Employee Pension and Other Postretirement Benefits (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 19	$ 23
Interest cost	25	26
Expected return on plan assets	(64)	(60)
Amortization of prior service cost	(7)	(4)
Recognized net actuarial loss	19	19
Total net periodic benefit cost	(8)	4
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	2	2
Interest cost	2	3
Expected return on plan assets	(5)	(5)
Amortization of prior service cost	(5)	(2)
Recognized net actuarial loss	3	2
Total net periodic benefit cost	(3)	0
Superannuation, SERP and ESBP
Components of net periodic benefit cost:
Service cost	0	1
Interest cost	1	0
Expected return on plan assets	0	0
Amortization of prior service cost	0	0
Recognized net actuarial loss	1	1
Total net periodic benefit cost	$ 2	$ 2