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Other Comprehensive Income/Loss - Schedule of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Mar. 31, 2019 | Dec. 31, 2018 | Mar. 31, 2018 | Dec. 31, 2017
Investment securities available-for sale: |  |  |  |  |  |  |  |
Change in net unrealized gain during period, pre-tax | $ 20.2 | $ (3.9) | $ 53.8 | $ (27.6) |  |  |  |
Change in net unrealized gain during period, tax expense (benefit) | 5.1 | (1.0) | 14.0 | (7.2) |  |  |  |
Change in net unrealized gain during period, net of tax | 15.1 | (2.9) | 39.8 | (20.4) |  |  |  |
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale, pre-tax | 0.0 | 0.0 | (6.0) | 0.0 |  |  |  |
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale, tax expense (benefit) |  |  | (1.6) | 0.0 |  |  |  |
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale, net of tax |  |  | (4.4) | 0.0 |  |  |  |
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, pre-tax | 0.0 | 0.5 | 0.0 | 1.0 |  |  |  |
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, tax expense (benefit) | 0.0 | 0.1 | 0.0 | 0.3 |  |  |  |
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, net of tax | 0.0 | 0.4 | 0.0 | 0.7 |  |  |  |
Defined benefits post-retirement benefit plan: |  |  |  |  |  |  |  |
Change in net actuarial (gain) loss, pre-tax | (0.2) | (0.2) | (0.3) | (0.3) |  |  |  |
Change in net actuarial (gain) loss, tax expense (benefit) | 0.0 |  | 0.0 | (0.1) |  |  |  |
Change in net actuarial (gain) loss, net of tax | (0.2) | (0.2) | (0.3) | (0.2) |  |  |  |
Other comprehensive income (loss), before tax | 20.0 | (3.6) | 47.5 | (26.9) |  |  |  |
Total other comprehensive income, tax expense (benefit) | 5.1 | (0.9) | 12.4 | (7.0) |  |  |  |
Total other comprehensive income, net of tax amount | 14.9 | (2.7) | 35.1 | (19.9) |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |  |  |  |
Net accumulated other comprehensive gain (loss) | 1,948.6 | 1,458.3 | 1,948.6 | 1,458.3 | $ 1,736.9 | $ 1,693.9 | $ 1,432.4 | $ 1,427.6
Net unrealized gain (loss) on investment securities available-for-sale |  |  |  |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |  |  |  |
Net accumulated other comprehensive gain (loss) | 9.9 |  | 9.9 |  |  | (25.5) |  |
Net accumulated other comprehensive gain (loss) |  |  |  |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |  |  |  |
Net accumulated other comprehensive gain (loss) | 10.5 | $ (35.0) | 10.5 | $ (35.0) | $ (4.4) | (24.6) | $ (32.3) | $ (12.0)
Postretirement Health Coverage | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] |  |  |  |  |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |  |  |  |  |
Net actuarial gain (loss) on defined benefit post-retirement benefit plans | $ 0.6 |  | $ 0.6 |  |  | $ 0.9 |  |

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