Basis of Presentation and New Accounting Pronouncements (Policies)	9 Months Ended
Sep. 26, 2020
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of PresentationThe accompanying condensed consolidated financial information of FormFactor, Inc. is unaudited and has been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and pursuant to the rules and regulations of the Securities and Exchange Commission ("SEC"). However, such information reflects all adjustments, consisting only of normal recurring adjustments, which are, in the opinion of management, necessary for a fair presentation of the financial position, results of operations and cash flows for the interim periods. The financial information as of December 28, 2019 is derived from our 2019 Annual Report on Form 10-K. The condensed consolidated financial statements included herein should be read in conjunction with the consolidated financial statements and the notes thereto included in our 2019 Annual Report on Form 10-K filed with the SEC on February 21, 2020. The results of operations for the interim periods presented are not necessarily indicative of the results to be expected for the full year.
Fiscal Year	Fiscal Year We operate on a 52/53 week fiscal year, whereby the fiscal year ends on the last Saturday of December. Fiscal 2020 and 2019 each contain 52 weeks and the nine months ended September 26, 2020 and September 28, 2019 each contained 39 weeks. Fiscal 2020 will end on December 26, 2020.
New Accounting Pronouncements	New Accounting Pronouncements ASU 2016-13 In June 2016, the Financial Accounting Standard Board ("FASB") issued Accounting Standards Update ("ASU") No. 2016-13, "Measurement of Credit Losses on Financial Instruments (Topic 326)." The provisions of this standard require financial assets measured at amortized cost to be presented at the net amount expected to be collected. An allowance account would be established to present the net carrying value at the amount expected to be collected. ASU 2016-13 also provides that credit losses relating to available-for-sale debt securities should be recorded through an allowance for credit losses. The guidance was amended through various ASU's subsequent to ASU 2016-13, all of which was effective beginning fiscal 2020. We adopted ASU 2016-13 on a prospective basis on December 29, 2019, the first day of fiscal 2020. The adoption did not have a material effect on our financial position, results of operations or cash flows. ASU 2018-15 In August 2018, the FASB issued ASU 2018-15, "Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract." The new guidance clarifies the accounting for implementation costs in cloud computing arrangements. ASU 2018-15 is effective for fiscal years, including interim periods within those fiscal years, beginning after December 15, 2019. We adopted ASU 2018-15 on a prospective basis on December 29, 2019, the first day of fiscal 2020. The adoption did not have a material effect on our financial position, results of operations or cash flows. ASU 2019-12 In December 2019, the FASB issued ASU 2019-12, “Income Taxes (Topic 740),” which simplifies the accounting for income taxes by removing certain exceptions to the general principles in Topic 740. The amendments also improve consistent application of and simplify GAAP for other areas of Topic 740 by clarifying and amending existing guidance. This guidance is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2020. Early adoption of the amendments is permitted, including adoption in any interim period for which financial statements have not yet been issued. Depending on the amendment, adoption may be applied on the retrospective, modified retrospective or prospective basis. We have not yet determined the impact of this standard on our financial position, results of operations or cash flows. ASU 2020-04 In March 2020, the FASB issued ASU 2020-04, "Referenced Rate Reform (Topic 848) - Facilitation of the Effects of Reference Rate Reform on Financial Reporting." The amendments in this update apply only to contracts and hedging relationships that reference LIBOR or another reference rate expected to be discontinued due to reference rate reform. The expedients and exceptions provided by the amendments do not apply to contract modifications made and hedging relationships entered into or evaluated after December 31, 2022. The amendments in this update are elective and are effective upon issuance for all entities. We have not yet evaluated the transition approach for our LIBOR indexed contracts and have not determined whether we will be electing such expedients and exceptions. Release No. 33-10786 In May 2020, the SEC issued Final Rule Release No. 33-10786, which amends the financial statement requirements for acquisitions and dispositions of businesses and related pro forma financial information required under SEC Regulation S-X, Rule 3-05. The final rule modifies the significance test required in SEC Regulation S-X, Rule 1-02(w) by raising the significance threshold for reporting dispositions of a business from 10% to 20% and by modifying the calculation of the investment and income tests. The modifications are effective for fiscal years starting after December 31, 2020 with early adoption permitted. We early adopted these modifications on June 28, 2020, the first day of the third quarter.
Fair Value Policy	Whenever possible, the fair values of our financial assets and liabilities are determined using quoted market prices of identical securities or quoted market prices of similar securities from active markets. The three levels of inputs that may be used to measure fair value are as follows: •Level 1 valuations are obtained from real-time quotes for transactions in active exchange markets involving identical securities; •Level 2 valuations utilize significant observable inputs, such as quoted prices for similar assets or liabilities, quoted prices near the reporting date in markets that are less active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities; and •Level 3 valuations utilize unobservable inputs to the valuation methodology and include our own data about assumptions market participants would use in pricing the asset or liability based on the best information available under the circumstances. We did not have any transfers of assets or liabilities measured at fair value on a recurring basis to or from Level 1, Level 2 or Level 3 during the three and nine months ended September 26, 2020 or the year ended December 28, 2019. The carrying values of Cash, Accounts receivable, net, Restricted cash, Prepaid expenses and other current assets, Accounts payable, Accrued liabilities, and Current portion of term loans, net of unamortized issuance costs, approximate fair value due to their short maturities. No changes were made to our valuation techniques during the first nine months of fiscal 2020. Assets and Liabilities Measured at Fair Value on a Non-Recurring BasisWe measure and report our non-financial assets such as Property, plant and equipment, Goodwill and Intangible assets at fair value on a non-recurring basis if we determine these assets to be impaired or in the period when we make a business acquisition. Other than as discussed in Note 4, Acquisition, t

Basis of Presentation and New Accounting Pronouncements (Policies)

9 Months Ended

Sep. 26, 2020

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Basis of PresentationThe accompanying condensed consolidated financial information of FormFactor, Inc. is unaudited and has been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) and pursuant to the rules and regulations of the Securities and Exchange Commission ("SEC"). However, such information reflects all adjustments, consisting only of normal recurring adjustments, which are, in the opinion of management, necessary for a fair presentation of the financial position, results of operations and cash flows for the interim periods. The financial information as of December 28, 2019 is derived from our 2019 Annual Report on Form 10-K. The condensed consolidated financial statements included herein should be read in conjunction with the consolidated financial statements and the notes thereto included in our 2019 Annual Report on Form 10-K filed with the SEC on February 21, 2020. The results of operations for the interim periods presented are not necessarily indicative of the results to be expected for the full year.

