Fitbit 10-Q 6 Aug 18 2018 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2018	Jul. 26, 2018
Entity Information [Line Items]
Entity Registrant Name	FITBIT INC
Entity Central Index Key	1,447,599
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-Q
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q2
Amendment Flag	false
Common Class A [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		214,090,259
Common Class B [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		31,281,638

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 328,116	$ 341,966
Marketable securities	252,347	337,334
Accounts receivable, net	242,038	406,019
Inventories	140,430	123,895
Income tax receivable	78,385	77,882
Prepaid expenses and other current assets	63,227	97,269
Total current assets	1,104,543	1,384,365
Property and equipment, net	110,185	104,908
Goodwill	61,058	51,036
Intangible assets, net	27,740	22,356
Deferred tax assets	3,528	3,990
Other assets	16,147	15,420
Total assets	1,323,201	1,582,075
Current liabilities:
Accounts payable	155,970	212,731
Accrued liabilities	377,058	452,137
Deferred revenue	28,116	35,504
Income taxes payable	22,729	928
Total current liabilities	583,873	701,300
Long-term deferred revenue	4,719	6,928
Other liabilities	55,650	49,884
Total liabilities	644,242	758,112
Commitments and contingencies (Note 6)
Stockholders’ equity:
Class A and Class B common stock	25	24
Additional paid-in capital	1,006,639	956,060
Accumulated other comprehensive income (loss)	4,679	(9)
Accumulated deficit	(332,384)	(132,112)
Total stockholders’ equity	678,959	823,963
Total liabilities and stockholders’ equity	$ 1,323,201	$ 1,582,075

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Income Statement [Abstract]
Revenue	$ 299,344	$ 353,299	$ 547,209	$ 652,241
Cost of revenue	180,329	204,054	314,071	384,697
Gross profit	119,015	149,245	233,138	267,544
Operating expenses:
Research and development	87,047	80,543	176,383	168,301
Sales and marketing	100,845	100,732	172,897	191,906
General and administrative	30,211	31,379	66,299	62,125
Total operating expenses	218,103	212,654	415,579	422,332
Operating loss	(99,088)	(63,409)	(182,441)	(154,788)
Interest income, net	2,177	193	3,527	1,289
Other income, net	2,258	303	2,775	836
Loss before income taxes	(94,653)	(62,913)	(176,139)	(152,663)
Income tax expense (benefit)	23,615	(4,673)	23,006	(34,344)
Net loss	$ (118,268)	$ (58,240)	$ (199,145)	$ (118,319)
Net loss per share:
Basic (in dollars per share)	$ (0.49)	$ (0.25)	$ (0.83)	$ (0.52)
Diluted (in dollars per share)	$ (0.49)	$ (0.25)	$ (0.83)	$ (0.52)
Shares used to compute net loss per share:
Basic (in shares)	242,898	230,322	241,227	228,788
Diluted (in shares)	242,898	230,322	241,227	228,788

Condensed Consolidated Stateme4

Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Statement of Comprehensive Income [Abstract]
Net loss	$ (118,268)	$ (58,240)	$ (199,145)	$ (118,319)
Cash flow hedges:
Change in unrealized gain (loss) on cash flow hedges, net of tax benefit (expense) of $(666), $404, $(666) and $202, respectively	5,248	(12,629)	5,912	(13,789)
Less: reclassification for realized net gains included in net loss, net of tax expense (benefit) of $130, $(46), $130 and $(8), respectively	(1,028)	1,430	(1,028)	1,647
Net change, net of tax	4,220	(11,199)	4,884	(12,142)
Change in foreign currency translation adjustment	0	585	0	314
Change in unrealized loss on available-for-sale investments, net of tax	118	9	(208)	85
Less reclassification for realized net gains	12	(3)	12	(12)
Net change, net of tax	130	6	(196)	73
Comprehensive loss	$ (113,918)	$ (68,848)	$ (194,457)	$ (130,074)

