Accounting Policies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Net cash from financing activities	$ (133)	$ 751
Net cash for operating activities	$ (149)	(70)
New Accounting Pronouncement, Early Adoption, Effect | Accounting Standards Update 2016-09, Statutory Tax Withholding Component
Debt Instrument [Line Items]
Net cash from financing activities		(30)
Net cash for operating activities		$ (30)	$ (62)	$ (111)