Fiscal Year

We operate on a 52/53 week fiscal year, whereby the fiscal year ends on the last Saturday of December. Fiscal 2020 and 2019 each contain 52 weeks and the nine months ended September 26, 2020 and September 28, 2019 each contained 39 weeks. Fiscal 2020 will end on December 26, 2020.

New Accounting Pronouncements

ASU 2016-13

In June 2016, the Financial Accounting Standard Board ("FASB") issued Accounting Standards Update ("ASU") No. 2016-13, "Measurement of Credit Losses on Financial Instruments (Topic 326)." The provisions of this standard require financial assets measured at amortized cost to be presented at the net amount expected to be collected. An allowance account would be established to present the net carrying value at the amount expected to be collected. ASU 2016-13 also provides that credit losses relating to available-for-sale debt securities should be recorded through an allowance for credit losses. The guidance was amended through various ASU's subsequent to ASU 2016-13, all of which was effective beginning fiscal 2020. We adopted ASU 2016-13 on a prospective basis on December 29, 2019, the first day of fiscal 2020. The adoption did not have a material effect on our financial position, results of operations or cash flows.

ASU 2018-15

In August 2018, the FASB issued ASU 2018-15, "Intangibles-Goodwill and Other-Internal-Use Software (Subtopic 350-40): Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract." The new guidance clarifies the accounting for implementation costs in cloud computing arrangements. ASU 2018-15 is effective for fiscal years, including interim periods within those fiscal years, beginning after December 15, 2019. We adopted ASU 2018-15 on a prospective basis on December 29, 2019, the first day of fiscal 2020. The adoption did not have a material effect on our financial position, results of operations or cash flows.

ASU 2019-12

In December 2019, the FASB issued ASU 2019-12, “Income Taxes (Topic 740),” which simplifies the accounting for income taxes by removing certain exceptions to the general principles in Topic 740. The amendments also improve consistent application of and simplify GAAP for other areas of Topic 740 by clarifying and amending existing guidance. This guidance is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2020. Early adoption of the amendments is permitted, including adoption in any interim period for which financial statements have not yet been issued. Depending on the amendment, adoption may be applied on the retrospective, modified retrospective or prospective basis. We have not yet determined the impact of this standard on our financial position, results of operations or cash flows.

ASU 2020-04

In March 2020, the FASB issued ASU 2020-04, "Referenced Rate Reform (Topic 848) - Facilitation of the Effects of Reference Rate Reform on Financial Reporting." The amendments in this update apply only to contracts and hedging relationships that reference LIBOR or another reference rate expected to be discontinued due to reference rate reform. The expedients and exceptions provided by the amendments do not apply to contract modifications made and hedging relationships entered into or evaluated after December 31, 2022. The amendments in this update are elective and are effective upon issuance for all entities. We have not yet evaluated the transition approach for our LIBOR indexed contracts and have not determined whether we will be electing such expedients and exceptions.

Release No. 33-10786

In May 2020, the SEC issued Final Rule Release No. 33-10786, which amends the financial statement requirements for acquisitions and dispositions of businesses and related pro forma financial information required under SEC Regulation S-X, Rule 3-05. The final rule modifies the significance test required in SEC Regulation S-X, Rule 1-02(w) by raising the significance threshold for reporting dispositions of a business from 10% to 20% and by modifying the calculation of the investment and income tests. The modifications are effective for fiscal years starting after December 31, 2020 with early adoption permitted. We early adopted these modifications on June 28, 2020, the first day of the third quarter.

Fair Value Policy

Whenever possible, the fair values of our financial assets and liabilities are determined using quoted market prices of identical securities or quoted market prices of similar securities from active markets. The three levels of inputs that may be used to measure fair value are as follows:

•Level 1 valuations are obtained from real-time quotes for transactions in active exchange markets involving identical securities;

•Level 2 valuations utilize significant observable inputs, such as quoted prices for similar assets or liabilities, quoted prices near the reporting date in markets that are less active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities; and

•Level 3 valuations utilize unobservable inputs to the valuation methodology and include our own data about assumptions market participants would use in pricing the asset or liability based on the best information available under the circumstances.

We did not have any transfers of assets or liabilities measured at fair value on a recurring basis to or from Level 1, Level 2 or Level 3 during the three and nine months ended September 26, 2020 or the year ended December 28, 2019.

The carrying values of Cash, Accounts receivable, net, Restricted cash, Prepaid expenses and other current assets, Accounts payable, Accrued liabilities, and Current portion of term loans, net of unamortized issuance costs, approximate fair value due to their short maturities.

No changes were made to our valuation techniques during the first nine months of fiscal 2020.

Assets and Liabilities Measured at Fair Value on a Non-Recurring BasisWe measure and report our non-financial assets such as Property, plant and equipment, Goodwill and Intangible assets at fair value on a non-recurring basis if we determine these assets to be impaired or in the period when we make a business acquisition. Other than as discussed in Note 4, Acquisition, t