Accounting Pronouncements - Defined benefits and cash flow (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 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		$ (6)	$ 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
Decrease in cash flows from operating activities		$ (3)