Balance Sheet Details - Accounts Receivable and Inventory (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 25, 2016	Dec. 27, 2015	Dec. 28, 2014
Cash and Cash Equivalents
Cash and cash equivalents	$ 129.6	$ 69.1	$ 28.5	$ 33.5
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total current accounts receivable	269.0	231.1
Allowance for doubtful accounts	(0.6)	(1.7)
Total accounts receivable, net	268.4	229.4
Retainages receivable	6.6	6.2
Inventory, Net of Allowances, Customer Advances and Progress Billings [Abstract]
Raw materials	35.4	31.9
Work in process	11.4	22.1
Finished goods	2.3	1.4
Supplies and other	1.9	1.8
Subtotal inventoried costs	51.0	57.2
Less customer advances and progress payments	(0.6)	(1.8)
Total inventoried costs	50.4	55.4
Billed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total current accounts receivable	101.5	102.8
Unbilled
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total current accounts receivable	167.5	128.3
U.S. Government
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total U.S. Government contract receivables	72.6	54.9
U.S. Government | Billed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total U.S. Government contract receivables	17.3	15.6
U.S. Government | Unbilled
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total U.S. Government contract receivables	$ 55.3	$ 39.3