Fair Value Of Financial Instruments (Estimated Fair Value Of Senior Notes) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Short-term Debt [Line Items]
Recorded Amount	$ 744,108	$ 1,242,815
Estimated Fair Value	734,217	1,274,224
7% Senior Notes Due 2012 [Member]
Short-term Debt [Line Items]
Recorded Amount		149,650
Estimated Fair Value		160,493
Rate on Senior Notes	7.00%
5 1/2% Senior Notes Due 2013 [Member]
Short-term Debt [Line Items]
Recorded Amount		349,748
Estimated Fair Value		362,198
Rate on Senior Notes	5.50%
5 3/8% Medium-Term Senior Notes Due 2014 [Member]
Short-term Debt [Line Items]
Recorded Amount	249,438	249,266
Estimated Fair Value	254,667	255,683
Rate on Senior Notes	5.375%
5 3/8% Medium-Term Senior Notes Due 2015 [Member]
Short-term Debt [Line Items]
Recorded Amount	249,857	249,821
Estimated Fair Value	252,083	251,450
Rate on Senior Notes	5.375%
5 5/8% Medium-Term Senior Notes Due 2020 [Member]
Short-term Debt [Line Items]
Recorded Amount	244,813	244,330
Estimated Fair Value	$ 227,467	$ 244,400
Rate on Senior Notes	5.625%