Defined Benefit Postretirement Plans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Amortization:
Net settlement loss	$ 8	$ 5	$ 11	$ 16
Estimated future contributions over the remainder of 2018	6		6
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	5	13	16
Interest cost	7	7	20	22
Expected return on plan assets	(9)	(10)	(26)	(32)
Amortization:
prior service cost (credit)	(2)	(3)	(7)	(7)
actuarial loss	3	3	9	7
Net settlement loss	10	8	16	25
Net periodic benefit cost	13	10	25	31
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	1	1
Interest cost	2	2	6	6
Expected return on plan assets	0	0	0	0
Amortization:
prior service cost (credit)	(2)	(2)	(6)	(5)
actuarial loss	0	0	1	0
Net settlement loss	0	0	0	0
Net periodic benefit cost	$ 0	$ 0	2	$ 2
Payment for other postretirement benefit plans			17
Funded Plan
Amortization:
Payment for pension benefits			58
Unfunded Plan
Amortization:
Payment for pension benefits			$ 12