Acquisitions and Divestitures - Narrative (Details)		3 Months Ended							12 Months Ended
	Jul. 02, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jul. 01, 2018 USD ($)	Apr. 01, 2018 USD ($) employee	Dec. 31, 2017 USD ($) employee	Oct. 01, 2017 USD ($)	Apr. 02, 2017 USD ($)	Mar. 27, 2016 USD ($)	Sep. 30, 2018 USD ($)	Oct. 01, 2017 USD ($) employee	Oct. 02, 2016 USD ($) employee	Sep. 27, 2015 USD ($)
Business acquisition
Contingent earn-out liability		$ 35,290,000				$ 2,438,000			$ 35,290,000	$ 2,438,000	$ 8,757,000	$ 4,169,000
Aggregate maximum of contingent consideration		50,600,000							50,600,000
Proceeds from divestiture of business									35,348,000	905,000	3,076,000
Increase (decrease) in contingent consideration									4,300,000	(6,900,000)	2,800,000
Gain (loss) on change in contingent consideration									$ (4,300,000)	6,900,000	$ (2,800,000)
Impairment of goodwill	$ 0	$ 0				0
Minimum
Business acquisition
Earn-out period									2 years
Maximum
Business acquisition
Earn-out period									3 years
Existing customer contracts | Minimum
Business acquisition
Useful life of intangible assets									1 year
Existing customer contracts | Maximum
Business acquisition
Useful life of intangible assets									10 years
Technology and trade names | Minimum
Business acquisition
Useful life of intangible assets									3 years
Technology and trade names | Maximum
Business acquisition
Useful life of intangible assets									5 years
Non-core field services business | Disposed of by Sale
Business acquisition
Proceeds from divestiture of business			$ 30,200,000
Disposal group revenue										$ 70,000,000
Loss on disposition of business			$ 3,400,000						$ 3,400,000
Coffey
Business acquisition
Number of employees | employee											3,300
Aggregate fair value of purchase prices											$ 76,100,000
Assumed debt											65,100,000
Secured bank debt assumed											37,100,000
Unsecured corporate bonds assumed											28,000,000
INDUS
Business acquisition
Aggregate fair value of purchase prices											18,700,000
Cash paid to the sellers											14,000,000
Contingent earn-out liability							$ 0	$ 4,700,000			4,900,000
Aggregate maximum of contingent consideration								$ 8,000,000			$ 8,000,000
Earn-out period								2 years			2 years
Potential earn-out to be paid each year								$ 4,000,000
Estimated potential earn-out (as a percent)								50.00%			59.00%
Minimum operating income threshold to earn contingent consideration during year one							3,200,000	$ 3,200,000
Minimum operating income threshold to earn contingent consideration during year two							3,600,000	3,600,000
Operating income to earn maximum consideration during the first year								3,600,000
Operating income to earn maximum consideration during the second year								$ 4,000,000
Gain (loss) on change in contingent consideration							5,000,000
Impairment of goodwill							$ 0
ELA
Business acquisition
Number of employees | employee										160
Aggregate fair value of purchase prices										$ 9,900,000
Cash paid to the sellers										8,300,000
Contingent earn-out liability						1,600,000				1,600,000
Aggregate maximum of contingent consideration						$ 1,700,000				$ 1,700,000
Earn-out period										2 years
Glumac
Business acquisition
Number of employees | employee					300
Aggregate fair value of purchase prices					$ 38,400,000
Cash paid to the sellers					20,000,000
Contingent earn-out liability					18,400,000
Aggregate maximum of contingent consideration					$ 20,000,000
Earn-out period					3 years
Cash settlement of earn-out liability									$ 1,500,000
NDY
Business acquisition
Number of employees | employee				700
Aggregate fair value of purchase prices				$ 56,100,000
Cash paid to the sellers				46,900,000
Contingent earn-out liability				7,600,000
Aggregate maximum of contingent consideration				$ 20,200,000
Earn-out period				3 years
Escrow deposit				$ 1,600,000
CEG
Business acquisition
Increase (decrease) in contingent consideration										$ (1,800,000)
Other Acquisitions
Business acquisition
Contingent earn-out liability												0
Cash settlement of earn-out liability												$ 1,000,000