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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, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
Investment securities available-for sale: |  |  |  |
Change in net unrealized gain during period, pre-tax | $ (3.8) | $ 8.0 | $ (27.6) | $ 17.4
Change in net unrealized gain during period, tax expense (benefit) | (1.0) | 3.3 | (7.2) | 7.0
Change in net unrealized gain during period, net of tax | (2.8) | 4.7 | (20.4) | 10.4
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, pre-tax | 0.5 | 0.5 | 1.0 | 0.9
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, tax expense (benefit) | 0.1 | 0.2 | 0.3 | 0.4
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, net of tax | 0.4 | 0.3 | 0.7 | 0.5
Defined benefits post-retirement benefit plan: |  |  |  |
Change in net actuarial (gain) loss, pre-tax | (0.2) | (0.2) | (0.3) | (0.9)
Change in net actuarial (gain) loss, tax expense (benefit) | 0.0 | (0.1) | (0.1) | (0.4)
Change in net actuarial (gain) loss, net of tax amount | (0.2) | (0.1) | (0.2) | (0.5)
Other comprehensive income (loss), before tax | (3.5) | 7.8 | (26.9) | 16.9
Total other comprehensive income, tax expense (benefit) | (0.9) | 3.2 | (7.0) | 6.8
Total other comprehensive income, net of tax amount | (2.6) | 4.6 | (19.9) | 10.1
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax | 0.0 | (0.5) | 0.0 | (0.5)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax | 0.0 | (0.2) | 0.0 | (0.2)
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax | $ 0.0 | $ (0.3) | $ 0.0 | $ (0.3)

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