Condensed Consolidated Stateme5

Condensed Consolidated Statements of Comprehensive Income (Loss) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Statement of Comprehensive Income [Abstract]
Change in unrealized gain (loss) on cash flow hedges, tax	$ (666)	$ 404	$ (666)	$ 202
Reclassification for realized net gains included in net income, tax	$ 130	$ (46)	$ 130	$ (8)

Condensed Consolidated Stateme6

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017
Cash Flows from Operating Activities
Net loss	$ (199,145)	$ (118,319)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:
Provision for inventory obsolescence	8,014	8,409
Depreciation	23,572
Depreciation and amortization expense	23,600	19,199
Write-off of property and equipment	7,485	5,250
Amortization of intangible assets	3,805	2,753
Stock-based compensation	49,498	44,459
Deferred income taxes	(1,299)	16,137
Other	(419)	1,428
Changes in operating assets and liabilities, net of acquisition:
Accounts receivable	164,195	261,165
Inventories	(24,217)	81,486
Prepaid expenses and other assets	37,624	(50,324)
Fitbit Force recall reserve	(291)	(500)
Accounts payable	(64,184)	(216,959)
Accrued liabilities and other liabilities	(74,033)	(43,963)
Deferred revenue	(9,623)	(5,477)
Income taxes payable	21,801	(1,516)
Net cash (used in) provided by operating activities	(57,217)	3,228
Cash Flows from Investing Activities
Purchase of property and equipment	(28,524)	(39,817)
Purchases of marketable securities	(224,812)	(317,678)
Sales of marketable securities	73,770	13,806
Maturities of marketable securities	236,575	351,144
Acquisition, net of cash acquired	(13,646)	0
Net cash provided by investing activities	43,363	7,455
Cash Flows from Financing Activities
Repayment of debt	(747)	0
Proceeds from issuance of common stock	10,738	11,407
Taxes paid related to net share settlement of restricted stock units	(9,987)	(5,234)
Net cash provided by financing activities	4	6,173
Net (decrease) increase in cash and cash equivalents	(13,850)	16,856
Effect of exchange rate on cash and cash equivalents	0	532
Cash and cash equivalents at beginning of period	341,966	301,320
Cash and cash equivalents at end of period	$ 328,116	$ 318,708

