Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	$ 693.0	$ 376.4
Fair value | Ark 2007 Notes
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt, fair value	27.6	0.0
Fair value | Ark 2021 Notes
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt, fair value	162.8	0.0
Fair value | NSM Bank Facility
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt, fair value	275.8	279.3
Fair value | Other NSM
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt, fair value	1.0	1.3
Fair value | Kudu Credit Facility
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt, fair value	246.8	0.0
Fair value | Kudu Bank Facility
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt, fair value	0.0	89.3
Fair value | Other Operations
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt, fair value	17.7	18.8
Carrying Value | Ark 2007 Notes
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	30.0	0.0
Carrying Value | Ark 2021 Notes
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	155.9	0.0
Carrying Value | NSM Bank Facility
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	271.2	271.3
Carrying Value | Other NSM
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	0.9	1.3
Carrying Value | Kudu Credit Facility
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	218.2	0.0
Carrying Value | Kudu Bank Facility
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	0.0	86.3
Carrying Value | Other Operations
Balance Sheet Grouping, Financial Statement Captions [Line Items]
Debt	$ 16.8	$ 17.5