Employee Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 18	$ 24	$ 53	$ 69
Interest cost	25	25	75	77
Expected return on plan assets	(64)	(59)	(191)	(177)
Amortization of prior service credit	(7)	(5)	(20)	(14)
Recognized net actuarial loss	19	19	59	57
Total net periodic benefit cost	(9)	4	(24)	12
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	1	2	4	6
Interest cost	2	2	6	7
Expected return on plan assets	(5)	(5)	(14)	(14)
Amortization of prior service credit	(5)	(3)	(16)	(7)
Recognized net actuarial loss	2	2	6	7
Total net periodic benefit cost	(5)	(2)	(14)	(1)
Superannuation, SERP and ESBP
Components of net periodic benefit cost:
Service cost	0	1	0	2
Interest cost	1	0	2	1
Expected return on plan assets	0	0	0	0
Amortization of prior service credit	0	0	0	0
Recognized net actuarial loss	0	1	2	2
Total net periodic benefit cost	$ 1	$ 2	$ 4	$ 5