Other Comprehensive Income/Loss - Schedule of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	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