Retirement Benefits and Trusteed Assets - VEBA Contributions (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended
	Mar. 07, 2018	Mar. 31, 2018
Defined Contribution Plan Disclosure [Line Items]
Contribution of common stock to pension plan		$ 175
Pension Benefits
Defined Contribution Plan Disclosure [Line Items]
Shares contributed to VEBA trust (in shares)	1,751,401
Shares contributed to VEBA trust (in dollars per share)	$ 99.92
Contribution of common stock to pension plan	$ 175