Employee Pension and Other Postretirement Benefits - Weighted Average Assumptions, PV of Benefit Obligations and Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Plan and Other Postretirement Benefits Plan
Discount rate in determining net periodic benefit cost after Plan amendments, interest cost	2.43%	3.80%
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Discount rate in determining net periodic benefit cost after Plan amendments, service cost	4.13%	3.90%
Discount rate in determining net periodic benefit cost after Plan amendments, interest cost	3.72%	3.90%
Discount rate in determining benefit obligations at year end	3.98%	4.29%
Rate of increase in future compensation levels for determining net periodic benefit cost	4.70%	4.70%
Rate of increase in future compensation levels for determining benefit obligations at year end	4.70%	4.70%
Expected return on plan assets	7.50%	7.50%
Service cost	$ 91	$ 96	$ 81
Interest cost	103	110	105
Expected return on plan assets	(235)	(216)	(193)
Amortization of prior service credit	(18)	(17)	(12)
Recognized net actuarial loss	75	106	59
Curtailment gain	0	0	0
Total net periodic benefit cost	$ 16	$ 79	40
Defined Benefit Plan, Service Cost Associated with Transferred Employees			6
Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Discount rate in determining net periodic benefit cost prior to Plan amendments, service cost	4.23%
Discount rate in determining net periodic benefit cost prior to Plan amendments, interest cost	3.43%
Discount rate in determining net periodic benefit cost after Plan amendments, service cost	3.22%	3.80%
Discount rate in determining benefit obligations at year end	3.81%	4.11%
Expected return on plan assets	7.50%	7.50%
Service cost	$ 8	$ 10	10
Interest cost	9	12	11
Expected return on plan assets	(19)	(19)	(18)
Amortization of prior service credit	(12)	(8)	(5)
Recognized net actuarial loss	10	12	1
Curtailment gain	0	0	0
Total net periodic benefit cost	(4)	7	(1)
Superannuation, SERP and ESBP
Pension Plan and Other Postretirement Benefits Plan
Service cost	2	1	1
Interest cost	2	2	4
Expected return on plan assets	0	0	0
Amortization of prior service credit	0	0	0
Recognized net actuarial loss	3	3	2
Curtailment gain	(2)	0	0
Total net periodic benefit cost	$ 5	$ 6	$ 7