Other Comprehensive Income/Loss - Schedule of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Investment securities available-for sale:
Change in net unrealized gain during period, pre-tax	$ 20.2	$ (3.9)	$ 53.8	$ (27.6)
Change in net unrealized gain during period, tax expense (benefit)	5.1	(1.0)	14.0	(7.2)
Change in net unrealized gain during period, net of tax	15.1	(2.9)	39.8	(20.4)
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	1.0
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, tax expense (benefit)	0.0	0.1	0.0	0.3
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, net of tax	0.0	0.4	0.0	0.7
Defined benefits post-retirement benefit plan:
Change in net actuarial (gain) loss, pre-tax	(0.2)	(0.2)	(0.3)	(0.3)
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)	(0.3)	(0.2)
Other comprehensive income (loss), before tax	20.0	(3.6)	47.5	(26.9)
Total other comprehensive income, tax expense (benefit)	5.1	(0.9)	12.4	(7.0)
Total other comprehensive income, net of tax amount	14.9	(2.7)	35.1	(19.9)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net accumulated other comprehensive gain (loss)	1,948.6	1,458.3	1,948.6	1,458.3	$ 1,736.9	$ 1,693.9	$ 1,432.4	$ 1,427.6
Net unrealized gain (loss) on investment securities available-for-sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net accumulated other comprehensive gain (loss)	9.9		9.9			(25.5)
Net accumulated other comprehensive gain (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net accumulated other comprehensive gain (loss)	10.5	$ (35.0)	10.5	$ (35.0)	$ (4.4)	(24.6)	$ (32.3)	$ (12.0)
Postretirement Health Coverage | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial gain (loss) on defined benefit post-retirement benefit plans	$ 0.6		$ 0.6			$ 0.9