Accounting Pronouncements - Defined benefits and cash flow (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2015
ASU 2017-07
Pension and Postretirement Benefit Costs
Other non-service components of pension and postretirement benefit costs	$ (4)		$ 16	$ 21
Amount of adjustments to non-service costs of defined benefit costs		$ 0
ASU 2016-18
New Accounting Pronouncements, Retrospective adjustments
Escrow cash account					$ 2,000
Impact on cash flows from operating activities due to use of escrow funds			$ (3)