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Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) | 12 Months Ended
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Dec. 31, 2020 USD ($) Investments | Dec. 31, 2019 USD ($) Investments
Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | $ 51,600,000 | $ 92,300,000
Available-for-Sale and Held-to-Maturity: |  |
Investment securities in an unrealized loss position (number of securities) | Investments | 181 | 338
Impairment losses | $ 0 | $ 0
Less than 12 Months | 838,600,000 | 925,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 5,500,000 | 5,400,000
12 Months or More | 16,600,000 | 202,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 200,000 | 1,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 855,200,000 | 1,127,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 5,700,000 | 7,200,000
Gross Unrealized Losses |  |
Less than 12 Months | (5,500,000) | (5,400,000)
12 Months or More | 16,600,000 | 202,500,000
12 Months or More | (200,000) | (1,800,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 855,200,000 | 1,127,900,000
Total | (5,700,000) | (7,200,000)
Less than 12 Months | 838,600,000 | 925,400,000
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |  |
Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | 1,000,000.0 | 1,200,000
Available-for-Sale and Held-to-Maturity: |  |
Less than 12 Months | 434,000,000.0 | 740,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 2,400,000 | 4,600,000
12 Months or More | 12,300,000 | 155,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 100,000 | 1,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 446,300,000 | 896,000,000.0
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 2,500,000 | 6,000,000.0
Gross Unrealized Losses |  |
Less than 12 Months | (2,400,000) | (4,600,000)
12 Months or More | 12,300,000 | 155,900,000
12 Months or More | (100,000) | (1,400,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 446,300,000 | 896,000,000.0
Total | (2,500,000) | (6,000,000.0)
Less than 12 Months | 434,000,000.0 | 740,100,000
US Government Agencies Debt Securities [Member] |  |
Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity |  | 19,800,000
Available-for-Sale and Held-to-Maturity: |  |
Less than 12 Months | 235,600,000 | 185,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 2,000,000.0 | 800,000
12 Months or More | 0 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 235,600,000 | 185,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 2,000,000.0 | 800,000
Gross Unrealized Losses |  |
Less than 12 Months | (2,000,000.0) | (800,000)
12 Months or More | 0 | 0
12 Months or More | 0 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 235,600,000 | 185,300,000
Total | (2,000,000.0) | (800,000)
Less than 12 Months | 235,600,000 | 185,300,000
Corporate Debt Securities [Member] |  |
Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | 3,900,000 | 13,900,000
Available-for-Sale and Held-to-Maturity: |  |
Less than 12 Months | 20,900,000 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 100,000 |
12 Months or More | 0 |
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 20,900,000 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 100,000 |
Gross Unrealized Losses |  |
Less than 12 Months | (100,000) |
12 Months or More | 0 |
12 Months or More | 0 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 20,900,000 |
Total | (100,000) |
Less than 12 Months | 20,900,000 |
Mortgage-backed Securities, Issued by Private Enterprises [Member] |  |
Available-for-Sale and Held-to-Maturity: |  |
Less than 12 Months | 0 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 0 | 0
12 Months or More | 4,300,000 | 46,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 100,000 | 400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 4,300,000 | 46,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 100,000 | 400,000
Gross Unrealized Losses |  |
Less than 12 Months | 0 | 0
12 Months or More | 4,300,000 | 46,600,000
12 Months or More | (100,000) | (400,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 4,300,000 | 46,600,000
Total | (100,000) | (400,000)
Less than 12 Months | 0 | 0
State, county and municipal securities |  |
Investments, Debt and Equity Securities [Abstract] |  |
Debt Securities, Held-to-maturity | 46,600,000 | $ 57,300,000
Available-for-Sale and Held-to-Maturity: |  |
Less than 12 Months | 148,100,000 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | 1,000,000.0 |
12 Months or More | 0 |
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | 0 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 148,100,000 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | 1,000,000.0 |
Gross Unrealized Losses |  |
Less than 12 Months | (1,000,000.0) |
12 Months or More | 0 |
12 Months or More | 0 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 148,100,000 |
Total | (1,000,000.0) |
Less than 12 Months | $ 148,100,000 |

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