Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in net unrealized gains during period	$ 61.8	$ 54.9	$ (13.9)
Reclassification adjustment for net gains included in net income	(0.3)	(0.1)	$ 0.1
Accumulated other comprehensive income (loss)	1,959.8	2,013.9
Net unrealized gain on investment securities available-for-sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	56.8	10.6
Total other comprehensive income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	56.6	11.0
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	$ 0.0	$ 0.4