Selected Quarterly Financial Data (Schedule Of Selected Quarterly Financial Data) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Selected Quarterly Financial Data [Abstract]
Revenues, net	$ 140,160	$ 134,213	$ 134,213	$ 111,005	$ 106,547	$ 111,655	$ 111,437	$ 104,202	$ 519,591	$ 433,841	$ 354,073
Operating income	54,232	61,723	59,892	50,487	23,774	49,062	50,557	47,100	226,334	170,493	146,045
Net income	$ 37,771	$ 40,514	$ 36,715	$ 32,335	$ 17,513	$ 33,398	$ 29,640	$ 27,345	$ 147,335	$ 107,896	$ 89,052
Basic earnings per share	$ 0.46	$ 0.50	$ 0.46	$ 0.40	$ 0.43	$ 0.85	$ 0.74	$ 0.67	$ 1.83	$ 3.00	$ 2.17
Diluted earnings per share	$ 0.45	$ 0.48	$ 0.44	$ 0.39	$ 0.22	$ 0.41	$ 0.37	$ 0.34	$ 1.76	$ 1.34	$ 1.13
Basic weighted average shares outstanding	81,512	80,819	80,151	79,937	39,612	34,076	33,988	34,011	80,610	35,434	33,802
Diluted weighted average shares outstanding	84,035	83,649	83,548	83,378	80,931	80,880	80,701	80,489	83,654	80,751	78,854