Goodwill and Intangible Assets, net - Changes in Goodwill (Details) $ in Millions	9 Months Ended
	Mar. 31, 2025 USD ($)
Goodwill [Roll Forward]
Balance at June 30, 2024	$ 2,353.6
Additions and other adjustments	947.3
Currency translation adjustments	(0.7)
Balance at March 31, 2025	3,300.2
Employer Services
Goodwill [Roll Forward]
Balance at June 30, 2024	2,348.8
Additions and other adjustments	947.3
Currency translation adjustments	(0.7)
Balance at March 31, 2025	3,295.4
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 March 31, 2025	$ 4.8