Financial Instruments and Risk Concentration (Details)	Dec. 31, 2015	Dec. 31, 2013	Sep. 30, 2013	Aug. 31, 2011
2.35% senior notes due September 2016
Interest rate on senior notes due (as a percent)	2.35%		2.35%
5.10% senior notes due September 2023
Interest rate on senior notes due (as a percent)	5.10%		5.10%
6.15% senior notes due February 2018
Interest rate on senior notes due (as a percent)	6.15%
9.25% senior notes due January 2019
Interest rate on senior notes due (as a percent)	9.25%	9.25%	9.25%
5.00% senior notes due September 2020
Interest rate on senior notes due (as a percent)	5.00%
4.625% senior notes due September 2021
Interest rate on senior notes due (as a percent)	4.625%			4.625%