Accounting Policies - Narrative (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Net cash for operating activities	$ 155	$ 190
Net cash from financing activities	$ (305)	(738)
Accounting Standards Update 2016-09, Statutory Tax Withholding Component
Debt Instrument [Line Items]
Net cash for operating activities		36	$ 62	$ 111
Net cash from financing activities		$ (36)	$ (62)	$ (111)