Mortgage Servicing Rights - Schedule of Mortgage Servicing Rights (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Servicing Asset at Amortized Value, Balance [Roll Forward]
Balance at end of year, net of valuation reserve	$ 27.7	$ 24.8
Mortgage loans
Servicing Asset at Amortized Value, Balance [Roll Forward]
Servicing asset at amortized cost, before valuation reserve, beginning	24.8	18.7	$ 15.9
Acquisitions of mortgage servicing rights	0.0	3.5	0.0
Originations of mortgage servicing rights	6.0	5.6	5.8
Amortization expense	(3.1)	(3.0)	(3.0)
Servicing asset at amortized cost, before valuation reserve, ending	27.7	24.8	18.7
Less valuation reserve	0.0	0.0	(0.2)
Balance at end of year, net of valuation reserve	27.7	24.8	18.5
Principal balance of serviced loans underlying mortgage servicing rights	$ 3,698.2	$ 3,636.7	$ 3,127.5
Mortgage servicing rights as a percentage of serviced loans	0.0075	0.0068	0.0059