Employee Benefit Plans - Benefit Obligations and Funded Status for Medical Plan (Details) - Postretirement Medical Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	$ 40	$ 51
Service cost - benefits earned during the year	1	0	$ 1
Interest cost on projected benefit obligation	1	2	2
Actuarial gain	5	9
Participant contributions	2	2
Benefits paid	(4)	(6)
Projected benefit obligation at end of year	34	40	$ 51
Funded status of the plan	(34)	(40)
Amounts recognized in the balance sheet consist of :
Current liabilities	(3)	(3)
Long-term liabilities	(31)	(37)
Net amount recognized	$ (34)	$ (40)
Discount rate assumption as of December 31	4.21%	3.52%