Other Comprehensive Income/Loss - Schedule of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Investment securities available-for sale:
Change in net unrealized gain during period, pre-tax	$ 39.2	$ 33.6
Change in net unrealized gain during period, tax expense (benefit)	10.3	8.8
Change in net unrealized gain during period, net of tax	28.9	24.8
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale, pre-tax	0.0	(6.0)
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale, tax expense (benefit)	0.0	(1.6)
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale, net of tax	0.0	(4.4)
Defined benefits post-retirement benefit plan:
Change in net actuarial (gain) loss, pre-tax	(0.2)	(0.2)
Change in net actuarial (gain) loss, tax expense (benefit)	(0.1)
Change in net actuarial (gain) loss, net of tax	(0.1)	(0.2)
Other comprehensive income, before tax	39.0	27.4
Total other comprehensive income, tax expense (benefit)	10.2	7.2
Total other comprehensive income, net of tax amount	$ 28.8	$ 20.2