Mortgage Servicing Rights - Schedule of Mortgage Servicing Rights (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Servicing Asset at Amortized Value, Balance [Roll Forward]
Balance at end of year, net of valuation reserve	$ 30.2	$ 27.7
Mortgage loans
Servicing Asset at Amortized Value, Balance [Roll Forward]
Servicing asset at amortized cost, before valuation reserve, beginning	27.7	24.8	$ 18.7
Acquisitions of mortgage servicing rights	0.0	0.0	3.5
Originations of mortgage servicing rights	7.3	6.0	5.6
Amortization expense	(4.4)	(3.1)	(3.0)
Servicing asset at amortized cost, before valuation reserve, ending	30.6	27.7	24.8
Less valuation reserve	(0.4)	0.0	0.0
Balance at end of year, net of valuation reserve	30.2	27.7	24.8
Principal balance of serviced loans underlying mortgage servicing rights	$ 3,710.1	$ 3,698.2	$ 3,636.7
Mortgage servicing rights as a percentage of serviced loans	0.0081	0.0075	0.0068