Restructuring and Related Charges - Additional Information (Detail) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Aug. 31, 2018 USD ($)	Jan. 31, 2016 USD ($)	Sep. 28, 2018 USD ($)	Sep. 29, 2017 USD ($)	Sep. 28, 2018 USD ($) Employees	Sep. 29, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) Employees	Aug. 01, 2018 USD ($)	Jan. 04, 2016 USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve			$ 6,365		$ 6,365		$ 8,043
Restructuring Reserve, Current			3,800		3,800		4,400
Restructuring Reserve, Noncurrent			2,600		2,600		3,600
Restructuring and related charges			987	$ 2,028	2,704	$ 4,084
Restructuring charges					3,056
Payments for Restructuring					5,517
Adjustments to restructuring liability					532
Harmonic 2017 Restructuring Plan [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges							2,500
Harmonic 2016 Restructuring Plan [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and related charges								$ 20,000
Facility Closing [Member] | Harmonic 2018 Restructuring Plan [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve			1,214		1,214
Restructuring charges					932
Payments for Restructuring					51
Adjustments to restructuring liability					1
Facility Closing [Member] | Harmonic 2017 Restructuring Plan [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve			187		187		296
Restructuring Reserve, Noncurrent			200		200
Restructuring charges							400
Payments for Restructuring					109
Facility Closing [Member] | Harmonic 2016 Restructuring Plan [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve			1,770		1,770		2,426
Restructuring and related charges								2,200
Payments for Restructuring					755
Adjustments to restructuring liability					99
Employee Severance And Benefit Arrangements [Member] | Harmonic 2016 Restructuring Plan [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and related charges								$ 17,800
Number of positions eliminated | Employees								118
Employee Severance [Member] | Harmonic 2018 Restructuring Plan [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve			6		6
Restructuring charges					$ 2,124
Number of positions eliminated | Employees					59
Payments for Restructuring					$ 2,046
Adjustments to restructuring liability					(72)
Employee Severance [Member] | Harmonic 2017 Restructuring Plan [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve			0		0		193
Restructuring charges							2,100
Payments for Restructuring					193
TVN Voluntary Departure Plan [Member] | Harmonic 2016 Restructuring Plan [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve			3,188		3,188		5,128
Payments for Restructuring					2,363
Adjustments to restructuring liability					504
TVN [Member]
Restructuring Cost and Reserve [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Curtailment								$ 2,000
Postemployment Benefits Liability			3,200		3,200		5,100
TVN [Member] | Employee Severance And Benefit Arrangements [Member] | Harmonic 2016 Restructuring Plan [Member]
Restructuring Cost and Reserve [Line Items]
Number of positions eliminated | Employees								83
TVN [Member] | TVN Voluntary Departure Plan [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges					500	$ 1,800
Postemployment Benefits Liability			$ 3,200		$ 3,200
San Jose CA Excess Facility [Member] | Facility Closing [Member] | Harmonic 2018 Restructuring Plan [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 900
San Jose CA Excess Facility [Member] | Facility Closing [Member] | Harmonic 2016 Restructuring Plan [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring charges		$ 1,400
Fair Value of Excess Facility Restructuring									$ 1,200	$ 2,500
Deferred Rent Credit									$ 300	$ 1,100
Adjustments to restructuring liability							$ 1,200