Acquisitions and Divestitures - Liabilities Assumed From Acquisition of Businesses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Combinations [Abstract]
Fair value of assets acquired	$ 83	$ 253	$ 2,576
Cash paid (net of cash acquired)	83	211	2,401
Liabilities assumed	$ 0	$ 42	$ 175