Debt (Consolidated Debt) (Details) - USD ($)		Dec. 31, 2015		Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Long-term Debt		$ 3,458,294,000		$ 2,982,895,000
Long-term Debt, Weighted Average Interest Rate	[1]	4.70%		4.90%	5.20%
Unamortized Debt Acquisition Costs - Contra Liability		$ (18,672,000)		$ (15,876,000)
Debt Instrument, Face Amount		3,400,000,000.0		2,900,000,000.0
Current portion of long-term debt		250,335,000		0
Long-term Debt, Excluding Current Maturities		3,189,287,000		2,967,019,000
5.65% Notes Due 2016 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 250,335,000	[2]	250,758,000
Long-term Debt, Weighted Average Interest Rate	[1]	5.70%
Debt Instrument, Face Amount		$ 250,000,000.0		250,000,000.0
Debt Instrument, Interest Rate, Stated Percentage		5.65%
6.40% Notes Due 2018 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 255,215,000		257,280,000
Long-term Debt, Weighted Average Interest Rate	[1]	5.50%
Debt Instrument, Face Amount		$ 250,000,000.0		250,000,000.0
Debt Instrument, Interest Rate, Stated Percentage		6.40%
6.55% Notes Due 2019 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 564,116,000		567,868,000
Long-term Debt, Weighted Average Interest Rate	[1]	5.70%
Debt Instrument, Face Amount		$ 550,000,000.0		550,000,000.0
Debt Instrument, Interest Rate, Stated Percentage		6.55%
4.25% Notes Due 2021 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 555,362,000		556,304,000
Long-term Debt, Weighted Average Interest Rate	[1]	4.00%
Debt Instrument, Face Amount		$ 550,000,000.0		550,000,000.0
Debt Instrument, Interest Rate, Stated Percentage		4.25%
3.20% Notes Due 2025 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 249,700,000		0
Long-term Debt, Weighted Average Interest Rate	[1],[3]	3.20%
Debt Instrument, Face Amount		$ 250,000,000.0		0
Debt Instrument, Interest Rate, Stated Percentage		3.20%
6.40% Notes Due 2037 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 249,036,000		249,017,000
Long-term Debt, Weighted Average Interest Rate	[1]	6.40%
Debt Instrument, Face Amount		$ 250,000,000.0		250,000,000.0
Debt Instrument, Interest Rate, Stated Percentage		6.40%
4.20% Notes Due 2042 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 248,437,000		248,406,000
Long-term Debt, Weighted Average Interest Rate	[1]	4.20%
Debt Instrument, Face Amount		$ 250,000,000.0		250,000,000.0
Debt Instrument, Interest Rate, Stated Percentage		4.20%
5.15% Notes Due 2043 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 556,218,000		556,320,000
Long-term Debt, Weighted Average Interest Rate	[1]	5.10%
Debt Instrument, Face Amount		$ 550,000,000.0		300,000,000.0
Debt Instrument, Interest Rate, Stated Percentage		5.15%
4.20% Notes Due 2045 [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 249,914,000		0
Long-term Debt, Weighted Average Interest Rate	[1],[3]	4.60%
Debt Instrument, Face Amount		$ 250,000,000.0		0
Debt Instrument, Interest Rate, Stated Percentage		4.20%
Commercial Paper [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 279,961,000		$ 296,942,000
Long-term Debt, Weighted Average Interest Rate	[1],[3]	0.50%
Line of Credit [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Long-term Debt		$ 0

[1] Weighted-average interest rate includes the amortization/accretion of discounts, premiums and gains/losses realized on historical cash flow and fair value hedges recognized as interest expense.

[2] These borrowings will mature in October 2016 and are included with current debt on our consolidated balance sheets at December 31, 2015.

[3] These borrowings were outstanding for only a portion of the year ending December 31, 2015. The weighted-average interest rate for these borrowings was calculated based on the number of days the borrowings were outstanding during the noted period.