Fair Value Measurements (Estimated Fair Value Of Senior Notes) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010
Recorded Amount	$ 1,243,273	$ 1,242,815
Fair Value, Inputs, Level 2 [Member] | 7% Senior Notes Due 2012 [Member]
Recorded Amount	149,705	149,650
Estimated Fair Value	162,000	160,493
7% Senior Notes Due 2012 [Member]
Recorded Amount	149,705	149,650
Interest rate	7.00%
Maturity date	2012
Fair Value, Inputs, Level 2 [Member] | 5 1/2% Senior Notes Due 2013 [Member]
Recorded Amount	349,810	349,748
Estimated Fair Value	374,500	362,198
5 1/2% Senior Notes Due 2013 [Member]
Recorded Amount	349,810	349,748
Interest rate	5.50%
Maturity date	2013
Fair Value, Inputs, Level 2 [Member] | 5 3/8% Medium-Term Senior Notes Due 2014 [Member]
Recorded Amount	249,351	249,266
Estimated Fair Value	265,700	255,683
5 3/8% Medium-Term Senior Notes Due 2014 [Member]
Recorded Amount	249,351	249,266
Interest rate	5.375%
Maturity date	2014
Fair Value, Inputs, Level 2 [Member] | 5 3/8% Medium-Term Senior Notes Due 2015 [Member]
Recorded Amount	249,839	249,821
Estimated Fair Value	259,050	251,450
5 3/8% Medium-Term Senior Notes Due 2015 [Member]
Recorded Amount	249,839	249,821
Interest rate	5.375%
Maturity date	2015
Fair Value, Inputs, Level 2 [Member] | 5 3/8% Senior Notes Due 2020 [Member]
Recorded Amount	244,568	244,330
Estimated Fair Value	244,525	244,400
5 3/8% Senior Notes Due 2020 [Member]
Recorded Amount	244,568	244,330
Interest rate	5.375%
Maturity date	2020
Fair Value, Inputs, Level 2 [Member]
Recorded Amount	1,243,273	1,242,815
Estimated Fair Value	$ 1,305,775	$ 1,274,224