Income Taxes - Narrative (Details) $ in Thousands, SFr in Millions		12 Months Ended
	Dec. 06, 2016 CHF (SFr)	Jan. 28, 2018 USD ($)	Jan. 29, 2017 USD ($)	Apr. 30, 2017 USD ($)	Jan. 31, 2016 USD ($)	Jan. 25, 2015 USD ($)
Income Tax [Line Items]
Tax benefit due to the decrease in the corporate tax rate		$ 2,600
Corporate tax rate		21.00%	35.00%
Provisional income tax expense due to mandatory transition tax on the deemed repatriation of undistributed foreign earnings		$ 2,100
Tax attributes utilized to reduce current tax liability in connection with mandatory transition tax		78,400
Current tax liability resulting from the transition tax		$ 1,100
Mandatory transition tax rate to the extent of cash and cash equivalents held by foreign affiliates		15.50%
Mandatory transition tax rate on remaining earnings and profits		8.00%
Foreign earnings that will continue to be reinvested indefinitely outside of the U.S.		$ 453,600
Foreign earnings that will be remitted in the foreseeable future		240,000
Deferred tax asset recognized for windfall tax benefits		8,400		$ 8,400
Gross research credits available to offset taxable income		11,707	$ 25,770
Net deferred tax assets		30,600	81,600
Valuation allowance		$ 41,050	82,961		$ 77,383	$ 75,536
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		$ 3,900	9,300
Unrecognized tax benefits, interest and penalties		$ 0	$ 300
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		$ 11,600
Federal
Income Tax [Line Items]
Operating loss carryforwards		0
Gross research credits available to offset taxable income		6,300
State
Income Tax [Line Items]
Operating loss carryforwards		111,400
Gross research credits available to offset taxable income		$ 14,600
Maximum | Foreign
Income Tax [Line Items]
Potential tax savings | SFr	SFr 44.0