Investment Securities (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Accrued interest available-for-sale debt securities	$ 38	$ 26
Available-for-sale debt securities:
Cost	9,442
Estimated Fair Value	9,487
Equity securities
Cost	58	56
Gross Unrealized Gains	46	27	$ 5
Gross Unrealized Losses	(2)	(2)
Estimated Fair Value	102	81
Total Cost	9,500	21,521
Total Gross Unrealized Gains	92	112
Total Gross Unrealized Losses	(3)	(2)
Total Estimated Fair Value	9,589	21,631
Investment Securities
Equity securities
Cost	7
Estimated Fair Value	51
State and municipal obligations
Available-for-sale debt securities:
Cost	102	172
Gross Unrealized Gains	5	7
Gross Unrealized Losses	(1)	0
Estimated Fair Value	106	179
U.S. Government agency obligations
Available-for-sale debt securities:
Cost	6	7
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	6	7
U.S. Government treasury obligations
Available-for-sale debt securities:
Cost	8,718	20,655
Gross Unrealized Gains	39	76
Gross Unrealized Losses	0	0
Estimated Fair Value	8,757	20,731
Mortgage-backed securities
Available-for-sale debt securities:
Cost	19	28
Gross Unrealized Gains	2	2
Gross Unrealized Losses	0	0
Estimated Fair Value	21	30
Foreign government bonds and obligations
Available-for-sale debt securities:
Cost	556	581
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	556	581
Other
Available-for-sale debt securities:
Cost	41	22
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 41	$ 22

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