Investment Securities - Rollforward of Fair Value Measurements by Level (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Effect of Distinguished Transaction	$ (7.1)	$ 0.0
Other long-term investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1,262.7	1,138.2
Net realized and unrealized gains	105.7	79.9
Purchases and contributions	119.6	3.1
Sales and distributions	(1.7)	(41.4)
Transfers in	0.0	0.0
Transfers out	0.0	0.0
Ending balance	$ 1,479.2	$ 1,179.8