Investment securities - Investments Held on Deposit or as Collateral and Unsettled investment purchases and sales (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Marketable securities
Investments held in trusts	$ 1,309.5	$ 792.4
Investments held in trusts - fair value	1,315.5	801.2
Accounts payable
Marketable securities
Unsettled investment purchases	2.3	3.2
Accounts receivable
Marketable securities
Unsettled investment sales	6.7	5.0
Interest rate cap \| Derivatives not designated as hedging instruments
Marketable securities
Collateral balances held	$ 0.2	$ 0.0

Amount payable on unsettled investment purchases.

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- Definition

Amount receivable on unsettled investment sales.

+ References

No definition available.

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- Definition

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

Investments which are not defined as or included in marketable (debt, equity, or other) securities whose use is restricted in whole or in part, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet.

+ References

No definition available.

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- Definition

The aggregate value of all restricted investments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-12.6(c))
-URI http://asc.fasb.org/extlink&oid=120401592&loc=d3e611133-123010

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 3
-Subparagraph (SX 210.12-12B(Footnote 11(c)))
-URI http://asc.fasb.org/extlink&oid=120401592&loc=d3e611197-123010

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- Definition

The carrying amount (fair value) as of the balance sheet date of securities held that had been pledged by counterparties under financing and lending arrangements, pursuant to which the entity has the right by agreement or custom to sell or re-pledge such securities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=121611835&loc=d3e5086-111524

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