Pension and Other Benefit Programs - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions	$ 136	$ 10	$ 10
Period of amortization	4 years
Corridor approach percentage	10.00%
Net periodic post-retirement benefit costs	$ (15)	$ (16)	(10)
Discount rate	3.40%	3.90%
Weighted-average discount rate for determining interest costs (pension/SERP plans)	3.20%	3.30%
Health care cost trend rate assumed	7.70%
Ultimate health care cost trend rate	4.50%
Percentage of assumed health care cost trend	1.00%
Contribution plan cost	$ 38	$ 31	21
Supplemental Employee Retirement Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cash surrender value of life insurance	55	53
Net periodic post-retirement benefit costs	1	1	2
Post-retirement Benefit Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic post-retirement benefit costs	5	$ 7	$ 8
Anticipated medicare subsidy receipts	$ 10
Maximum | Equity Securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocation, percentage	65.00%
Maximum | Fixed Income Securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocation, percentage	95.00%
Minimum | Supplemental Employee Retirement Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Weighted-average discount rate for determining interest costs (pension/SERP plans)	260.00%	2.50%	3.20%
Minimum | Equity Securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocation, percentage	5.00%
Minimum | Fixed Income Securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target allocation, percentage	35.00%