Business, Basis of Presentation and Summary of Significant Accounting Policies - New Accounting Pronouncements (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for Credit Losses as a Percentage of Amortized Costs		0.50%
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ 13,052			$ 1,722	$ 7,427	$ 5,366
Retained Earnings (Accumulated Deficit)	33,078			$ 28,926
Operating Lease, Right-of-Use Asset	1,488
Operating Lease, Right-of-Use Liability	$ 1,654
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
Maximum | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for Credit Losses as a Percentage of Amortized Costs		1.00%