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Investments (Narrative) (Details) |  |  | 12 Months Ended
--- | --- | --- | ---
Nov. 01, 2020 USD ($) | Nov. 01, 2019 USD ($) | Dec. 31, 2020 USD ($) investment | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Schedule of Investments [Line Items] |  |  |  |  |
Investment securities available for sale transferred to held to maturity fair value, net of tax | $ 5,900,000 |  | $ 5,894,000 | $ 72,512,000 | $ (109,507,000)
Reclassification of unrealized losses, net of tax | 4,500,000 |  | 4,157,000 | 71,149,000 | $ (71,149,000)
Interest receivable, available-for-sale debt securities |  |  | 17,600,000 |  |
Interest receivable, held-to-maturity debt securities |  |  | 5,400,000 |  |
Debt securities, accrued interest, writeOff |  |  | $ 0 |  |
Number of investments in continuous unrealized loss position for more than twelve months | investment |  |  | 39 |  |
Unrealized losses related to marketability of securities or issuers ability to honor redemption obligations |  |  | $ 0 |  |
Investment value deemed to be OTTI |  |  | 0 |  |
Investment securities, aggregate carrying value, pledged as collateral |  |  | 4,640,000,000 | $ 3,930,000,000 |
Debt securities, held-to-maturity, nonaccrual |  |  | 0 |  |
Debt securities, held-to-maturity, past due |  |  | $ 0 |  |
Mortgage Backed Securities |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |
Investment securities available for sale transferred to held to maturity amortized cost | 1,460,000,000 |  |  |  |
Investment securities available for sale transferred to held to maturity fair value | $ 1,470,000,000 |  |  |  |
Weighted average contractual maturity (years) | 18 years |  |  |  |
Accounting Standards Update 2019-04 | Mortgage Backed Securities |  |  |  |  |
Schedule of Investments [Line Items] |  |  |  |  |
Investment securities available for sale transferred to held to maturity fair value |  | $ 2,150,000,000 |  |  |
Mortgage backed securities, held-to-maturity, transferred to available-for-sale |  | 2,080,000,000.00 |  |  |
Investment securities available for sale transferred to held to maturity fair value, net of tax |  | 72,500,000 |  |  |
Reclassification of unrealized losses, net of tax |  | $ 55,800,000 |  |  |

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