Financial Instruments and Risk Concentration (Details)	Dec. 31, 2018	Feb. 28, 2018	Jan. 31, 2018
6.15% senior notes due February 2018
Debt Instrument, Interest Rate, Stated Percentage	6.15%	6.15%
9.25% senior notes due January 2019
Debt Instrument, Interest Rate, Stated Percentage	9.25%
5.00% senior notes due September 2020
Debt Instrument, Interest Rate, Stated Percentage	5.00%
4.625% senior notes due September 2021
Debt Instrument, Interest Rate, Stated Percentage	4.625%
Senior Notes 5.50 Percentage Due Two Zero Two Three [Member]
Debt Instrument, Interest Rate, Stated Percentage	5.50%
5.10% senior notes due September 2023
Debt Instrument, Interest Rate, Stated Percentage	5.10%
5.75% senior notes due February 2025
Debt Instrument, Interest Rate, Stated Percentage	5.75%		5.75%