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Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2019 USD ($) | Dec. 31, 2021 USD ($) Investments | Dec. 31, 2020 USD ($) Investments
Investments, Debt and Equity Securities [Abstract] |  |  |
Debt Securities, Held-to-maturity |  | $ 1,687,600,000 | $ 51,600,000
Available-for-Sale and Held-to-Maturity: |  |  |
Investment securities in an unrealized loss position (number of securities) | Investments |  | 285 | 181
Less than 12 Months |  | $ 1,067,700,000 |
Less than 12 Months |  | (35,700,000) |
12 Months or More |  | 0 |
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | 0 |
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value |  | 1,067,700,000 |
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss |  | (35,700,000) |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months |  | 2,760,300,000 | $ 838,600,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 68,700,000 | 16,600,000
Debt Securities, Available-for-sale, Unrealized Loss Position |  | 2,829,000,000 | 855,200,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss |  | (59,400,000) | (5,700,000)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | (58,300,000) | (5,500,000)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | (1,100,000) | (200,000)
Other than Temporary Impairment Losses, Investments | $ 0 |  |
Related Allowance |  | 0 |
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |  |  |
Investments, Debt and Equity Securities [Abstract] |  |  |
Debt Securities, Held-to-maturity |  | 1,609,000,000 | 1,000,000.0
Available-for-Sale and Held-to-Maturity: |  |  |
Less than 12 Months |  | 1,038,700,000 |
Less than 12 Months |  | (35,300,000) |
12 Months or More |  | 0 |
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | 0 |
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value |  | 1,038,700,000 |
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss |  | (35,300,000) |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months |  | 1,262,800,000 | 434,000,000.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 26,400,000 | 12,300,000
Debt Securities, Available-for-sale, Unrealized Loss Position |  | 1,289,200,000 | 446,300,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss |  | (23,300,000) | (2,500,000)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | (23,000,000.0) | (2,400,000)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | (300,000) | (100,000)
US Government Agencies Debt Securities [Member] |  |  |
Available-for-Sale and Held-to-Maturity: |  |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months |  | 297,000,000.0 | 235,600,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 16,400,000 | 0
Debt Securities, Available-for-sale, Unrealized Loss Position |  | 313,400,000 | 235,600,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss |  | (9,200,000) | (2,000,000.0)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | (8,900,000) | (2,000,000.0)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | (300,000) | 0
Corporate Debt Securities [Member] |  |  |
Investments, Debt and Equity Securities [Abstract] |  |  |
Debt Securities, Held-to-maturity |  | 11,000,000.0 | 3,900,000
Available-for-Sale and Held-to-Maturity: |  |  |
Less than 12 Months |  | 29,000,000.0 |
Less than 12 Months |  | (400,000) |
12 Months or More |  | 0 |
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | 0 |
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value |  | 29,000,000.0 |
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss |  | (400,000) |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months |  | 109,900,000 | 20,900,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 20,900,000 | 0
Debt Securities, Available-for-sale, Unrealized Loss Position |  | 130,800,000 | 20,900,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss |  | (3,600,000) | (100,000)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | (3,300,000) | (100,000)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | (300,000) | 0
Mortgage-backed Securities, Issued by Private Enterprises [Member] |  |  |
Available-for-Sale and Held-to-Maturity: |  |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months |  | 127,200,000 | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 0 | 4,300,000
Debt Securities, Available-for-sale, Unrealized Loss Position |  | 127,200,000 | 4,300,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss |  | (1,100,000) | (100,000)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | (1,100,000) | 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | 0 | (100,000)
US Treasury notes |  |  |
Available-for-Sale and Held-to-Maturity: |  |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months |  | 684,700,000 |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 0 |
Debt Securities, Available-for-sale, Unrealized Loss Position |  | 684,700,000 |
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss |  | (12,900,000) |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | (12,900,000) |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | 0 |
State, county and municipal securities |  |  |
Investments, Debt and Equity Securities [Abstract] |  |  |
Debt Securities, Held-to-maturity |  | 67,600,000 | 46,600,000
Available-for-Sale and Held-to-Maturity: |  |  |
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months |  | 278,700,000 | 148,100,000
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer |  | 5,000,000.0 | 0
Debt Securities, Available-for-sale, Unrealized Loss Position |  | 283,700,000 | 148,100,000
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss |  | (9,300,000) | (1,000,000.0)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss |  | (9,100,000) | (1,000,000.0)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss |  | $ (200,000) | $ 0

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