Income Taxes (Narrative) (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2018	Jan. 31, 2018
Tax Credit Carryforward [Line Items]
Income Tax Examination, pre-tax adjustments proposed	$ 192	$ 198
Income Tax Examination, Estimate of Possible Loss	$ 136

Estimated amount of loss resulting from an adverse tax position.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (d)(3)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32718-109319

Name:	us-gaap_IncomeTaxExaminationEstimateOfPossibleLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration