Long term Debt (Schedule of Long Term Debt) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Aug. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Notes payable		$ 2,800,000		$ 2,850,000
Unamortized discount on notes payable		(23,835)		(19,223)
Total notes payable, net of unamortized discount		2,776,165		2,830,777
Capitalized lease obligation, due through June 2029		216,402		228,698
Total debt including current maturities		2,992,567		3,059,475
Current maturities		(569,817)		(18,881)	[1]
Total long-term debt		$ 2,422,750		3,040,594	[1]
Capital lease expiration date		Mar. 01, 2029
2.50% notes, due December 2017 [Member]
Debt Instrument [Line Items]
Notes payable	[2]	$ 550,000		$ 550,000
Notes payable, stated interest rate	[2]	2.50%		2.50%
Notes payable, due date	[2]	December 2017
Interest rate increase		1.00%
4.00% notes, due June 2022 [Member]
Debt Instrument [Line Items]
Notes payable		$ 500,000		$ 500,000
Notes payable, stated interest rate		4.00%
Notes payable, due date		June 2022
3.70% notes, due December 2022 [Member]
Debt Instrument [Line Items]
Notes payable	[2]	$ 600,000		$ 600,000
Notes payable, stated interest rate	[2]	3.70%		3.70%
Notes payable, due date	[2]	December 2022
Interest rate increase		1.00%
6.875% Notes Due August 15, 2024 [Member]
Debt Instrument [Line Items]
Notes payable		$ 550,000
Notes payable, stated interest rate		6.875%	6.875%
Maturity date		Aug. 15, 2024
7.05% notes, due May 2029 [Member]
Debt Instrument [Line Items]
Notes payable		$ 250,000		$ 250,000
Notes payable, stated interest rate		7.05%		7.05%
Notes payable, due date		May 2029
5.125% notes, due December 2042 [Member]
Debt Instrument [Line Items]
Notes payable	[2]	$ 350,000		$ 350,000
Notes payable, stated interest rate	[2]	5.125%		5.125%
Notes payable, due date	[2]	December 2042
Notes payable to banks, 1.4375% [Member]
Debt Instrument [Line Items]
Notes payable				$ 600,000
Notes payable, stated interest rate		1.4375%

[1] Reclassified to conform to current presentation. See Note B for additional information.

[2] The interest rate paid is 1.0% above rate shown due to a downgrade of the credit rating for the Company's notes in February 2016.