Debt (Consolidated Debt) (Details) (USD $)	1 Months Ended	9 Months Ended
	Aug. 31, 2011	Sep. 30, 2011		Dec. 31, 2010
Debt Instrument [Line Items]
Revolving credit facility				$ 15,000,000
Long-term debt, Total		2,153,437,000		1,906,148,000
Weighted-Average Interest Rate			[1]
Long-term debt		2,100,000,000
6.45% Notes Due 2014 [Member]
Debt Instrument [Line Items]
Notes payable		249,829,000		249,786,000
Weighted-Average Interest Rate		6.30%	[1]
Long-term debt		250,000,000
Debt instrument, maturity date		2014
Debt instrument interest rate		6.45%
5.65% Notes Due 2016 [Member]
Debt Instrument [Line Items]
Notes payable		252,144,000		252,466,000
Weighted-Average Interest Rate		5.70%	[1]
Long-term debt		250,000,000
Debt instrument, maturity date		2016
Debt instrument interest rate		5.65%
6.40% Notes Due 2018 [Member]
Debt Instrument [Line Items]
Notes payable		263,994,000		259,125,000
Weighted-Average Interest Rate		5.10%	[1]
Long-term debt		250,000,000
Debt instrument, maturity date		2018
Debt instrument interest rate		6.40%
6.55% Notes Due 2019 [Member]
Debt Instrument [Line Items]
Notes payable		579,372,000		581,890,000
Weighted-Average Interest Rate		5.90%	[1]
Long-term debt		550,000,000
Debt instrument, maturity date		2019
Debt instrument interest rate		6.55%
4.25% Notes Due 2021 [Member]
Debt Instrument [Line Items]
Notes payable		559,138,000		298,932,000
Weighted-Average Interest Rate		4.20%	[1]
Long-term debt	250,000,000	550,000,000
Debt instrument, maturity date	2021	2021
Debt instrument interest rate	4.25%	4.25%
6.40% Notes Due 2037 [Member]
Debt Instrument [Line Items]
Notes payable		248,960,000		248,949,000
Weighted-Average Interest Rate		6.30%	[1]
Long-term debt		$ 250,000,000
Debt instrument, maturity date		2037
Debt instrument interest rate		6.40%

[1] Weighted-average interest rate includes the impact of outstanding 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 9—Derivative Financial Instruments for detailed information regarding interest rate swaps).