) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting [Abstract]
Segment Profits	$ 58,782	$ 59,733	$ 126,112	$ 131,679
General and administrative	(18,592)	(13,774)	(37,565)	(29,380)
Gain (loss) on derivatives	445	4,905	(27)	2,736
Gain (loss) on sale of property	(84)	406	(95)	504
Depreciation and amortization	(34,673)	(32,889)	(68,417)	(65,085)
Operating income	$ 5,878	$ 18,381	$ 20,008	$ 40,454