Investment Securities - Credit Quality Indicators (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	$ 82,371	$ 16,592
Municipal bonds and notes
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	6,485	3,635
Municipal bonds and notes | Aaa
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	3,422	2,070
Municipal bonds and notes | Aa1
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	1,883	1,145
Municipal bonds and notes | Aa2
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	981	420
Municipal bonds and notes | Aa3
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	172	0
Municipal bonds and notes | A1
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	27	0
Corporate bonds
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	533	0
Corporate bonds | Aa1
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	29	0
Corporate bonds | Aa2
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	27	0
Corporate bonds | Aa3
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	43	0
Corporate bonds | A1
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	210	0
Corporate bonds | A2
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	$ 224	$ 0