Accounting Principles and Practices (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred tax assets	$ 296	$ 261	$ 389
Deferred tax liabilities	243	169	127
Other non-current assets	$ 439	$ 452	307
Adoption of new accounting guidance			492	$ 49
Accounting Standards Update 2016-16
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred tax assets			23
Deferred tax liabilities			12
Other non-current assets			(26)
Accumulated Other Comprehensive Loss, Net of Tax
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of new accounting guidance			(1)
Accumulated Other Comprehensive Loss, Net of Tax | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of new accounting guidance			(1)
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of new accounting guidance			493	$ 49
Retained Earnings | Accounting Standards Update 2016-16
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of new accounting guidance			(15)
Retained Earnings | Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of new accounting guidance			$ 1