Condensed Consolidating Financial Statements - Balance Sheet (Details) - USD ($)	Apr. 01, 2018	Dec. 31, 2017	Mar. 26, 2017	Dec. 25, 2016	May 31, 2014
Condensed Financial Statements, Captions [Line Items]
Principal	$ 300,500,000	$ 300,800,000
Current assets:
Cash, cash equivalents and restricted cash	128,200,000	130,900,000	$ 74,100,000	$ 70,700,000
Accounts receivable and unbilled receivables, net	208,900,000	212,300,000
Amounts due from affiliated companies	0	0
Inventoried costs	48,200,000	49,000,000
Other current assets	19,700,000	20,600,000
Current assets of discontinued operations	52,100,000	58,600,000
Total current assets	457,100,000	471,400,000
Property, plant and equipment, net	61,500,000	58,000,000
Goodwill	425,700,000	425,700,000
Intangible assets, net	20,300,000	22,000,000
Investment in subsidiaries	0	0
Other assets	7,800,000	8,100,000
Non-current assets of discontinued operations	38,800,000	38,800,000
Total assets	1,011,200,000	1,024,000,000
Current liabilities:
Accounts payable	31,300,000	34,700,000
Accrued expenses	40,400,000	42,600,000
Accrued compensation	34,100,000	30,200,000
Billings in excess of costs and earnings on uncompleted contracts	38,200,000	42,800,000
Amounts due to affiliated companies	0	0
Other current liabilities	7,800,000	9,400,000
Current liabilities of discontinued operations	23,200,000	29,200,000
Total current liabilities	175,000,000	188,900,000
Long-term debt, net of current portion	293,600,000	293,500,000
Other long-term liabilities	24,200,000	24,100,000
Non-current liabilities of discontinued operations	5,900,000	6,000,000
Total liabilities	498,700,000	512,500,000
Total stockholders’ equity	512,500,000	511,500,000
Total liabilities and stockholders’ equity	1,011,200,000	1,024,000,000
Eliminations
Current assets:
Cash, cash equivalents and restricted cash	0	0
Accounts receivable and unbilled receivables, net	0	0
Amounts due from affiliated companies	(240,300,000)	(238,100,000)
Inventoried costs	0	0
Other current assets	0	0
Current assets of discontinued operations	0	0
Total current assets	(240,300,000)	(238,100,000)
Property, plant and equipment, net	0	0
Goodwill	0	0
Intangible assets, net	0	0
Investment in subsidiaries	(548,100,000)	(541,100,000)
Other assets	0	0
Non-current assets of discontinued operations	0	0
Total assets	(788,400,000)	(779,200,000)
Current liabilities:
Accounts payable	0	0
Accrued expenses	0	0
Accrued compensation	0	0
Billings in excess of costs and earnings on uncompleted contracts	0	0
Amounts due to affiliated companies	(240,300,000)	(238,100,000)
Other current liabilities	0	0
Current liabilities of discontinued operations	0	0
Total current liabilities	(240,300,000)	(238,100,000)
Long-term debt, net of current portion	0	0
Other long-term liabilities	0	0
Non-current liabilities of discontinued operations	0	0
Total liabilities	(240,300,000)	(238,100,000)
Total stockholders’ equity	(548,100,000)	(541,100,000)
Total liabilities and stockholders’ equity	$ (788,400,000)	(779,200,000)
Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Ownership percentage	100.00%
Parent Company
Current assets:
Cash, cash equivalents and restricted cash	$ 118,200,000	121,300,000
Accounts receivable and unbilled receivables, net	0	0
Amounts due from affiliated companies	240,300,000	238,100,000
Inventoried costs	0	0
Other current assets	3,100,000	3,700,000
Current assets of discontinued operations	0	0
Total current assets	361,600,000	363,100,000
Property, plant and equipment, net	1,700,000	1,900,000
Goodwill	0	0
Intangible assets, net	0	0
Investment in subsidiaries	475,400,000	471,100,000
Other assets	800,000	800,000
Non-current assets of discontinued operations	0	0
Total assets	839,500,000	836,900,000
Current liabilities:
Accounts payable	2,200,000	2,300,000
