Debt (Consolidated Debt) (Details) (USD $)	3 Months Ended
	Mar. 31, 2012		Dec. 31, 2011
Debt Instrument [Line Items]
Revolving credit facility
Total debt	2,150,107,000		2,151,775,000
Weighted-Average Interest Rate		[1]
Weighted-Average Interest Rate	5.30%	[1]
Long-term debt	2,100,000,000		2,100,000,000
6.45% Notes Due 2014 [Member]
Debt Instrument [Line Items]
Notes payable	249,859,000		249,844,000
Weighted-Average Interest Rate	6.30%	[1]
Long-term debt	250,000,000
Debt instrument interest rate	6.45%
Debt instrument, maturity date	2014
5.65% Notes Due 2016 [Member]
Debt Instrument [Line Items]
Notes payable	251,930,000		252,037,000
Weighted-Average Interest Rate	5.60%	[1]
Long-term debt	250,000,000
Debt instrument interest rate	5.65%
Debt instrument, maturity date	2016
6.40% Notes Due 2018 [Member]
Debt Instrument [Line Items]
Notes payable	262,962,000		263,477,000
Weighted-Average Interest Rate	5.30%	[1]
Long-term debt	250,000,000
Debt instrument interest rate	6.40%
Debt instrument, maturity date	2018
6.55% Notes Due 2019 [Member]
Debt Instrument [Line Items]
Notes payable	577,665,000		578,521,000
Weighted-Average Interest Rate	5.60%	[1]
Long-term debt	550,000,000
Debt instrument interest rate	6.55%
Debt instrument, maturity date	2019
4.25% Notes Due 2021 [Member]
Debt Instrument [Line Items]
Notes payable	558,723,000		558,932,000
Weighted-Average Interest Rate	4.00%	[1]
Long-term debt	550,000,000
Debt instrument interest rate	4.25%
Debt instrument, maturity date	2021
6.40% Notes Due 2037 [Member]
Debt Instrument [Line Items]
Notes payable	248,968,000		248,964,000
Weighted-Average Interest Rate	6.40%	[1]
Long-term debt	$ 250,000,000
Debt instrument interest rate	6.40%
Debt instrument, maturity date	2037

[1] Weighted-average interest rate includes the impact of interest rate swaps, the amortization/accretion of discounts and premiums and the amortization/accretion of gains and losses realized on historical cash flow and fair value hedges (see Note 7—Derivative Financial Instruments for detailed information regarding fair value hedges and interest rate swaps).