Debt (Consolidated Debt) (Details) (USD $)	Mar. 31, 2014		Dec. 31, 2013
Debt Instrument [Line Items]
Total debt	$ 2,941,276,000		$ 2,685,287,000
Weighted-Average Interest Rate	5.20%	[1]
Debt Instrument, Face Amount	2,900,000,000		2,700,000,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Total debt	0		0
Weighted-Average Interest Rate	0.00%	[1]
Senior Notes [Member] | 6.45% Notes Due 2014 [Member]
Debt Instrument [Line Items]
Total debt	249,988,000		249,971,000
Weighted-Average Interest Rate	6.30%	[1]
Debt Instrument, Face Amount	250,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	6.45%
Senior Notes [Member] | 5.65% Notes Due 2016 [Member]
Debt Instrument [Line Items]
Total debt	251,076,000		251,183,000
Weighted-Average Interest Rate	5.70%	[1]
Debt Instrument, Face Amount	250,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	5.65%
Senior Notes [Member] | 6.40% Notes Due 2018 [Member]
Debt Instrument [Line Items]
Total debt	258,829,000		259,346,000
Weighted-Average Interest Rate	5.40%	[1]
Debt Instrument, Face Amount	250,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	6.40%
Senior Notes [Member] | 6.55% Notes Due 2019 [Member]
Debt Instrument [Line Items]
Total debt	570,613,000		571,515,000
Weighted-Average Interest Rate	5.70%	[1]
Debt Instrument, Face Amount	550,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	6.55%
Senior Notes [Member] | 4.25% Notes Due 2021 [Member]
Debt Instrument [Line Items]
Total debt	556,988,000		557,213,000
Weighted-Average Interest Rate	4.00%	[1]
Debt Instrument, Face Amount	550,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	4.25%
Senior Notes [Member] | 6.40% Notes Due 2037 [Member]
Debt Instrument [Line Items]
Total debt	249,003,000		248,998,000
Weighted-Average Interest Rate	6.40%	[1]
Debt Instrument, Face Amount	250,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	6.40%
Senior Notes [Member] | 4.20% Notes Due 2042 [Member]
Debt Instrument [Line Items]
Total debt	248,384,000		248,377,000
Weighted-Average Interest Rate	4.20%	[1]
Debt Instrument, Face Amount	250,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	4.20%
Senior Notes [Member] | 5.15% Notes Due 2043 [Member]
Debt Instrument [Line Items]
Total debt	556,395,000		298,684,000
Weighted-Average Interest Rate	5.20%	[1]
Debt Instrument, Face Amount	$ 550,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	5.15%

[1] Weighted-average interest rate includes 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.