New Accounting Guidance - Narrative (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 14,196	$ 13,504	$ 13,140
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		14	$ 14
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	364	364
Accounting Standards Update 2016-16
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		(14)
Accounting Standards Update 2016-16 | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption		$ (14)
Minimum | Real Estate
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, liability	1,700
Right-of-use asset obtained in exchange for operating lease liability	1,500
Maximum | Real Estate
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, liability	2,000
Right-of-use asset obtained in exchange for operating lease liability	1,700
Other Liabilities | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, liability	200
Other Liabilities | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, liability	$ 300