Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in net unrealized gain during period	$ 54.9	$ (13.9)	$ (6.5)
Reclassification adjustment for net gain included in net income	(0.1)	0.1	$ (0.7)
Accumulated other comprehensive income (loss)	2,013.9	1,693.9		$ 982.6
Net unrealized gain (loss) on investment securities available-for-sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	10.6	(25.5)
Total other comprehensive income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	11.0	(24.6)		$ (8.2)
Postretirement Healthcare Plan
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), before tax, related to the Plan	0.5
Postretirement Healthcare Plan | Net actuarial gain 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	$ 0.4	$ 0.9