Retirement Plans and Other Postretirement Benefits - Narrative (Details)	3 Months Ended
	Mar. 31, 2018 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amount of other postretirement benefit trust assets for union employee medical costs	$ 186,000,000
Pension Benefits
Contributions
Voluntary employer contributions to pension plan	50,000,000
Minimum employer contributions for the next three years	0
Maximum employer contributions for the next two years (up to)	250,000,000
Other Benefits
Contributions
Estimated future employer contributions in next three years	0
Retiree medical clain reimbursements	$ 22,000,000