Investment Securities - Rollforward of Fair Value Measurements by Level (Details) - Other long-term investments - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2024	Sep. 30, 2023
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 1,138.2	$ 911.6
Net realized and unrealized gains	79.9	74.0
Purchases and contributions	3.1	200.1
Sales and distributions	(41.4)	(110.4)
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Ending balance	$ 1,179.8	$ 1,075.3

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

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