Basis of Presentation and Summa

Basis of Presentation and Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation and Summary of Significant Accounting Policies	Basis of Presentation and Summary of Significant Accounting Policies The accompanying condensed consolidated financial statements of Fitbit, Inc. (the “Company”) are unaudited. The condensed consolidated balance sheet at December 31, 2017 has been derived from the audited financial statements of the Company. The accompanying condensed financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) for interim financial information, and in management’s opinion, includes all adjustments, consisting of only normal recurring adjustments, necessary for the fair statement of the Company’s financial position, its results of operations, and cash flows for the interim periods presented. The results of operations for the three and six months ended June 30, 2018 are not necessarily indicative of the results to be expected for the full fiscal year or any other period. The accompanying condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto contained in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2017, filed with the Securities and Exchange Commission (“SEC”) on March 1, 2018. The Company’s fiscal year ends on December 31 of each year. The Company is on a 4-4-5 week quarterly calendar. There were 91 days in each of the three months ended June 30, 2018 and July 1, 2017 , and 181 and 182 days in the six months ended June 30, 2018 and July 1, 2017 , respectively. The condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany transactions and balances have been eliminated. Use of Estimates The preparation of condensed consolidated financial statements in accordance with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported and disclosed in the condensed consolidated financial statements and accompanying notes. The primary estimates and assumptions made by management are related to revenue recognition, reserves for sales returns and incentives, reserves for warranty, valuation of stock options, fair value of derivative assets and liabilities, allowance for doubtful accounts, inventory valuation, fair value of goodwill and acquired tangible and intangible assets and liabilities assumed during acquisitions, the number of reportable segments, the recoverability of intangible assets and their useful lives, contingencies, and income taxes. Actual results could differ from those estimates, and such differences may be material to the condensed consolidated financial statements. Significant Accounting Policies There have been no significant changes in the Company’s accounting policies from those disclosed in its Annual Report on Form 10-K, except for the policies described below in relation to the adoption of Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers (Topic 606) , discussed below in the section titled “ Accounting Pronouncements Recently Adopted .” Revenue Recognition The Company recognizes revenue upon transfer of control of promised goods or services to customers in an amount that reflects the consideration the Company expects to receive in exchange for those goods or services. Products and Services The Company derives substantially all of its revenue from sales of its wearable devices, which includes both connected health and fitness devices and accessories and smartwatches. The Company also generates a small portion of revenue from its subscription-based services. The Company considers delivery of its products to have occurred once control has transferred and delivery of services to have occurred as control is transferred. The Company recognizes revenue, net of estimated sales returns, sales incentives, discounts, and sales tax. Arrangements with Multiple Performance Obligations The Company enters into contracts that have multiple performance obligations that include hardware, software, and services. The first performance obligation is the hardware and firmware essential to the functionality of the connected health and fitness device or smartwatch delivered at the time of sale. The second performance obligation is the software services included with the products, which are provided free of charge and enable users to sync, view, and access real-time data on the Company’s online dashboard and mobile apps. The third performance obligation is the embedded right included with the purchase of the device to receive, on a when-and-if-available basis, future unspecified firmware upgrades and features relating to the product’s essential firmware. In addition, the Company occasionally offers a fourth performance obligation in bundled arrangements that allows access to subscription-based services related to the Company’s Fitbit Coach offering. The Company allocates revenue to all performance obligations based on their relative standalone selling prices (“SSP”). The Company’s process for determining its SSP considers multiple factors including consumer behaviors, the Company’s internal pricing model, and cost-plus margin and may vary depending upon the facts and circumstances related to each deliverable. SSP for the health and fitness devices and smartwatches reflect the Company’s best estimate of the selling prices if they were sold regularly on a stand-alone basis and comprise the majority of the arrangement consideration. SSP for upgrade rights currently ranges from $1.00 to $3.00 . SSP for the online dashboard and mobile apps is currently estimated at $0.99 . SSP for access to Fitbit Coach subscription-based services is based on the price charged when sold separately. Amounts allocated to the delivered wearable devices are recognized at the time of delivery, provided the other conditions for revenue recognition have been met. Amounts allocated to the online dashboard and mobile apps and unspecified upgrade rights are deferred and recognized on a straight-line basis over the estimated usage period. The Company offers its users the ability to purchase subscription-based services, through which the users receive incremental features, including access to a digital personal trainer, in-depth analytics