Other Comprehensive Income/Loss - Schedule of Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity Attributable to Parent	$ 1,736.9	$ 1,432.4	$ 1,693.9	$ 1,427.6
Investment securities available-for sale:
Change in net unrealized gain during period, pre-tax	33.6	(23.7)
Change in net unrealized gain during period, tax expense (benefit)	8.8	(6.2)
Change in net unrealized gain during period, net of tax	24.8	(17.5)
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, pre-tax	6.0	0.0
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, tax expense (benefit)	(1.6)
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity, net of tax	(4.4)	0.0
Other Comprehensive Income (Loss), Transfers from Available-for-Sale to Held-to-Maturity Securities, before Tax	0.0	0.5
Other Comprehensive Income (Loss), Transfers from Available-for-Sale to Held-to-Maturity Securities, Tax	0.0	0.2
Other Comprehensive Income (Loss), Transfers from Available-for-Sale to Held-to-Maturity Securities, Net of Tax	0.0	0.3
Defined benefits post-retirement benefit plan:
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, Tax	0.0	0.1
Other comprehensive income (loss), before tax	27.4	(23.3)
Total other comprehensive income, tax expense (benefit)	7.2	(6.1)
Total other comprehensive income, net of tax amount	20.2	(17.2)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax	(0.2)	(0.1)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, after Tax	(0.2)	0.0
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity Attributable to Parent	(5.1)		(25.5)
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity Attributable to Parent	(4.4)	$ (32.3)	(24.6)	$ (12.0)
Postretirement Health Coverage [Member] | Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax	$ 0.7		$ 0.9