Long term Debt (Schedule of Long Term Debt) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017		Nov. 30, 2017	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Notes payable		$ 2,800,000				$ 2,800,000
Unamortized discount on notes payable		(27,433)				(23,835)
Total notes payable, net of unamortized discount		2,772,567				2,776,165
Capitalized lease obligation, due through June 2029		143,855				216,402
Total debt including current maturities		2,916,422				2,992,567
Current maturities		(9,902)				(569,817)
Total long-term debt		$ 2,906,520				2,422,750
Capital lease expiration date		Mar. 01, 2029
3.50% notes, due December 2017 [Member]
Debt Instrument [Line Items]
Notes payable						$ 550,000
Interest rate		3.50%			3.50%	3.50%
Notes payable, due date		December 2017
4.00% notes, due June 2022 [Member]
Debt Instrument [Line Items]
Notes payable		$ 500,000				$ 500,000
Interest rate		4.00%				4.00%
Notes payable, due date		June 2022
4.45% notes, due December 2022 [Member]
Debt Instrument [Line Items]
Notes payable	[1]	$ 600,000				$ 600,000
Interest rate		4.45%	[1]	4.70%		4.45%	[1]
Notes payable, due date	[1]	December 2022
6.875% notes, due August 15, 2024 [Member]
Debt Instrument [Line Items]
Notes payable		$ 550,000				$ 550,000
Interest rate		6.875%				6.875%
Notes payable, due date		August 2024
5.75% notes, due August 2025 [Member]
Debt Instrument [Line Items]
Notes payable		$ 550,000
Interest rate		5.75%
Notes payable, due date		August 2025
7.05% notes, due May 2029 [Member]
Debt Instrument [Line Items]
Notes payable		$ 250,000				$ 250,000
Interest rate		7.05%				7.05%
Notes payable, due date		May 2029
5.875% notes, due December 2042 [Member]
Debt Instrument [Line Items]
Notes payable	[1]	$ 350,000				$ 350,000
Interest rate		5.875%	[1]	6.125%		5.875%	[1]
Notes payable, due date	[1]	December 2042

[1] Due to a series of ratings changes by credit agencies, the paying interest rates on the notes due December 2022 and December 2042 decreased from 4.7% to 4.45% and 6.125% to 5.875%, respectively, at December 2017.