Goodwill and Intangible Assets, net - Changes in Goodwill (Details) $ in Millions	6 Months Ended
	Dec. 31, 2024 USD ($)
Goodwill [Roll Forward]
Balance at June 30, 2024	$ 2,353.6
Additions and other adjustments	846.4
Currency translation adjustments	(16.1)
Balance at December 31, 2024	3,183.9
Employer Services
Goodwill [Roll Forward]
Balance at June 30, 2024	2,348.8
Additions and other adjustments	846.4
Currency translation adjustments	(16.1)
Balance at December 31, 2024	3,179.1
PEO Services
Goodwill [Roll Forward]
Balance at June 30, 2024	4.8
Additions and other adjustments	0.0
Currency translation adjustments	0.0
Balance at December 31, 2024	$ 4.8