Loans - Schedule of Retail Loans By Risk Ratings (Details) - Retail $ in Millions	Mar. 31, 2020 USD ($)
Residential mortgage
Financing Receivable, Recorded Investment [Line Items]
2020	$ 210.6
2019	1,120.7
2018	1,376.9
2017	1,642.5
2016	2,163.6
Prior	3,567.6
Revolving Loans	0.0
Total	10,081.9
Residential mortgage | Low risk
Financing Receivable, Recorded Investment [Line Items]
2020	51.5
2019	400.9
2018	423.9
2017	560.7
2016	1,130.5
Prior	1,937.1
Revolving Loans	0.0
Total	4,504.6
Residential mortgage | Moderate risk
Financing Receivable, Recorded Investment [Line Items]
2020	132.7
2019	640.0
2018	835.4
2017	967.7
2016	928.3
Prior	1,335.1
Revolving Loans	0.0
Total	4,839.2
Residential mortgage | High risk
Financing Receivable, Recorded Investment [Line Items]
2020	26.4
2019	79.8
2018	117.6
2017	114.1
2016	104.8
Prior	295.4
Revolving Loans	0.0
Total	738.1
Home equity
Financing Receivable, Recorded Investment [Line Items]
2020	2.4
2019	43.2
2018	95.1
2017	74.1
2016	30.7
Prior	81.2
Revolving Loans	2,023.2
Total	2,349.9
Home equity | Low risk
Financing Receivable, Recorded Investment [Line Items]
2020	0.0
2019	0.0
2018	0.0
2017	0.0
2016	0.0
Prior	31.7
Revolving Loans	335.2
Total	366.9
Home equity | Moderate risk
Financing Receivable, Recorded Investment [Line Items]
2020	0.4
2019	15.3
2018	12.1
2017	6.1
2016	3.2
Prior	35.2
Revolving Loans	689.5
Total	761.8
Home equity | High risk
Financing Receivable, Recorded Investment [Line Items]
2020	2.0
2019	27.9
2018	83.0
2017	68.0
2016	27.5
Prior	14.3
Revolving Loans	998.5
Total	1,221.2
Other consumer
Financing Receivable, Recorded Investment [Line Items]
2020	4.9
2019	68.3
2018	49.0
2017	5.4
2016	1.5
Prior	13.1
Revolving Loans	0.0
Total	142.2
Other consumer | Low risk
Financing Receivable, Recorded Investment [Line Items]
2020	1.6
2019	10.0
2018	6.3
2017	1.5
2016	0.5
Prior	4.8
Revolving Loans	0.0
Total	24.7
Other consumer | Moderate risk
Financing Receivable, Recorded Investment [Line Items]
2020	0.0
2019	0.1
2018	0.2
2017	0.1
2016	0.2
Prior	5.3
Revolving Loans	0.0
Total	5.9
Other consumer | High risk
Financing Receivable, Recorded Investment [Line Items]
2020	3.3
2019	58.2
2018	42.5
2017	3.8
2016	0.8
Prior	3.0
Revolving Loans	0.0
Total	$ 111.6