Loans - Schedule of Commercial Loans By Risk Ratings (Details) - Commercial $ in Millions	Mar. 31, 2020 USD ($)
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
2020	$ 337.6
2019	1,800.3
2018	1,844.7
2017	1,760.3
2016	1,629.2
Prior	6,893.5
Revolving Loans	386.0
Total	14,651.6
Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
2020	335.3
2019	1,762.3
2018	1,730.4
2017	1,703.1
2016	1,584.0
Prior	6,615.0
Revolving Loans	380.7
Total	14,110.8
Commercial real estate | Special mention
Financing Receivable, Recorded Investment [Line Items]
2020	0.0
2019	8.7
2018	91.4
2017	28.0
2016	38.6
Prior	115.3
Revolving Loans	2.4
Total	284.4
Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
2020	2.3
2019	29.3
2018	22.9
2017	29.2
2016	6.6
Prior	161.6
Revolving Loans	2.9
Total	254.8
Commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
2020	0.0
2019	0.0
2018	0.0
2017	0.0
2016	0.0
Prior	1.6
Revolving Loans	0.0
Total	1.6
Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
2020	676.9
2019	1,899.3
2018	1,094.6
2017	845.3
2016	688.9
Prior	2,120.6
Revolving Loans	1,772.1
Total	9,097.7
Commercial and industrial | Pass
Financing Receivable, Recorded Investment [Line Items]
2020	674.1
2019	1,876.3
2018	1,015.3
2017	801.6
2016	635.6
Prior	1,930.1
Revolving Loans	1,635.0
Total	8,568.0
Commercial and industrial | Special mention
Financing Receivable, Recorded Investment [Line Items]
2020	2.1
2019	11.5
2018	22.3
2017	6.1
2016	39.8
Prior	63.1
Revolving Loans	41.9
Total	186.8
Commercial and industrial | Substandard
Financing Receivable, Recorded Investment [Line Items]
2020	0.7
2019	11.5
2018	57.0
2017	37.0
2016	12.3
Prior	127.4
Revolving Loans	93.5
Total	339.4
Commercial and industrial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
2020	0.0
2019	0.0
2018	0.0
2017	0.6
2016	1.2
Prior	0.0
Revolving Loans	1.7
Total	3.5
Equipment financing
Financing Receivable, Recorded Investment [Line Items]
2020	615.1
2019	2,037.9
2018	1,186.8
2017	655.1
2016	320.6
Prior	197.2
Revolving Loans	0.0
Total	5,012.7
Equipment financing | Pass
Financing Receivable, Recorded Investment [Line Items]
2020	556.4
2019	1,843.0
2018	1,068.9
2017	587.7
2016	303.1
Prior	164.3
Revolving Loans	0.0
Total	4,523.4
Equipment financing | Special mention
Financing Receivable, Recorded Investment [Line Items]
2020	13.5
2019	33.7
2018	16.1
2017	10.9
2016	4.4
Prior	1.7
Revolving Loans	0.0
Total	80.3
Equipment financing | Substandard
Financing Receivable, Recorded Investment [Line Items]
2020	45.2
2019	161.2
2018	101.8
2017	56.5
2016	13.1
Prior	31.2
Revolving Loans	0.0
Total	409.0
Equipment financing | Doubtful
Financing Receivable, Recorded Investment [Line Items]
2020	0.0
2019	0.0
2018	0.0
2017	0.0
2016	0.0
Prior	0.0
Revolving Loans	0.0
Total	0.0
MWABL
Financing Receivable, Recorded Investment [Line Items]
2020	2.6
2019	18.1
2018	15.0
2017	8.5
2016	21.7
Prior	64.6
Revolving Loans	2,817.5
Total	2,948.0
MWABL | Pass
Financing Receivable, Recorded Investment [Line Items]
2020	2.6
2019	10.8
2018	15.0
2017	6.6
2016	21.0
Prior	62.7
Revolving Loans	2,697.0
Total	2,815.7
MWABL | Special mention
Financing Receivable, Recorded Investment [Line Items]
2020	0.0
2019	7.3
2018	0.0
2017	0.0
2016	0.0
Prior	0.0
Revolving Loans	68.6
Total	75.9
MWABL | Substandard
Financing Receivable, Recorded Investment [Line Items]
2020	0.0
2019	0.0
2018	0.0
2017	1.9
2016	0.7
Prior	1.9
Revolving Loans	51.9
Total	56.4
MWABL | Doubtful
Financing Receivable, Recorded Investment [Line Items]
2020	0.0
2019	0.0
2018	0.0
2017	0.0
2016	0.0
Prior	0.0
Revolving Loans	0.0
Total	$ 0.0