Other Comprehensive Income/Loss - Schedule of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Investment securities available-for sale:
Change in net unrealized gain during period, pre-tax	$ 4.2	$ (6.3)	$ 58.0	$ (33.9)
Change in net unrealized gain during period, tax expense (benefit)	1.0	(1.7)	15.0	(8.9)
Change in net unrealized gain during period, net of tax	3.2	(4.6)	43.0	(25.0)
Reclassification adjustment for net (gains) losses included in income	(0.1)	0.0	(0.1)	0.0
Reclassification adjustment for net (gains) losses included in income, Tax Expense (Benefit)	0.0	0.0	0.0	0.0
Reclassification adjustment for net (gains) losses included in income, Net of Tax	(0.1)	0.0	(0.1)	0.0
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	1.5
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, tax expense (benefit)	0.0	0.1	0.0	0.4
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, net of tax	0.0	0.4	0.0	1.1
Defined benefits post-retirement benefit plan:
Change in net actuarial (gain) loss, pre-tax	(0.2)	(0.2)	(0.5)	(0.5)
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)	(0.5)	(0.4)
Other comprehensive income (loss), before tax	3.9	(6.0)	51.4	(32.9)
Total other comprehensive income, tax expense (benefit)	1.0	(1.6)	13.4	(8.6)
Total other comprehensive income, net of tax amount	$ 2.9	$ (4.4)	$ 38.0	$ (24.3)