Balance Sheet Details - Accounts Receivable and Inventory (Details) - USD ($) $ in Millions	Dec. 25, 2016	Dec. 27, 2015	Dec. 28, 2014	Dec. 29, 2013
Cash and Cash Equivalents [Abstract]
Cash and cash equivalents	$ 69.1	$ 28.5	$ 33.5	$ 54.2
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total current accounts receivable	231.1	208.6
Allowance for doubtful accounts	(1.7)	(1.8)
Total accounts receivable, net	229.4	206.8
Retainages receivable	6.2	6.4
Inventory, Net of Allowances, Customer Advances and Progress Billings [Abstract]
Raw materials	31.9	32.9
Work in process	22.1	19.2
Finished goods	1.4	2.6
Supplies and other	1.8	1.6
Subtotal inventoried costs	57.2	56.3
Less customer advances and progress payments	(1.8)	(0.7)
Total inventoried costs	55.4	55.6
Billed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total current accounts receivable	102.8	96.6
Unbilled
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total current accounts receivable	128.3	112.0
U.S. Government
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total U.S. Government contract receivables	54.9	44.5
U.S. Government | Billed
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total U.S. Government contract receivables	15.6	13.5
U.S. Government | Unbilled
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total U.S. Government contract receivables	$ 39.3	$ 31.0