Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Line Items]
Income tax expense			$ 550	$ 186	$ 209
Effective income tax rate		35.00%
Tax Cuts and Jobs Act of 2017, incomplete accounting, provisional income tax expense			49
Tax Cuts and Jobs Act of 2017, incomplete accounting, transition tax for accumulated foreign earnings, provisional income tax expense			190
Foreign earnings repatriated	$ 89
Cash and cash equivalents	1,200		1,200
Operating loss carryforwards	4,100		4,100
Operating loss carryforwards, not subject to expiration	2,800		2,800
Foreign tax credit carryforwards	0		0	310
U.S. general business credit carryforwards, including Energy Tax Credits	927		927	964
Valuation allowance, deferred tax assets	178		178	150
Valuation allowance, increase in deferred tax assets				83
Positions assumed in acquisitions			0	0	0
Unrecognized tax benefits, income tax penalties and interest expense (benefit)			(8)	(19)	5
Unrecognized tax benefits, income tax penalties and interest accrued	45		45	42	$ 63
Unrecognized tax benefits	18		18
United States Pension Benefits
Cash and Cash Equivalents [Line Items]
Operating loss carryforwards	842		842
Net Operating Loss Carryforward
Cash and Cash Equivalents [Line Items]
Valuation allowance, deferred tax assets	156		156	128
Other Deferred Tax Assets
Cash and Cash Equivalents [Line Items]
Valuation allowance, deferred tax assets	$ 22		22	$ 22
Additional Valuation Allowances
Cash and Cash Equivalents [Line Items]
Valuation allowance, increase in deferred tax assets			$ 15