regarding the user’s personal metrics, or video-based customized workouts. Amounts paid for subscriptions are deferred and recognized ratably over the service period, which is typically one year. Revenue from subscription-based services was less than 1% of revenue for all periods presented. In addition, the Company offers subscription-based software and services to certain customers in the corporate wellness program, which includes a real-time dashboard, and the ability to create corporate challenges. SSP for the corporate wellness subscription is determined based on the Company’s internal pricing model for anticipated renewals for existing customers and pricing for new customers. Revenue allocated to the corporate wellness subscription is deferred and recognized on a straight-line basis over the estimated access period of one year, which is the typical service period. Revenue for corporate wellness software and services was less than 1% of revenue for all periods presented. The Company applies a practical expedient to expense costs to obtain a contract with a customer as incurred when the amortization period would be one year or less. The Company applies a practical expedient to not consider the effect of a significant financing component as it expects that the period between transfer of control and payment from customer to be one year or less. The Company accounts for shipping and handling fees billed to customers as revenue. Sales taxes and value added taxes (“VAT”) collected from customers which are remitted to governmental authorities are not included in revenue, and are reflected as a liability on the consolidated balance sheets. Rights of Return, Stock Rotation Rights, and Price Protection The Company offers limited rights of return, stock rotation rights, and price protection under various policies and programs with its retailer and distributor customers and end-users. Below is a summary of the general provisions of such policies and programs: • Retailers and distributors are generally allowed to return products that were originally sold through to an end-user under provisions of their contracts, called “open-box” returns, and such returns may be made at any time after the original sale. • All purchases through Fitbit.com are covered by a 45-day right of return. • Certain distributors are allowed stock rotation rights which are limited rights of return of products purchased during a prior period, generally one quarter. • Certain distributors and retailers are allowed return rights for defective products. • Certain distributors are offered price protection that allows for the right to a partial credit for unsold inventory held by the distributor if the Company reduces the selling price of a product. The Company estimates reserves for these policies and programs based on historical experience, and records the reserves as a reduction of revenue and an accrued liability. Through June 30, 2018 , actual returns have primarily been open-box returns. In addition, through June 30, 2018 , the Company has had limited price protection claims. On a quarterly basis, the amount of revenue that is reserved for future returns is calculated based on historical trends and data specific to each reporting period. The historical trends consider product life cycles, new product introductions, market acceptance of products, product sell-through, the type of customer, seasonality, and other factors. Return rates can fluctuate over time, but have been sufficiently predictable to allow the Company to estimate expected future product returns. The Company reviews the actual returns evidenced in prior quarters as a percent of related revenue to determine the historical rate of returns. The Company then applies the historical rate of returns to the current period revenue as a basis for estimating future returns. When necessary, the Company also provides a specific reserve for products in the distribution channel in excess of estimated requirements. This estimate can be affected by the amount of a particular product in the channel, the rate of sell-through, product plans, and other factors. The Company also considers whether there are circumstances which may result in anticipated returns higher than the historical return rate from direct customers and records an additional specific reserve as necessary. The estimates and assumptions used to reserve for rights of return, stock rotation rights, and price protection have been accurate in all material respects and have not materially changed in the past. Sales Incentives The Company offers sales incentives through various programs, consisting primarily of cooperative advertising and marketing development fund programs. The Company records advertising and marketing development fund programs with customers as a reduction to revenue unless it receives a distinct benefit in exchange for credits claimed by the customer and can reasonably estimate the fair value of the distinct benefit received, in which case the Company records it as a marketing expense. The Company recognizes a liability and reduces revenue for rebates or other incentives based on the estimated amount of rebates or credits that will be claimed by customers. Refer to Note 10 for disaggregated revenue by geographic region, based on ship-to destinations. Customer Bankruptcy In September 2017, Wynit Distribution (“Wynit”) filed for bankruptcy protection under Chapter 11 of the United States Bankruptcy Code. Wynit was the Company’s largest customer, historically representing 11% of total revenue during the six months ended July 1, 2017 and 19% of total accounts receivables as of July 1, 2017. In connection with Wynit’s bankruptcy filing, the Company believed that the collectability of the product shipments to Wynit during the third quarter of 2017 was not reasonably assured. However, as of July 1, 2017, collectability of accounts receivables from Wynit was reasonably assured. The Company ceased to recognize revenue from Wynit, which totaled $8.1 million during the third quarter of 2017. Additionally, the Company recorded a charge of $35.8 million during the third quarter ended