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Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Change in net unrealized gains during period | $ 61.8 | $ 54.9 | $ (13.9)
Reclassification adjustment for net gains included in net income | (0.3) | (0.1) | $ 0.1
Accumulated other comprehensive income (loss) | 1,959.8 | 2,013.9 |
Net unrealized gain on investment securities available-for-sale |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Accumulated other comprehensive income (loss) | 56.8 | 10.6 |
Total other comprehensive income |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Accumulated other comprehensive income (loss) | 56.6 | 11.0 |
Postretirement Healthcare Plan | Net actuarial gain on defined benefit post-retirement benefit plans |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |
Accumulated other comprehensive income (loss), before tax, related to the Plan | $ 0.0 | $ 0.4 |

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