Acquisitions and Dispositions of Businesses - Dispositions (Details) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) disposal	Dec. 31, 2017 USD ($) disposal	Dec. 31, 2016 USD ($) disposal
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal of business		$ (6)	$ (16)	$ 39
Disposal Group, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of dispositions | disposal		4	9	5
Disposal Group, Held-for-sale or Disposed
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal group assets	$ 47
Disposal group liabilities	41
Amortization and Impairment of Intangible Assets | Disposal Group, Held-for-sale or Disposed
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Noncash impairment charge	$ 176