Fair value measurements - Reconciliation of investments measured at fair value using Level 3 inputs (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2023	Mar. 31, 2022
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 240.0	$ 340.1
Transfers in to (out of) Level 3	0.0	(40.6)
Purchases	2.9	34.2
Sales & Settlements	(2.1)	(11.7)
Realized and Unrealized Gains (Losses)	1.1	(5.8)
Ending balance	241.9	316.2
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(30.0)	(60.4)
Transfers in to (out of) Level 3	0.0	0.0
Purchases	(3.2)	(1.9)
Sales & Settlements	1.5	0.0
Realized and Unrealized Gains (Losses)	(23.7)	15.4
Ending balance	(55.4)	(46.9)
Liability-classified capital instruments
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(21.4)	(57.2)
Transfers in to (out of) Level 3	0.0	0.0
Purchases	0.0	0.0
Sales & Settlements	0.0	0.0
Realized and Unrealized Gains (Losses)	(25.6)	15.1
Ending balance	(47.0)	(42.1)
Derivative liabilities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(8.6)	(3.2)
Transfers in to (out of) Level 3	0.0	0.0
Purchases	(3.2)	(1.9)
Sales & Settlements	1.5	0.0
Realized and Unrealized Gains (Losses)	1.9	0.3
Ending balance	(8.4)	(4.8)
Preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	3.2	2.8
Transfers in to (out of) Level 3	0.0	0.0
Purchases	0.0	0.0
Sales & Settlements	0.0	0.0
Realized and Unrealized Gains (Losses)	0.0	0.5
Ending balance	3.2	3.3
Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	227.3	336.9
Transfers in to (out of) Level 3	0.0	(40.6)
Purchases	0.4	34.2
Sales & Settlements	0.0	(10.0)
Realized and Unrealized Gains (Losses)	(0.3)	(5.2)
Ending balance	227.4	315.3
Derivative assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	9.5	0.4
Transfers in to (out of) Level 3	0.0	0.0
Purchases	2.5	0.0
Sales & Settlements	(2.1)	(1.7)
Realized and Unrealized Gains (Losses)	1.4	(1.1)
Ending balance	$ 11.3	$ (2.4)

Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales And Settlements

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Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Sales And Settlements

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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

No definition available.

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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 https://asc.fasb.org/extlink&oid=126976982&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 https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258

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

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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

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)
-URI https://asc.fasb.org/extlink&oid=126976982&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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- Definition

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

+ References

No definition available.

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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

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)
-URI https://asc.fasb.org/extlink&oid=126976982&loc=d3e19207-110258

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