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Other Comprehensive Income/Loss - Schedule of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2018 | Mar. 31, 2017
Investment securities available-for sale: |  |
Change in net unrealized gain during period, pre-tax | $ (23.7) | $ 9.3
Change in net unrealized gain during period, tax expense (benefit) | (6.2) | 3.7
Change in net unrealized gain during period, net of tax | (17.5) | 5.6
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, pre-tax | 0.5 | 0.5
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, tax expense (benefit) | 0.2 | 0.2
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, net of tax | 0.3 | 0.3
Defined benefits post-retirement benefit plan: |  |
Change in net actuarial (gain) loss, pre-tax | (0.1) | (0.7)
Change in net actuarial (gain) loss, tax expense (benefit) | (0.1) | (0.3)
Change in net actuarial (gain) loss, net of tax amount | 0.0 | (0.4)
Other comprehensive income, before tax | (23.3) | 9.1
Total other comprehensive income, tax expense (benefit) | (6.1) | 3.6
Total other comprehensive income, net of tax amount | $ (17.2) | $ 5.5

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