Accounting Policies - Narrative (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Net cash for operating activities	$ 467	$ 483
Net cash from financing activities	(518)	(860)
Accounting Standards Update 2016-09, Statutory Tax Withholding Component
Debt Instrument [Line Items]
Net cash for operating activities		61	$ 62	$ 111
Net cash from financing activities		$ (61)	(62)	$ (111)
Accounting Standards Update 2017-07
Debt Instrument [Line Items]
Non-operating income (expense), expected to be adjusted	$ 22		$ 31