Revenue from Contracts with Customers (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended			12 Months Ended
		Sep. 24, 2021	Sep. 25, 2020	Sep. 24, 2021	Sep. 25, 2020		Dec. 25, 2020	Dec. 27, 2019
Disaggregation of Revenue [Line Items]
Revenue Reserves		$ 292.9	$ 861.4	$ 292.9	$ 861.4		$ 235.4	$ 337.4
Revenue Reserve Provision				1,649.3	1,520.3
Revenue Reserve Payments or Credits				(1,591.8)	(1,531.4)
Capitalized Contract Cost, Gross		26.5		26.5			25.8
Capitalized Contract Cost, Amortization				4.4	4.0
Net sales (includes refined estimate of the retrospective one-time charge of $0.7 million and $535.1 million related to the Medicaid lawsuit for the three and nine months ended September 25, 2020)		507.2	698.3	1,611.6	1,530.6
Medicaid lawsuit (Note 12)							(641.1)
Sales [Member]
Disaggregation of Revenue [Line Items]
Medicaid lawsuit (Note 12)		0.0	0.7	0.0	535.1	[1]	(536.0)
Operating Expense [Member]
Disaggregation of Revenue [Line Items]
Medicaid lawsuit (Note 12)		$ 0.0	$ 0.2	$ 0.0	$ (105.1)
Transferred at Point in Time [Member]
Disaggregation of Revenue [Line Items]
Revenue		80.30%	79.50%	78.70%	78.50%
Transferred over Time [Member]
Disaggregation of Revenue [Line Items]
Revenue		19.70%	20.50%	21.30%	21.50%
Royalty [Member]
Disaggregation of Revenue [Line Items]
Net sales (includes refined estimate of the retrospective one-time charge of $0.7 million and $535.1 million related to the Medicaid lawsuit for the three and nine months ended September 25, 2020)		$ 27.3	$ 20.4	$ 82.2	$ 52.3
Rebates and Chargebacks [Member]
Disaggregation of Revenue [Line Items]
Revenue Reserves		249.4	823.3	249.4	823.3		196.5	295.8
Revenue Reserve Provision				1,588.6	1,453.7
Revenue Reserve Payments or Credits				(1,535.7)	(1,461.3)
Rebates and Chargebacks [Member] | Sales [Member]
Disaggregation of Revenue [Line Items]
Medicaid lawsuit (Note 12)	[1]				(535.1)
Allowance for Sales Returns [Member]
Disaggregation of Revenue [Line Items]
Revenue Reserves		21.3	26.4	21.3	26.4		26.6	28.4
Revenue Reserve Provision				18.2	22.3
Revenue Reserve Payments or Credits				(23.5)	(24.3)
Medicaid lawsuit (Note 12)					0.0
Other Sales Deductions [Member]
Disaggregation of Revenue [Line Items]
Revenue Reserves		$ 22.2	$ 11.7	22.2	11.7		$ 12.3	$ 13.2
Revenue Reserve Provision				42.5	44.3
Revenue Reserve Payments or Credits				$ (32.6)	(45.8)
Medicaid lawsuit (Note 12)					$ 0.0

[1] Excludes the $105.1 million that is reflected as a component of operating expenses as it represents a pre-acquisition contingency related to the portion of the liability that arose from sales of Acthar Gel prior to the Company's acquisition of Questcor Pharmaceuticals Inc. ("Questcor") in August 2014. See Note 12 for further detail on the status of the Medicaid lawsuit.