Fair value measurements - Reconciliation of Investments Measured at Fair Value using Level 3 Inputs (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 114.8	$ 172.0	$ 87.5	$ 185.4
Transfers in to (out of) Level 3	0.0	0.0	0.0	0.0
Purchases	0.0	0.0	0.0	0.0
Sales & Settlements	0.0	(0.4)	0.0	(5.0)
Realized and Unrealized Gains (Losses)	(8.7)	(44.7)	18.6	(53.5)
Ending balance	106.1	126.9	106.1	126.9
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(6.1)	(103.8)	(14.3)	(73.7)
Transfers in to (out of) Level 3	0.0	0.0	0.0	0.0
Purchases	0.0	0.0	0.0	0.0
Sales & Settlements	0.0	2.2	5.4	1.1
Realized and Unrealized Gains (Losses)	(6.2)	10.3	(3.4)	(18.7)
Ending balance	(12.3)	(91.3)	(12.3)	(91.3)
Liability-classified capital instruments
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		(83.2)		(67.3)
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)		10.6		(5.3)
Ending balance		(72.6)		(72.6)
Derivative liabilities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(6.1)	(20.6)	(14.3)	(6.4)
Transfers in to (out of) Level 3	0.0	0.0	0.0	0.0
Purchases	0.0	0.0	0.0	0.0
Sales & Settlements	0.0	2.2	5.4	1.1
Realized and Unrealized Gains (Losses)	(6.2)	(0.3)	(3.4)	(13.4)
Ending balance	(12.3)	(18.7)	(12.3)	(18.7)
Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	87.0	170.6	86.6	169.7
Transfers in to (out of) Level 3	0.0	0.0	0.0	0.0
Purchases	0.0	0.0	0.0	0.0
Sales & Settlements	0.0	0.0	0.0	0.0
Realized and Unrealized Gains (Losses)	0.0	(51.0)	0.4	(50.1)
Ending balance	87.0	119.6	87.0	119.6
Derivative assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	27.8	1.4	0.9	15.7
Transfers in to (out of) Level 3	0.0	0.0	0.0	0.0
Purchases	0.0	0.0	0.0	0.0
Sales & Settlements	0.0	(0.4)	0.0	(5.0)
Realized and Unrealized Gains (Losses)	(8.7)	6.3	18.2	(3.4)
Ending balance	$ 19.1	$ 7.3	$ 19.1	$ 7.3

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

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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.

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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.

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

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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.

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