Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019		Jun. 30, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity	$ 48,039	$ 44,699	$ 47,371	$ 45,344		$ 43,818	$ 41,035
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity	$ 24,440	$ 19,185	$ 23,052	20,162		$ 17,834	15,088
Adjustment upon Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity				(30)	[1]		(15)
Adjustment upon Adoption | Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' equity				$ (30)	[1]		$ (15)

[1] See Note 3 for further information about the Company's adoption of new credit loss guidance (ASU 2016-13).