Employee Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3	$ 2	$ 5	$ 4
Interest cost	6	8	12	14
Expected return on plan assets	(12)	(11)	(22)	(21)
Net amortization	(3)	(4)	(6)	(7)
Net periodic benefit cost	(6)	(5)	(11)	(10)
Domestic Plan [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	6	10	12
Interest cost	26	29	52	58
Expected return on plan assets	(41)	(40)	(82)	(80)
Net amortization	7	8	15	15
Net periodic benefit cost	(3)	3	(5)	5
UNITED KINGDOM | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	7	10	13
Interest cost	14	15	28	29
Expected return on plan assets	(26)	(25)	(53)	(49)
Defined Benefit Plan, Plan Assets, Payment for Settlement	24	0	24	0
Net amortization	14	16	29	33
Net periodic benefit cost	31	$ 13	38	$ 26
Accounting Standards Update 2017-07 [Member] | Other Pension, Postretirement and Supplemental Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Prior Period Reclassification Adjustment	$ 4		$ 8