Other Comprehensive Income/Loss - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net accumulated other comprehensive gains	$ 1,956.1	$ 2,013.9	$ 1,736.9	$ 1,693.9
Net unrealized gains on investment securities available-for-sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net accumulated other comprehensive gains	39.6	10.6
Net accumulated other comprehensive gains
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net accumulated other comprehensive gains	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	$ 0.2	$ 0.4