Accrued expenses	8,700,000	5,700,000
Accrued compensation	5,000,000	5,600,000
Billings in excess of costs and earnings on uncompleted contracts	0	0
Amounts due to affiliated companies	0	0
Other current liabilities	700,000	1,400,000
Current liabilities of discontinued operations	600,000	1,000,000
Total current liabilities	17,200,000	16,000,000
Long-term debt, net of current portion	293,600,000	293,500,000
Other long-term liabilities	12,400,000	12,100,000
Non-current liabilities of discontinued operations	3,800,000	3,800,000
Total liabilities	327,000,000	325,400,000
Total stockholders’ equity	512,500,000	511,500,000
Total liabilities and stockholders’ equity	839,500,000	836,900,000
Subsidiary Guarantors on a Combined Basis
Current assets:
Cash, cash equivalents and restricted cash	(1,600,000)	(1,100,000)
Accounts receivable and unbilled receivables, net	181,800,000	183,800,000
Amounts due from affiliated companies	0	0
Inventoried costs	29,900,000	30,800,000
Other current assets	12,100,000	13,500,000
Current assets of discontinued operations	52,100,000	58,600,000
Total current assets	274,300,000	285,600,000
Property, plant and equipment, net	53,000,000	49,300,000
Goodwill	382,800,000	382,800,000
Intangible assets, net	14,600,000	15,800,000
Investment in subsidiaries	72,700,000	70,000,000
Other assets	7,000,000	7,300,000
Non-current assets of discontinued operations	38,800,000	38,800,000
Total assets	843,200,000	849,600,000
Current liabilities:
Accounts payable	25,200,000	27,900,000
Accrued expenses	29,800,000	33,600,000
Accrued compensation	25,600,000	20,700,000
Billings in excess of costs and earnings on uncompleted contracts	36,000,000	38,200,000
Amounts due to affiliated companies	207,600,000	206,400,000
Other current liabilities	2,300,000	4,300,000
Current liabilities of discontinued operations	22,500,000	28,100,000
Total current liabilities	349,000,000	359,200,000
Long-term debt, net of current portion	0	0
Other long-term liabilities	4,700,000	5,100,000
Non-current liabilities of discontinued operations	2,100,000	2,200,000
Total liabilities	355,800,000	366,500,000
Total stockholders’ equity	487,400,000	483,100,000
Total liabilities and stockholders’ equity	843,200,000	849,600,000
Non-Guarantors on a Combined Basis
Current assets:
Cash, cash equivalents and restricted cash	11,600,000	10,700,000
Accounts receivable and unbilled receivables, net	27,100,000	28,500,000
Amounts due from affiliated companies	0	0
Inventoried costs	18,300,000	18,200,000
Other current assets	4,500,000	3,400,000
Current assets of discontinued operations	0	0
Total current assets	61,500,000	60,800,000
Property, plant and equipment, net	6,800,000	6,800,000
Goodwill	42,900,000	42,900,000
Intangible assets, net	5,700,000	6,200,000
Investment in subsidiaries	0	0
Other assets	0	0
Non-current assets of discontinued operations	0	0
Total assets	116,900,000	116,700,000
Current liabilities:
Accounts payable	3,900,000	4,500,000
Accrued expenses	1,900,000	3,300,000
Accrued compensation	3,500,000	3,900,000
Billings in excess of costs and earnings on uncompleted contracts	2,200,000	4,600,000
Amounts due to affiliated companies	32,700,000	31,700,000
Other current liabilities	4,800,000	3,700,000
Current liabilities of discontinued operations	100,000	100,000
Total current liabilities	49,100,000	51,800,000
Long-term debt, net of current portion	0	0
Other long-term liabilities	7,100,000	6,900,000
Non-current liabilities of discontinued operations	0	0
Total liabilities	56,200,000	58,700,000
Total stockholders’ equity	60,700,000	58,000,000
Total liabilities and stockholders’ equity	116,900,000	$ 116,700,000
Senior notes | 7% Senior Notes due 2019
Condensed Financial Statements, Captions [Line Items]
Principal	$ 300,000,000				$ 625,000,000.0