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Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) $ in Millions | Sep. 30, 2020 USD ($) Investments | Dec. 31, 2019 USD ($) Investments
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Available-for-Sale: |  |
Available-for-Sale, Less than 12 Months, Fair Value | $ 266.9 | $ 925.4
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (1.7) | (5.4)
Available-for-Sale, 12 Months or More, Fair Value | 24.1 | 202.5
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (0.2) | (1.8)
Available-for-Sale, Total Fair Value | 291.0 | 1,127.9
Available-for-Sale, Total Gross Unrealized Losses | (1.9) | (7.2)
Held-to-Maturity: |  |
Gross Unrealized Losses | 0.0 | 0.0
Investment securities classified as held to maturity | 55.0 | 92.3
Investment debt securities held-to-maturity | 58.4 | 94.5
Debt Securities, Available-for-sale, Amortized Cost | 3,369.2 | 2,945.5
Gross Unrealized Gains | 86.2 | 21.7
Gross Unrealized Losses | (1.9) | (7.2)
Gross Unrealized Gains | $ 3.4 | $ 2.2
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Percent Debt Securities | 66.30% |
Investment securities in an unrealized loss position (securities) | Investments | 127 | 331
State, county and municipal securities |  |
Available-for-Sale: |  |
Available-for-Sale, Less than 12 Months, Fair Value | $ 22.6 |
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (0.2) |
Available-for-Sale, 12 Months or More, Fair Value | 0.0 |
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | 0.0 |
Available-for-Sale, Total Fair Value | 22.6 |
Available-for-Sale, Total Gross Unrealized Losses | (0.2) |
Held-to-Maturity: |  |
Gross Unrealized Losses | 0.0 | $ 0.0
Investment securities classified as held to maturity | 49.9 | 57.3
Investment debt securities held-to-maturity | 53.1 | 59.4
Debt Securities, Available-for-sale, Amortized Cost | 233.6 | 80.1
Gross Unrealized Gains | 2.7 | 0.8
Gross Unrealized Losses | (0.2) | 0.0
Gross Unrealized Gains | 3.2 | 2.1
Obligations of U.S. government agencies |  |
Available-for-Sale: |  |
Available-for-Sale, Less than 12 Months, Fair Value | 102.0 | 185.3
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (0.6) | (0.8)
Available-for-Sale, 12 Months or More, Fair Value | 0.0 | 0.0
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | 0.0 | 0.0
Available-for-Sale, Total Fair Value | 102.0 | 185.3
Available-for-Sale, Total Gross Unrealized Losses | (0.6) | (0.8)
Held-to-Maturity: |  |
Gross Unrealized Losses |  | 0.0
Investment securities classified as held to maturity |  | 19.8
Investment debt securities held-to-maturity |  | 19.8
Debt Securities, Available-for-sale, Amortized Cost | 278.4 | 367.5
Gross Unrealized Gains | 1.0 | 0.1
Gross Unrealized Losses | (0.6) | (0.8)
Gross Unrealized Gains |  | 0.0
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations |  |
Available-for-Sale: |  |
Available-for-Sale, Less than 12 Months, Fair Value | 88.2 | 740.1
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (0.4) | (4.6)
Available-for-Sale, 12 Months or More, Fair Value | 11.4 | 155.9
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (0.1) | (1.4)
Available-for-Sale, Total Fair Value | 99.6 | 896.0
Available-for-Sale, Total Gross Unrealized Losses | (0.5) | (6.0)
Held-to-Maturity: |  |
Gross Unrealized Losses | 0.0 | 0.0
Investment securities classified as held to maturity | 1.0 | 1.2
Investment debt securities held-to-maturity | 1.1 | 1.2
Debt Securities, Available-for-sale, Amortized Cost | 2,535.3 | 2,303.6
Gross Unrealized Gains | 76.9 | 19.6
Gross Unrealized Losses | (0.5) | (6.0)
Gross Unrealized Gains | 0.1 | 0.0
Private mortgage-backed securities |  |
Available-for-Sale: |  |
Available-for-Sale, Less than 12 Months, Fair Value | 0.0 | 0.0
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | 0.0 | 0.0
Available-for-Sale, 12 Months or More, Fair Value | 12.7 | 46.6
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (0.1) | (0.4)
Available-for-Sale, Total Fair Value | 12.7 | 46.6
Available-for-Sale, Total Gross Unrealized Losses | (0.1) | (0.4)
Held-to-Maturity: |  |
Debt Securities, Available-for-sale, Amortized Cost | 20.0 | 47.6
Gross Unrealized Gains | 0.2 | 0.0
Gross Unrealized Losses | (0.1) | (0.4)
Corporate securities |  |
Available-for-Sale: |  |
Available-for-Sale, Less than 12 Months, Fair Value | 54.1 |
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (0.5) |
Available-for-Sale, 12 Months or More, Fair Value | 0.0 |
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | 0.0 |
Available-for-Sale, Total Fair Value | 54.1 |
Available-for-Sale, Total Gross Unrealized Losses | (0.5) |
Held-to-Maturity: |  |
Gross Unrealized Losses | 0.0 | 0.0
Investment securities classified as held to maturity | 4.0 | 13.9
Investment debt securities held-to-maturity | 4.1 | 14.0
Debt Securities, Available-for-sale, Amortized Cost | 292.9 | 134.5
Gross Unrealized Gains | 5.4 | 1.2
Gross Unrealized Losses | (0.5) | 0.0
Gross Unrealized Gains | $ 0.1 | $ 0.1

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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=121645371&loc=d3e27161-111563 + Details
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