Financial Instruments and Risk Concentration (Details)	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2013
Senior Notes Due September 2016
Financial Instruments and Risk Concentration
Interest rate on senior notes due (as a percent)	2.35%		2.35%
Senior Notes Due September 2023
Financial Instruments and Risk Concentration
Interest rate on senior notes due (as a percent)	5.10%		5.10%
Senior Notes Due February 2018
Financial Instruments and Risk Concentration
Interest rate on senior notes due (as a percent)	6.15%
Senior Notes Due January 2019
Financial Instruments and Risk Concentration
Interest rate on senior notes due (as a percent)	9.25%	9.25%	9.25%
Senior Notes Due September 2020
Financial Instruments and Risk Concentration
Interest rate on senior notes due (as a percent)	5.00%
Senior Notes Due September 2021
Financial Instruments and Risk Concentration
Interest rate on senior notes due (as a percent)	4.625%