Fair Value Measurements - Financial Instruments (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2016	Dec. 31, 2015
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, excluding current portion and medium-term fixed rate note due September 2016	$ 9,299	$ 8,753
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt, excluding current portion and medium-term fixed rate note due September 2016	$ 10,115	$ 9,101

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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