Loan Servicing - Mortgage Servicing Rights Activity (Details) - Mortgage Servicing Rights - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Servicing Assets at Fair Value [Line Items]
Mortgage loans sold with servicing retained	$ 4,855.0	$ 4,653.0
Mortgage loans sold with servicing retained	1,762.0	1,636.0	$ 1,381.0
Pretax gains resulting from above loan sales	43.0	70.0	32.0
Mortgage banking operations	12.0	12.0	11.0
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	35.6	42.6	36.8
Additions	19.2	16.0	14.3
Payoffs and curtailments	(12.8)	(14.8)	(5.0)
(Impairment charge) / recovery	4.8	(5.8)	(1.0)
Amortization	(2.4)	(2.4)	(2.5)
Balance at end of period	44.4	35.6	42.6
Fair value, beginning of period	35.6	45.2	41.1
Fair value, end of period	46.0	35.6	$ 45.2
Valuation allowance for impairment of recognized servicing assets, balance	$ 2.5	$ 7.3