Fair value measurements - Reconciliation of investments measured at fair value using Level 3 inputs (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 5.2	$ 4.0
Transfers in to (out of) Level 3	(1.2)	0.0
Purchases	122.6	0.0
Assets Acquired	294.9	0.0
Sales	(79.1)	0.0
Realized and Unrealized Gains (Losses)	(2.3)	1.2
Ending balance	340.1	5.2
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(1.0)	0.0
Transfers in to (out of) Level 3	28.2	0.0
Purchases	(138.0)	0.0
Assets Acquired	(2.7)	0.0
Sales	1.9	(1.0)
Realized and Unrealized Gains (Losses)	51.2	0.0
Ending balance	(60.4)	(1.0)
Liability-classified capital instruments
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0.0
Transfers in to (out of) Level 3	27.0
Purchases	(137.6)
Assets Acquired	0.0
Sales	0.0
Realized and Unrealized Gains (Losses)	53.4
Ending balance	(57.2)	0.0
Contingent consideration
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0.0
Transfers in to (out of) Level 3	0.0
Purchases	0.0
Assets Acquired	(0.7)
Sales	1.9
Realized and Unrealized Gains (Losses)	(1.2)
Ending balance	0.0	0.0
Derivative liabilities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(1.0)	0.0
Transfers in to (out of) Level 3	1.2	0.0
Purchases	(0.4)	0.0
Assets Acquired	(2.0)	0.0
Sales	0.0	(1.0)
Realized and Unrealized Gains (Losses)	(1.0)	0.0
Ending balance	(3.2)	(1.0)
Preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0.0
Transfers in to (out of) Level 3	0.0
Purchases	10.0
Assets Acquired	2.8
Sales	(10.0)
Realized and Unrealized Gains (Losses)	0.0
Ending balance	2.8	0.0
Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	4.0	4.0
Transfers in to (out of) Level 3	0.0	0.0
Purchases	103.6	0.0
Assets Acquired	259.0	0.0
Sales	(24.9)	0.0
Realized and Unrealized Gains (Losses)	(4.8)	0.0
Ending balance	336.9	4.0
Derivative assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1.2	0.0
Transfers in to (out of) Level 3	(1.2)	0.0
Purchases	0.0	0.0
Assets Acquired	0.3
Sales	(1.4)	0.0
Realized and Unrealized Gains (Losses)	1.5	1.2
Ending balance	0.4	1.2
Loan participations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0.0
Transfers in to (out of) Level 3	0.0
Purchases	9.0
Assets Acquired	32.8
Sales	(42.8)
Realized and Unrealized Gains (Losses)	1.0
Ending balance	$ 0.0	$ 0.0

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Acquisition

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- Definition

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Acquisition

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

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- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

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Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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- Definition

Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

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No definition available.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

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- Definition

Amount of gain (loss) recognized in income from liability measured at fair value on recurring basis using unobservable input (level 3).

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Amount of purchases of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of sales of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Amount of transfers of financial instrument classified as a liability into (out of) level 3 of the fair value hierarchy.

+ References

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Fair value of financial instrument classified as a liability measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258

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