Other Comprehensive Income/Loss - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax	$ 0.2	$ 0.0	$ 0.2	$ 0.0
Net accumulated other comprehensive gains	1,998.9	1,948.6	1,998.9	1,948.6	$ 1,956.1	$ 2,013.9	$ 1,736.9	$ 1,693.9
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	(0.2)		(0.2)			0.0
Net unrealized gains on investment securities available-for-sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net accumulated other comprehensive gains	65.3		65.3			10.6
Net accumulated other comprehensive gains
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net accumulated other comprehensive gains	65.2	$ 10.5	65.2	$ 10.5	$ 39.8	11.0	$ (4.4)	$ (24.6)
Postretirement Health Coverage | Net actuarial gains on defined benefit post-retirement benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial gains on defined benefit post-retirement benefit plans	$ 0.1		$ 0.1			$ 0.4