Acquisitions and Divestitures - Pro Forma (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Pro forma:
Revenues	$ 1,376,393	$ 1,336,227
Net income	$ 83,940	$ 229,272
Net income per common share:
Basic	$ 1.75	$ 4.81
Diluted	$ 1.74	$ 4.78