Pension and Other Employee Benefits - Components of Net Periodic Benefit Cost and Related Actuarial Assumptions (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 26, 2020	Sep. 28, 2019	Sep. 29, 2018
Other Liabilities Disclosure [Abstract]
Service cost	$ 0.0	$ 0.0	$ 0.0
Interest cost	0.1	0.2	0.2
Expected return on plan assets	0.0	0.0	0.0
Amortization of prior service cost	0.0	0.0	0.0
Recognized net actuarial gain	0.2	0.1	0.1
Net periodic benefit cost	$ 0.3	$ 0.3	$ 0.3