Fair Value Measurements - Financial Instruments (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Fixed rate medium term note due 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt Instrument, Face Amount	$ 1,000
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Medium-term fixed rate note due September 2016	999	$ 999
Long-term debt, excluding current portion and medium-term fixed rate note due September 2016	8,927	8,753
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Medium-term fixed rate note due September 2016	1,002	1,003
Long-term debt, excluding current portion and medium-term fixed rate note due September 2016	$ 9,559	$ 9,101