Benefit Plans (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Dec. 31, 2017	Mar. 31, 2018
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Additional employer contributions		$ 7
Pension [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Anticipated additional employer contributions		35
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Contribution by employer	$ 270
Other Postretirement Benefits Plan [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Anticipated additional employer contributions		$ 70