Summary of Significant Accounting Policies - Narrative (Details) $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($) segment	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accounting Policies [Abstract]
Number of operating segments | segment	2
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Adoption of accounting standard (Note 1)	$ 12,547	$ 11,053	$ 11,316		$ 12,726	$ 12,688	$ 12,385	$ 11,984	$ 11,659
Cumulative Effect, Period of Adoption, Adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of accounting standard (Note 1)					(643)
Allowance For Credit Losses [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of accounting standard (Note 1)				$ 801
Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of accounting standard (Note 1)	$ 5,489	$ 4,596	$ 4,868		5,744	$ 5,813	$ 5,342	$ 4,939	$ 4,667
Retained Earnings [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of accounting standard (Note 1)				$ (643)	$ (643)