Acquisitions and Divestitures - Pro Forma (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pro forma:
Revenues	$ 335,394	$ 354,320	$ 1,041,350	$ 964,165
Net income	$ 47,186	$ 60,897	$ 140,670	$ 171,445
Net income per common share:
Basic	$ 0.98	$ 1.28	$ 2.94	$ 3.60
Diluted	$ 0.98	$ 1.27	$ 2.92	$ 3.58