Employee Benefit Plans - Accumulated Other Comprehensive Income and Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 01, 2017	Nov. 30, 2017
Postretirement Medical Plan
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax [Abstract]
Actuarial (loss) gain	$ (12)	$ (8)
AOCI	(12)	(8)
Net periodic benefit (income) expense:
Service cost - benefits earned during the year	1	0	$ 1
Interest cost	1	2	2
Amortization of actuarial gain	(1)	0	0
Net periodic benefit (income) expense	$ 1	$ 2	$ 3
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]
Discount rate - net periodic benefit costs		3.90%	4.20%
Discount rate - benefit obligations	4.21%	3.52%
Minimum | Postretirement Medical Plan
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]
Discount rate - net periodic benefit costs	3.11%
Maximum | Postretirement Medical Plan
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]
Discount rate - net periodic benefit costs	3.67%
United Kingdom
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax [Abstract]
Actuarial (loss) gain	$ 5	$ 44
Prior-service credit	19	39
AOCI	24	83
Net periodic benefit (income) expense:
Interest cost	28	34	$ 41
Expected return on plan assets	(67)	(60)	(59)
Amortization of prior-service credit	(2)	(1)	(1)
Recognized AOCI loss due to settlements	0	0	0
Net periodic benefit (income) expense	(41)	(27)	(19)
Amounts recognized in Other comprehensive (loss) income
Actuarial loss (gain)	40	(72)	29
Prior-service cost	19	0	(42)
Reclassification of recognized AOCI gain due to settlements	0	0	0
Reclassification of prior-service credit to net periodic benefit (income) expense	2	1	1
Loss (gain) recognized in Other comprehensive (loss) income	$ 61	$ (71)	$ (12)
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]
Discount rate - net periodic benefit costs	2.21%	2.70%	3.75%
Discount rate - benefit obligations	2.85%	2.53%	2.70%	3.83%	4.35%
Expected long-term rate of return on plan assets	4.95%	5.00%	5.40%
Qualified Plan | United States
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax [Abstract]
Actuarial (loss) gain	$ (50)	$ 13
Prior-service credit	0	0
AOCI	(50)	13
Net periodic benefit (income) expense:
Interest cost	57	74	$ 76
Expected return on plan assets	(92)	(93)	(88)
Amortization of prior-service credit	0	0	0
Recognized AOCI loss due to settlements	0	(1)	0
Net periodic benefit (income) expense	(35)	(20)	(12)
Amounts recognized in Other comprehensive (loss) income
Actuarial loss (gain)	63	(47)	11
Prior-service cost	0	0	0
Reclassification of recognized AOCI gain due to settlements	0	1	0
Reclassification of prior-service credit to net periodic benefit (income) expense	0	0	0
Loss (gain) recognized in Other comprehensive (loss) income	$ 63	$ (46)	$ 11
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]
Discount rate - net periodic benefit costs			4.65%
Discount rate - benefit obligations			4.35%
Expected long-term rate of return on plan assets			5.58%
Qualified Plan | United States | Minimum
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]
Discount rate - net periodic benefit costs	3.14%	3.83%
Discount rate - benefit obligations	4.18%	3.55%
Expected long-term rate of return on plan assets	3.00%	2.35%
Qualified Plan | United States | Maximum
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]
Discount rate - net periodic benefit costs	3.38%	4.35%
Discount rate - benefit obligations	4.39%	3.71%
Expected long-term rate of return on plan assets	5.40%	5.65%
Nonqualified Plan | United States
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax [Abstract]
Actuarial (loss) gain	$ (3)	$ (8)
Prior-service credit	0	0
AOCI	(3)	(8)
Net periodic benefit (income) expense:
Interest cost	2	3	$ 3
Expected return on plan assets	0	0	0
Amortization of prior-service credit	0	0	0
Recognized AOCI loss due to settlements	0	0	0
Net periodic benefit (income) expense	2	3	3
Amounts recognized in Other comprehensive (loss) income
Actuarial loss (gain)	(5)	6	3
Prior-service cost	0	0	0
Reclassification of recognized AOCI gain due to settlements	0	0	0
Reclassification of prior-service credit to net periodic benefit (income) expense	0	0	0
Loss (gain) recognized in Other comprehensive (loss) income	$ (5)	$ 6	$ 3
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]
Discount rate - net periodic benefit costs		4.35%	4.65%
Discount rate - benefit obligations			4.35%
Nonqualified Plan | United States | Minimum
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]
Discount rate - net periodic benefit costs		2.84%
Discount rate - benefit obligations	3.93%	3.21%
Nonqualified Plan | United States | Maximum
Defined Benefit Plan, Assumptions Used in Calculations [Abstract]
Discount rate - net periodic benefit costs		3.21%
Discount rate - benefit obligations	4.28%	3.60%