Mortgage Servicing Rights - Schedule of Mortgage Servicing Rights (Details) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Servicing Asset at Amortized Value, Balance [Roll Forward]
Balance at end of year, net of valuation reserve	$ 28.3	$ 31.1
Mortgage loans
Servicing Asset at Amortized Value, Balance [Roll Forward]
Servicing asset at amortized cost, before valuation reserve, beginning	31.1	31.6	$ 34.3
Originations of mortgage servicing rights	1.1	2.5	3.7
Amortization expense	(3.9)	(4.3)	(6.4)
Servicing asset at amortized cost, before valuation reserve, ending	28.3	31.1	31.6
Acquisition of mortgage servicing rights	0.0	1.3	0.0
Less valuation reserve	0.0	0.0	(3.4)
Balance at end of year, net of valuation reserve	28.3	31.1	28.2
Principal balance of serviced loans underlying mortgage servicing rights	$ 3,091.1	$ 3,259.8	$ 3,203.7
Mortgage servicing rights as a percentage of serviced loans	0.0092	0.0095	0.0088