Fair Value Measurements - Changes in Level 3 Fair Value Measurements (Details) - Liability [Member] - Estimate of Fair Value Measurement [Member] - Fair Value, Measurements, Recurring [Member] - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance as of beginning of period	$ 86	$ 112
Change in fair value recorded in Restructuring and support expenses and other	(1)	(10)
Change due to resolved claims in Disputed claims liability	(12)	(29)
Balance as of end of period	73	73
CEC Convertible Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Balance as of beginning of period	831	1,016
Change in fair value recorded in Restructuring and support expenses and other	(97)	(282)
Change due to resolved claims in Disputed claims liability	4	4
Balance as of end of period	$ 738	$ 738