Long term Debt (Schedule of Long Term Debt) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Notes payable	$ 2,850,000	$ 2,700,000	[1]
Unamortized discount on notes payable	(19,223)	(23,522)	[1]
Total notes payable, net of unamortized discount	2,830,777	2,676,478	[1]
Capitalized lease obligation, due through June 2029	228,698	306,579	[1]
Total debt including current maturities	3,059,475	2,983,057	[1]
Current maturities	(18,881)	(465,388)	[1],[2]
Total long-term debt	$ 3,040,594	2,517,669	[1],[2]
Capital lease expiration date	Jun. 30, 2029
2.50% notes, due December 2017 [Member]
Debt Instrument [Line Items]
Notes payable	$ 550,000	$ 550,000	[1]
Notes payable, stated interest rate	2.50%	2.50%
Notes payable, due date	December 2017
4.00% notes, due June 2022 [Member]
Debt Instrument [Line Items]
Notes payable	$ 500,000	$ 500,000	[1]
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	$ 600,000	$ 600,000	[1]
Notes payable, stated interest rate	3.70%	3.70%
Notes payable, due date	December 2022
7.05% notes, due May 2029 [Member]
Debt Instrument [Line Items]
Notes payable	$ 250,000	$ 250,000	[1]
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	$ 350,000	$ 350,000	[1]
Notes payable, stated interest rate	5.125%	5.125%
Notes payable, due date	December 2042
Notes payable to banks, 1.4375% [Member]
Debt Instrument [Line Items]
Notes payable	$ 600,000	$ 450,000	[1]
Notes payable, stated interest rate	1.4375%

[1] Reclassified to current presentation

[2] Reclassified to conform to current presentation.