Employee Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 20	$ 16	$ 39	$ 35
Interest cost	25	25	50	50
Expected return on plan assets	(67)	(63)	(134)	(127)
Amortization of prior service credit	(7)	(6)	(13)	(13)
Recognized net actuarial loss	22	21	44	40
Total net periodic benefit (income) cost	(7)	(7)	(14)	(15)
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	2	1	3	3
Interest cost	2	2	4	4
Expected return on plan assets	(5)	(4)	(10)	(9)
Amortization of prior service credit	(4)	(6)	(8)	(11)
Recognized net actuarial loss	1	1	2	4
Total net periodic benefit (income) cost	(4)	(6)	(9)	(9)
Superannuation Supplemental Executives Retirement Plan Defined Benefit and Executive Supplemental Defined Benefit Plan [Member]
Components of net periodic benefit cost:
Service cost			0	0
Interest cost			1	1
Expected return on plan assets			0	0
Amortization of prior service credit			0	0
Recognized net actuarial loss			2	2
Superannuation, SERP and ESBP
Components of net periodic benefit cost:
Service cost	0	0
Interest cost	0	0
Expected return on plan assets	0	0
Amortization of prior service credit	0	0
Recognized net actuarial loss	1	1
Total net periodic benefit (income) cost	$ 1	$ 1	$ 3	$ 3