Summary of Significant Accounting Policies - Balance Sheet (Details) - USD ($) $ in Millions	Apr. 01, 2018	Jan. 01, 2018	Dec. 31, 2017	Dec. 25, 2016
Assets
Accounts receivable, net	$ 57.7		$ 74.2
Unbilled receivable, net	151.2		138.1
Inventoried costs	48.2		49.0
Liabilities
Accrued expenses	33.8		40.9
Billings in excess of costs and earnings on uncompleted contracts	38.2		42.8
Stockholders’ equity:
Accumulated deficit	(723.2)		(720.8)
ASC 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impact from adoption of ASC 606			$ (0.2)	$ 0.0
Assets
Unbilled receivable, net		$ 139.4
Inventoried costs		48.7
Liabilities
Accrued expenses		40.3
Billings in excess of costs and earnings on uncompleted contracts		44.6
Stockholders’ equity:
Accumulated deficit		(721.0)
ASC 606 | Balances Without Adoption of ASC 606
Assets
Accounts receivable, net	55.6
Unbilled receivable, net	147.1
Inventoried costs	52.4
Liabilities
Billings in excess of costs and earnings on uncompleted contracts	39.7
Stockholders’ equity:
Accumulated deficit	(726.7)
ASC 606 | Effect of Change Higher/(Lower)
Assets
Accounts receivable, net	2.1
Unbilled receivable, net	4.1
Inventoried costs	(4.2)
Liabilities
Billings in excess of costs and earnings on uncompleted contracts	(1.5)
Stockholders’ equity:
Accumulated deficit	$ 3.5
Balance at January 1, 2018 | ASC 606
Assets
Unbilled receivable, net		138.1
Inventoried costs		49.0
Liabilities
Accrued expenses		40.9
Billings in excess of costs and earnings on uncompleted contracts		42.8
Stockholders’ equity:
Accumulated deficit		(720.8)
ASC 606 Adjustment | ASC 606
Assets
Unbilled receivable, net		1.3
Inventoried costs		(0.3)
Liabilities
Accrued expenses		(0.6)
Billings in excess of costs and earnings on uncompleted contracts		1.8
Stockholders’ equity:
Accumulated deficit		$ (0.2)