Pension and Other Postretirement Benefits (Funded Status and Amounts Recognized on the Consolidated Balance Sheets) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets [Abstract]
Projected benefit obligation	$ 4,558	$ 4,795
Accumulated benefit obligation	4,485	4,717
Fair value of plan assets	3,129	3,602
Accumulated benefit obligation for all defined benefit pension plans	4,590	4,830
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Noncurrent liabilities	(1,769)	(1,575)
Pension benefits [Member]
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets [Abstract]
Funded status at end of year	(1,416)	(1,177)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Other assets	13	17
Current liabilities	(53)	(51)
Noncurrent liabilities	(1,376)	(1,143)
Net amounts recognized	(1,416)	(1,177)
Amounts recognized in accumulated other comprehensive income (loss) on the Consolidated Balance Sheets
Net actuarial (loss) gain	(1,827)	(1,848)
Net prior service (cost) credit	(7)	(9)
Share of equity investee	(1)	(1)
Net amount recognized in accumulated other comprehensive income (loss) before tax	(1,835)	(1,858)
Deferred income taxes	725	734
Net amount recognized in accumulated other comprehensive income (loss)	(1,110)	(1,124)
Postretirement benefits [Member]
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets [Abstract]
Funded status at end of year	(443)	(482)
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]
Other assets	0	0
Current liabilities	(50)	(50)
Noncurrent liabilities	(393)	(432)
Net amounts recognized	(443)	(482)
Amounts recognized in accumulated other comprehensive income (loss) on the Consolidated Balance Sheets
Net actuarial (loss) gain	230	246
Net prior service (cost) credit	0	0
Share of equity investee	0	0
Net amount recognized in accumulated other comprehensive income (loss) before tax	230	246
Deferred income taxes	(38)	(44)
Net amount recognized in accumulated other comprehensive income (loss)	192	202
Qualified Pension Plans [Member]
Defined Benefit Plan, Pension Plans with Accumulated Benefit Obligations in Excess of Plan Assets [Abstract]
Funded status at end of year	$ (742)	$ (533)