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Investment Securities - Schedule of Amortized Cost, Unrealized Gains/(Losses) and Fair Value (Details) - USD ($) $ in Millions | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | $ 8,471 |
Securities available for sale at fair value | 8,494 | $ 8,047
Securities pledged as collateral | 7,500 | 6,400
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 304 | 10
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (1) | 0
Fair Value | 303 | 10
U.S. treasuries |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 603 | 613
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Securities available for sale at fair value | 603 | 613
Government agency issued MBS |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 4,324 | 3,722
Gross Unrealized Gains | 58 | 92
Gross Unrealized Losses | (37) | (2)
Securities available for sale at fair value | 4,345 | 3,812
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 229 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | (1) |
Fair Value | 228 |
Government agency issued CMO |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 2,234 | 2,380
Gross Unrealized Gains | 14 | 29
Gross Unrealized Losses | (31) | (3)
Securities available for sale at fair value | 2,217 | 2,406
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost | 75 |
Gross Unrealized Gains | 0 |
Gross Unrealized Losses | 0 |
Fair Value | 75 |
Other U.S. government agencies |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 794 | 672
Gross Unrealized Gains | 6 | 12
Gross Unrealized Losses | (12) | 0
Securities available for sale at fair value | 788 | 684
Corporate and other debt |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost |  | 40
Gross Unrealized Gains |  | 1
Gross Unrealized Losses |  | (1)
Securities available for sale at fair value |  | 40
Schedule of Held-to-maturity Securities [Line Items] |  |
Amortized Cost |  | 10
Gross Unrealized Gains |  | 0
Gross Unrealized Losses |  | 0
Fair Value |  | 10
States and municipalities |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 516 | 445
Gross Unrealized Gains | 10 | 15
Gross Unrealized Losses | (1) | 0
Securities available for sale at fair value | 525 | 460
Securities available-for-sale, excluding interest only strip: |  |
Debt Securities, Available-for-sale [Line Items] |  |
Amortized Cost | 8,471 | 7,872
Gross Unrealized Gains | 88 | 149
Gross Unrealized Losses | (81) | (6)
Securities available for sale at fair value | 8,478 | 8,015
Interest- only strips- AFS |  |
Debt Securities, Available-for-sale [Line Items] |  |
Securities available for sale at fair value | $ 16 | $ 32

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563 + Details
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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563 + Details
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