Investment Securities - Credit Quality Indicators (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	$ 59,997	$ 16,592
Municipal bonds and notes
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	5,265	3,635
Municipal bonds and notes | Aaa
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	2,680	2,070
Municipal bonds and notes | Aa1
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	1,619	1,145
Municipal bonds and notes | Aa2
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	802	420
Municipal bonds and notes | Aa3
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	152	0
Municipal bonds and notes | A1
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	12	0
Corporate bonds
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	373	0
Corporate bonds | Aa1
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	10	0
Corporate bonds | Aa2
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	28	0
Corporate bonds | Aa3
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	24	0
Corporate bonds | A1
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	158	0
Corporate bonds | A2
Financing Receivable, Credit Quality Indicator [Line Items]
Held-to-maturity securities, amortized cost	$ 153	$ 0