Employee Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 16	$ 22	$ 35	$ 45
Interest cost	25	26	50	52
Expected return on plan assets	(63)	(58)	(127)	(118)
Amortization of prior service credit	(6)	(5)	(13)	(9)
Recognized net actuarial loss	21	19	40	38
Total net periodic benefit cost	(7)	4	(15)	8
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	1	2	3	4
Interest cost	2	2	4	5
Expected return on plan assets	(4)	(4)	(9)	(9)
Amortization of prior service credit	(6)	(2)	(11)	(4)
Recognized net actuarial loss	1	3	4	5
Total net periodic benefit cost	(6)	1	(9)	1
Superannuation, SERP and ESBP
Components of net periodic benefit cost:
Service cost	0	0	0	1
Interest cost	0	1	1	1
Expected return on plan assets	0	0	0	0
Amortization of prior service credit	0	0	0	0
Recognized net actuarial loss	1	0	2	1
Total net periodic benefit cost	$ 1	$ 1	$ 3	$ 3