Interim Financial Information (Schedule of Quarterly Financial Information) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information Disclosure [Abstract]
Revenues, Net	$ 366,712	$ 332,056	$ 327,570	$ 326,239	$ 338,959	$ 283,914	$ 291,965	$ 272,903	$ 1,352,577	$ 1,187,741	$ 1,078,300
Operating Expenses	(286,048)	(245,197)	(229,294)	(229,200)	(259,840)	(189,470)	(209,691)	(202,224)	(989,739)	(861,225)	(798,464)
Operating Income	80,664	86,859	98,276	97,039	79,119	94,444	82,274	70,679	362,838	326,516	279,836
Income From Continuing Operations	15,191	36,543	41,405	43,077	29,265	40,317	27,058	29,722	136,216	126,362	118,189
Income (Loss) From Discontinued Operations, Net of Income Taxes	0	105	105	104	213	(314)	97	96	314	92	(38,090)
Net Income	$ 15,191	$ 36,648	$ 41,510	$ 43,181	$ 29,478	$ 40,003	$ 27,155	$ 29,818	$ 136,530	$ 126,454	$ 80,099
Earnings Per Share, Basic [Abstract]
Income From Continuing Operations	$ 0.22	$ 0.52	$ 0.59	$ 0.62	$ 0.42	$ 0.58	$ 0.39	$ 0.43	$ 1.94	$ 1.82	$ 1.71
Income From Discontinued Operations	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ (0.55)
Net Income	$ 0.22	$ 0.52	$ 0.59	$ 0.62	$ 0.42	$ 0.58	$ 0.39	$ 0.43	$ 1.94	$ 1.83	$ 1.16
Earnings Per Share, Diluted [Abstract]
Income From Continuing Operations	$ 0.21	$ 0.51	$ 0.57	$ 0.60	$ 0.40	$ 0.56	$ 0.39	$ 0.43	$ 1.89	$ 1.79	$ 1.71
Income From Discontinued Operations	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ (0.55)
Net Income	$ 0.21	$ 0.51	$ 0.58	$ 0.60	$ 0.41	$ 0.55	$ 0.39	$ 0.43	$ 1.89	$ 1.79	$ 1.16