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Debt - Schedule of the Change in Debt under the NSM Bank Facility (Details) £ in Millions, $ in Millions			3 Months Ended
	Apr. 07, 2020 USD ($)	Apr. 07, 2020 GBP (£)	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)
Debt Instrument
Beginning balance			$ 376.4
Repayments			(93.7)	$ (1.0)
Ending balance			429.7
Term Loan | NSM Bank Facility
Debt Instrument
Beginning balance			271.3
Ending balance			271.5
Specialty Insurance Distribution (NSM)
Debt Instrument
Beginning balance			272.6
Ending balance			272.7
Specialty Insurance Distribution (NSM) | NSM Bank Facility
Debt Instrument
Beginning balance			277.4	221.3
Ending balance			277.3	220.7
Specialty Insurance Distribution (NSM) | Term Loan | NSM Bank Facility
Debt Instrument
Borrowings	$ 52.4	£ 42.5	0.0	0.0
Repayments			(0.7)	(0.6)
Foreign currency translation			0.6	0.0
Specialty Insurance Distribution (NSM) | Revolving credit loan | NSM Bank Facility
Debt Instrument
Borrowings			0.0	0.0
Repayments			$ 0.0	$ 0.0

X

- Definition

Amount of increase (decrease) in debt instruments, classified as other.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(f))
-URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ 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)
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-Publisher FASB
-Name Accounting Standards Codification
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- Definition

The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
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- Definition

The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

+ References

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-Publisher FASB
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