Loan Servicing - Mortgage Servicing Rights Activity (Details) - Mortgage Servicing Rights - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Servicing Assets at Fair Value [Line Items]
SBA loans sold to investors with servicing retained	$ 4,653.0	$ 4,686.0
Mortgage loans sold with servicing retained	1,636.0	1,381.0	$ 1,060.0
Pretax gains resulting from above loan sales	70.0	32.0	19.0
Mortgage banking operations	12.0	11.0	9.0
Valuation allowance for impairment of recognized servicing assets, balance	7.3	1.5
Servicing Asset at Amortized Cost, Balance [Roll Forward]
Balance at beginning of period	42.6	36.8	29.1
Additions	16.0	14.3	12.5
Payoffs and curtailments	(14.8)	(5.0)	(1.8)
(Impairment) charge / recovery	(5.8)	(1.0)	(0.5)
Amortization	(2.4)	(2.5)	(2.5)
Balance at end of period	35.6	42.6	36.8
Fair value, beginning of period	45.2	41.1	32.4
Fair value, end of period	$ 35.6	$ 45.2	$ 41.1