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Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Balance Sheet Grouping, Financial Statement Captions
Carrying Value	$ 710.4	$ 376.4
Other NSM debt
Balance Sheet Grouping, Financial Statement Captions
Carrying Value	1.3	1.3
Kudu Credit Facility
Balance Sheet Grouping, Financial Statement Captions
Carrying Value	195.6
Other Operations debt
Balance Sheet Grouping, Financial Statement Captions
Carrying Value	19.1
Fair Value | Ark 2007 Notes
Balance Sheet Grouping, Financial Statement Captions
Fair Value	42.0	0.0
Fair Value | Ark 2021 Notes
Balance Sheet Grouping, Financial Statement Captions
Fair Value	163.1	0.0
Fair Value | NSM Bank Facility
Balance Sheet Grouping, Financial Statement Captions
Fair Value	300.6	279.3
Fair Value | Other NSM debt
Balance Sheet Grouping, Financial Statement Captions
Fair Value	1.3	1.3
Fair Value | Kudu Credit Facility
Balance Sheet Grouping, Financial Statement Captions
Fair Value	218.2	0.0
Fair Value | Kudu Bank Facility
Balance Sheet Grouping, Financial Statement Captions
Fair Value	0.0	89.3
Fair Value | Other Operations debt
Balance Sheet Grouping, Financial Statement Captions
Fair Value	20.5	18.8
Carrying Value | Ark 2007 Notes
Balance Sheet Grouping, Financial Statement Captions
Carrying Value	43.9	0.0
Carrying Value | Ark 2021 Notes
Balance Sheet Grouping, Financial Statement Captions
Carrying Value	156.8	0.0
Carrying Value | NSM Bank Facility
Balance Sheet Grouping, Financial Statement Captions
Carrying Value	293.7	271.3
Carrying Value | Other NSM debt
Balance Sheet Grouping, Financial Statement Captions
Carrying Value	1.3	1.3
Carrying Value | Kudu Credit Facility
Balance Sheet Grouping, Financial Statement Captions
Carrying Value	195.6	0.0
Carrying Value | Kudu Bank Facility
Balance Sheet Grouping, Financial Statement Captions
Carrying Value	0.0	86.3
Carrying Value | Other Operations debt
Balance Sheet Grouping, Financial Statement Captions
Carrying Value	$ 19.1	$ 17.5

X

- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

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+ References

No definition available.

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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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