Fair Value Measurments (Details 1) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Carrying Amount	$ 1,903,000	$ 1,560,000
Estimated Fair Value	2,058,506	1,561,343
5.45% Notes Due 2015 [Member]
Debt Instrument [Line Items]
Carrying Amount	100,000	100,000
Estimated Fair Value	111,475	108,572
7.31% Notes Due 2016 [Member]
Debt Instrument [Line Items]
Carrying Amount	62,000	62,000
Estimated Fair Value	74,817	72,153
4.98% Notes Due 2017 [Member]
Debt Instrument [Line Items]
Carrying Amount	116,000	116,000
Estimated Fair Value	128,570	119,385
5.92% Notes Due 2018 [Member]
Debt Instrument [Line Items]
Carrying Amount	200,000	200,000
Estimated Fair Value	231,091	212,660
7.77% Notes Due 2019 [Member]
Debt Instrument [Line Items]
Carrying Amount	173,000	173,000
Estimated Fair Value	219,552	203,051
5.50% Notes Due 2020[Member]
Debt Instrument [Line Items]
Carrying Amount	207,000	207,000
Estimated Fair Value	229,423	206,233
4.51% Notes Due 2020 [Member]
Debt Instrument [Line Items]
Carrying Amount	315,000	315,000
Estimated Fair Value	318,925	284,207
5.60% Notes Due 2022[Member]
Debt Instrument [Line Items]
Carrying Amount	87,000	87,000
Estimated Fair Value	94,165	84,818
4.66% Notes Due 2022 [Member]
Debt Instrument [Line Items]
Carrying Amount	35,000	35,000
Estimated Fair Value	34,631	30,989
5.85% Notes Due 2025 [Member]
Debt Instrument [Line Items]
Carrying Amount	90,000	90,000
Estimated Fair Value	99,022	87,211
4.91% Notes Due 2025 [Member]
Debt Instrument [Line Items]
Carrying Amount	175,000	175,000
Estimated Fair Value	173,835	152,064
Credit Facility [Member]
Debt Instrument [Line Items]
Carrying Amount	343,000	0
Estimated Fair Value	$ 343,000	$ 0