Retirement Plans (Amounts Recognized as a Component of AOCI) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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		$ 584	$ 532
Net actuarial gain (loss) recognized during year	[1],[2]	(40)	(29)	$ (20)
Net actuarial loss (gain) occurring during year		103	81
Net amount recognized in comprehensive income (loss) for the year		63	52
Accumulated other comprehensive (gain) loss at end of year		647	584	532
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		(20)	119
Net actuarial gain (loss) recognized during year		(1)	(8)
Prior service (cost) credit recognized during year		9	5
Net actuarial loss (gain) occurring during year		(17)	(136)
Net amount recognized in comprehensive income (loss) for the year		(9)	(139)
Accumulated other comprehensive (gain) loss at end of year		$ (29)	$ (20)	$ 119

[1] Amounts in parentheses indicate losses.

[2] These accumulated other comprehensive loss components are included in the computation of net periodic pension and OPEB costs (see Note 17 - Retirement Plans for additional details).