Income Taxes - Narrative (Details) SFr in Millions				3 Months Ended		12 Months Ended
	May 19, 2019	May 19, 2019	Dec. 06, 2016 CHF (SFr)	Jan. 26, 2020 USD ($)	Apr. 28, 2019 USD ($)	Jan. 26, 2020 USD ($)	Jan. 27, 2019 USD ($)	Jan. 28, 2018 USD ($)	Jan. 29, 2017 USD ($)
Income Tax [Line Items]
Tax benefit due to the decrease in the corporate tax rate								$ 2,600,000
GILTI effective tax rate						10.50%
Reduction of valuation allowance					$ 15,800,000	$ 2,722,000	$ 23,029,000	41,911,000
Provisional income tax expense due to mandatory transition tax on the deemed repatriation of undistributed foreign earnings						2,100,000
Tax attributes used to completely offset any cash tax liability resulting from the transition tax							76,500,000
Increase to tax provision expense				$ 1,900,000
Foreign earnings that will continue to be reinvested indefinitely outside of the U.S.				547,900,000		547,900,000
Foreign earnings that will be remitted in the foreseeable future						240,000,000.0
Foreign earnings remitted				76,100,000		76,100,000	80,000,000.0
Gross research credits available to offset taxable income				6,063,000		6,063,000	6,693,000
Net deferred tax assets				32,700,000		32,700,000	29,800,000
Valuation allowance				16,189,000		$ 16,189,000	18,912,000	$ 41,050,000	$ 82,961,000
Valuation allowance, methodologies and assumptions						The valuation allowances established relate to all U. S. and state deferred tax assets, for which we have determined that it is more likely than not that a benefit will not be realized. In considering in whether a valuation allowance was required for our U.S. deferred income tax assets, we considered all available positive and negative evidence. Positive evidence considered included reversing taxable temporary differences. Negative evidence considered included the cumulative taxable losses in the U.S. recorded during the three year period ended January 29, 2018, on both an annual and cumulative basis. Based on the weight of all available evidence, we concluded that the negative evidence outweighed the positive evidence and that it was more likely than not that the U.S. federal and state deferred tax assets that cannot be realized through the reversal of taxable temporary differences would not be realized. As a result, we have established a full valuation allowance against the deferred tax assets in the U.S. that will not be realized through the reversal of taxable temporary differences.
Percentage of uncertain tax positions evaluating criteria						50.00%
Net tax benefits, if recognized, would impact the effective tax rate				8,600,000		$ 8,600,000	4,500,000
Additional penalties or interest accrued						0
Unrecognized tax benefits, interest and penalties				300,000		$ 300,000	$ 0
Foreign
Income Tax [Line Items]
Description of income tax holiday						On December 6, 2016, the Company was granted a tax holiday ("Tax Holiday") with an effective date of January 30, 2017. This Tax Holiday replaces the current Swiss Ruling. The Tax Holiday provides Semtech (International) AG with a 70% reduction to the Cantonal tax rate, bringing the statutory Cantonal tax rate down from 12.56% to 3.77%. The maximum benefit under this Tax Holiday is CHF 500.0 million of cumulative after tax profit which equates to a maximum potential tax savings of CHF 44.0 million. The Tax Holiday is effective for five years and can be extended for an additional five years if the Company meets certain staffing targets by January 30, 2022.
Reduction in statutory Cantonal tax rate, percentage			70.00%
Statutory Cantonal tax rate before tax holiday, percentage			12.56%
Statutory Cantonal tax rate after tax holiday, percentage			3.77%
Maximum after-tax profit subject to potential savings | SFr			SFr 500.0
Income tax holiday, initial term			5 years
Income tax holiday, possible additional term			5 years
Gross research credits available to offset taxable income				500,000		$ 500,000
Federal
Income Tax [Line Items]
Gross research credits available to offset taxable income				11,000,000.0		11,000,000.0
State
Income Tax [Line Items]
Operating loss carryforwards				105,600,000		105,600,000
Gross research credits available to offset taxable income				$ 15,100,000		$ 15,100,000
Maximum | Foreign
Income Tax [Line Items]
Potential tax savings | SFr			SFr 44.0
Swiss Federal Tax Administration (FTA) [Member] | Foreign
Income Tax [Line Items]
Reduction in statutory Cantonal tax rate, percentage	70.00%
Statutory Cantonal tax rate after tax holiday, percentage	2.54%
Statutory Cantonal tax rate	8.46%	12.56%