Severance (Details) - Employee severance - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Restructuring reserve
Balance at the beginning of the period	$ 52	$ 6
Accrued to expense	14	89
Payments, net	(58)	(43)
Balance at the end of the period	$ 8	$ 52