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Other Comprehensive Income/Loss - Schedule of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Stockholders' Equity Attributable to Parent | $ 1,736.9 | $ 1,432.4 | $ 1,693.9 | $ 1,427.6
Investment securities available-for sale: |  |  |  |
Change in net unrealized gain during period, pre-tax | 33.6 | (23.7) |  |
Change in net unrealized gain during period, tax expense (benefit) | 8.8 | (6.2) |  |
Change in net unrealized gain during period, net of tax | 24.8 | (17.5) |  |
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, pre-tax | 6.0 | 0.0 |  |
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, tax expense (benefit) | (1.6) |  |  |
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, net of tax | (4.4) | 0.0 |  |
Other Comprehensive Income (Loss), Transfers from Available-for-Sale to Held-to-Maturity Securities, before Tax | 0.0 | 0.5 |  |
Other Comprehensive Income (Loss), Transfers from Available-for-Sale to Held-to-Maturity Securities, Tax | 0.0 | 0.2 |  |
Other Comprehensive Income (Loss), Transfers from Available-for-Sale to Held-to-Maturity Securities, Net of Tax | 0.0 | 0.3 |  |
Defined benefits post-retirement benefit plan: |  |  |  |
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, Tax | 0.0 | 0.1 |  |
Other comprehensive income (loss), before tax | 27.4 | (23.3) |  |
Total other comprehensive income, tax expense (benefit) | 7.2 | (6.1) |  |
Total other comprehensive income, net of tax amount | 20.2 | (17.2) |  |
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax | (0.2) | (0.1) |  |
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, after Tax | (0.2) | 0.0 |  |
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Stockholders' Equity Attributable to Parent | (5.1) |  | (25.5) |
AOCI Attributable to Parent [Member] |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Stockholders' Equity Attributable to Parent | (4.4) | $ (32.3) | (24.6) | $ (12.0)
Postretirement Health Coverage [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax | $ 0.7 |  | $ 0.9 |

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