Retirement Benefits and Trusteed Assets - VEBA Contributions (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended
	Mar. 05, 2019	Mar. 31, 2019	Mar. 31, 2018
Defined Contribution Plan Disclosure [Line Items]
Amount		$ 100	$ 175
Pension Benefits
Defined Contribution Plan Disclosure [Line Items]
Number of Shares (in shares)	814,597
Price per Share (in dollars per share)	$ 122.76
Amount	$ 100