Business, Basis of Presentation and Summary of Significant Accounting Policies (New Accounting Pronouncements) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ 6,911		$ 1,722	$ 5,634	$ 7,427
Retained Earnings (Accumulated Deficit)	29,944		$ 28,926
Operating Lease, Right-of-Use Asset	1,521
Operating Lease, Right-of-Use-Liability	$ 1,673
Accounting Standards Update 2017-12
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax		$ 18
Retained Earnings (Accumulated Deficit)		(18)
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained Earnings (Accumulated Deficit)		95
Operating Lease, Right-of-Use Asset		1,500
Operating Lease, Right-of-Use-Liability		$ 1,700