Goodwill and Intangible Assets, net - Changes in Goodwill (Details) $ in Millions	6 Months Ended
	Dec. 31, 2023 USD ($)
Goodwill [Roll Forward]
Balance at June 30, 2023	$ 2,339.4
Additions and other adjustments	27.5
Currency translation adjustments	2.9
Balance at December 31, 2023	2,369.8
Employer Services
Goodwill [Roll Forward]
Balance at June 30, 2023	2,334.6
Additions and other adjustments	27.5
Currency translation adjustments	2.9
Balance at December 31, 2023	2,365.0
PEO Services
Goodwill [Roll Forward]
Balance at June 30, 2023	4.8
Additions and other adjustments	0.0
Currency translation adjustments	0.0
Balance at December 31, 2023	$ 4.8