Retirement Plans (Amounts Recognized as a Component of AOCI) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits [Member]
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]
Accumulated other comprehensive (gain) loss at beginning of year	$ 647	$ 584
Net actuarial gain/(loss) recognized during year	(30)	(40)
Net actuarial loss/(gain) occuring during year	22	103
Settlement loss recognized	(83)
Net amount recognized in comprehensive income (loss) for the year	(91)	63
Accumulated other comprehensive (gain)/loss at end of year	556	647
Postretirement Benefits Other Than Pensions (OPEB) [Member]
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]
Accumulated other comprehensive (gain) loss at beginning of year	(29)	(20)
Net actuarial gain/(loss) recognized during year		(1)
Prior service (cost)/credit recognized during year	9	9
Net actuarial loss/(gain) occuring during year	53	(17)
Net amount recognized in comprehensive income (loss) for the year	62	(9)
Accumulated other comprehensive (gain)/loss at end of year	$ 33	$ (29)