Acquisitions and Dispositions of Businesses - Dispositions (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) disposal	Dec. 31, 2016 USD ($) disposal	Dec. 31, 2015 USD ($) disposal
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net gain (loss) on disposal of businesses | $	$ (16)	$ 39	$ 82
Disposal Group, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of dispositions | disposal	9	5	7