Business, Basis of Presentation and Summary of Significant Accounting Policies - New Accounting Pronouncements (Details) - USD ($) $ in Millions	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Jan. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained Earnings (Accumulated Deficit)	$ 36,491		$ 33,078
Operating Lease, Right-of-Use Asset	$ 1,314		$ 1,488
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
Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained Earnings (Accumulated Deficit)		$ (121)