Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in net unrealized loss during period	$ (13.9)	$ (6.5)	$ (19.4)
Reclassification adjustment for net loss included in net income	0.1	(0.7)	(0.3)
Accumulated other comprehensive income (loss)	1,693.9	1,427.6	982.6	$ 950.5
Net unrealized gain (loss) on investment securities available-for-sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	(25.5)	(13.3)
Net actuarial gain (loss) on defined benefit post-retirement benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)		1.3
Total other comprehensive income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	(24.6)	$ (12.0)	$ (8.2)	$ 0.4
Postretirement Healthcare Plan
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), before tax, related to the Plan	1.2
Postretirement Healthcare Plan | Net actuarial gain (loss) on defined benefit post-retirement benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), before tax, related to the Plan	$ 0.9