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	Dec. 31, 2013
Pension Benefits
Pension Plan and Other Postretirement Benefits Plan
Defined Benefit Plan, Service Cost Associated with Transferred Employees			$ 6
Discount rate in determining net periodic benefit cost prior to Plan amendments			4.90%
Discount rate in determining net periodic benefit cost after Plan amendments		3.90%	4.50%
Discount rate in determining benefit obligations at year end		4.29%	3.90%
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%	7.50%
Service cost		$ 96	$ 81	$ 87
Interest cost		110	105	99
Expected return on plan assets		(216)	(193)	(166)
Amortization of prior service credit		(17)	(12)
Recognized net actuarial loss		106	59	107
Total net periodic benefit cost		$ 79	$ 40	127
Other Benefits
Pension Plan and Other Postretirement Benefits Plan
Discount rate in determining net periodic benefit cost prior to Plan amendments			4.60%
Discount rate in determining net periodic benefit cost after Plan amendments		3.80%	4.20%
Discount rate in determining benefit obligations at year end		4.11%	3.80%
Expected return on plan assets		7.50%	7.50%
Service cost		$ 10	$ 10	14
Interest cost		12	11	11
Expected return on plan assets		(19)	(18)	(15)
Amortization of prior service credit		(8)	(5)	0
Recognized net actuarial loss		12	1	7
Total net periodic benefit cost		7	(1)	17
Superannuation, SERP and ESBP
Pension Plan and Other Postretirement Benefits Plan
Service cost		1	1	1
Interest cost		2	4	3
Expected return on plan assets		0	0
Amortization of prior service credit		0	0
Recognized net actuarial loss		3	2	2
Total net periodic benefit cost		$ 6	$ 7	$ 6