Debt (Consolidated Debt) (Details) (USD $)	6 Months Ended
	Jun. 30, 2012		Dec. 31, 2011
Debt Instrument [Line Items]
Total debt	$ 2,148,432,000		$ 2,151,775,000
Weighted-Average Interest Rate at June 30, 2012	5.30%	[1]
Long-term debt	2,100,000,000		2,100,000,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Revolving credit facility
Weighted-Average Interest Rate at June 30, 2012		[1]
6.45% Notes Due 2014 [Member]
Debt Instrument [Line Items]
Notes payable	249,874,000		249,844,000
Weighted-Average Interest Rate at June 30, 2012	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	251,823,000		252,037,000
Weighted-Average Interest Rate at June 30, 2012	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	262,445,000		263,477,000
Weighted-Average Interest Rate at June 30, 2012	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	576,804,000		578,521,000
Weighted-Average Interest Rate at June 30, 2012	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	558,514,000		558,932,000
Weighted-Average Interest Rate at June 30, 2012	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	248,972,000		248,964,000
Weighted-Average Interest Rate at June 30, 2012	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 on interest expense (see Note 7—Derivative Financial Instruments for detailed information regarding fair value hedges and interest rate swaps).