Other Comprehensive Income/Loss - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net accumulated other comprehensive gain (loss)	$ 1,982.6	$ 1,948.6	$ 1,693.9	$ 1,654.4	$ 1,458.3	$ 1,427.6
Net unrealized gain (loss) on investment securities available-for-sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net accumulated other comprehensive gain (loss)	12.9		(25.5)
Net accumulated other comprehensive gain (loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net accumulated other comprehensive gain (loss)	13.4	$ 10.5	(24.6)	$ (39.4)	$ (35.0)	$ (12.0)
Postretirement Health Coverage | Net actuarial gain (loss) on defined benefit post-retirement benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net actuarial gain (loss) on defined benefit post-retirement benefit plans	$ 0.5		$ 0.9