September 30, 2017 comprised of cost of revenue of $5.5 million associated with shipments to Wynit in the third quarter of 2017 and bad debt expense of $30.3 million associated with all of Wynit’s outstanding accounts receivables. The Company maintains credit insurance that covers a portion of the exposure related to its customer receivables. The Company recorded an insurance receivable based on an analysis of its insurance policies, including their exclusions, an assessment of the nature of the claim, and information from its insurance carrier. As of September 30, 2017, the Company had recorded an insurance receivable of $26.8 million , included in prepaid expenses and other current assets, associated with the amount it had concluded was probable related to the claim. The $26.8 million insurance receivable allowed the Company to recover $22.7 million of bad debt expense and $4.1 million of cost of revenue, resulting in a net charge of $9.0 million in the consolidated statement of operations comprised of net bad debt expense of $7.6 million and net cost of revenue of $1.4 million . The Company received $21.4 million of the insurance receivable during the fourth quarter of 2017 and the remaining $5.4 million in January 2018. During the six months ended June 30, 2018, the Company released $12.4 million in outstanding product return and rebate reserves related to Wynit, as it believed the possibility of future claims associated with these reserves was remote. This reserve release resulted in a $12.4 million increase in revenue during the six months ended June 30, 2018. Non-Monetary Transaction The Company entered into an agreement with a third party during 2016 to exchange inventory for advertising credits and cash. The Company recorded the transaction based on the estimated fair value of the products exchanged. For the year ended December 31, 2016, the Company recorded $15.0 million of revenue and $7.0 million of associated cost of goods sold upon exchange of the products for advertising credits of $13.0 million and cash of $2.0 million . The $13.0 million of unused advertising credits remaining as of December 31, 2016 were recorded in prepaid expenses and other current assets, and other assets. Such credits are expected to be used over the contractual period of four years, and will be expensed as advertising services are received. During the three months ended June 30, 2018 and July 1, 2017 , $1.0 million and $0.2 million , respectively, and during the six months ended June 30, 2018 and July 1, 2017 , $1.1 million and $0.3 million , respectively, of credits were utilized in each of these periods. The Company’s prepaid and other assets related to unused advertising credits as of June 30, 2018 and December 31, 2017 were $11.1 million and $12.2 million , respectively. Recent Accounting Pronouncements Accounting Pronouncements Not Yet Adopted In February 2016, the Financial Accounting Standards Board (the “FASB”) issued ASU 2016-02, Leases (Topic 842). ASU 2016-02 requires lessees to recognize right-of-use assets and lease liabilities for operating leases, initially measured at the present value of the lease payments, on the balance sheet. ASU 2016-02 will become effective for the Company on January 1, 2019, and requires adoption using a modified retrospective approach. The Company is currently evaluating the impact of this guidance on its consolidated financial statements. The Company anticipates that the adoption will have a material impact on its consolidated balance sheets, as it will now include a right of use asset and a lease liability for the obligation to make lease payments related to substantially all operating lease arrangements; however, the Company does not expect the adoption to have a material impact on its consolidated statements of operations. In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. ASU 2016-13 provides for a new impairment model which requires measurement and recognition of expected credit losses for most financial assets and certain other instruments, including but not limited to accounts receivable and available for sale debt securities. ASU 2016-13 will become effective for the Company on January 1, 2020 and early adoption is permitted. The Company is currently evaluating the impact of this guidance on its consolidated financial statements. In January 2017, the FASB issued ASU 2017-04, Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment. ASU 2017-04 simplifies the subsequent measurement of goodwill by eliminating the second step of the goodwill impairment test. The second step measures a goodwill impairment loss by comparing the implied fair value of a reporting unit’s goodwill with the carrying amount of that goodwill. Under ASU 2017-04, a company will record an impairment charge based on the excess of a reporting unit’s carrying amount over its fair value. ASU 2017-04 will be applied prospectively and is effective for annual or interim goodwill impairment tests in fiscal years beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. The Company is currently evaluating the impact of this guidance on its consolidated financial statements. In June 2018, the FASB issued ASU 2018-07, Compensation-Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting. ASU 2018-07 expands the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from nonemployees. ASU 2018-07 will become effective for the Company on January 1, 2019 and early adoption is permitted. The Company is currently evaluating the impact of this guidance on its consolidated financial statements. Accounting Pronouncements Recently Adopted In May 2014, the FASB, issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606), which affects any entity that either enters into contracts with customers to transfer goods and services or enters into contracts for the transfer of nonfinancial assets. The standard’s core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The Company adopted ASU 2014-09 effective January 1, 2018, utilizing the modified retrospective transition method. Prior periods were not retrospectively adjusted. Upon adoption, the Company recognized an immaterial cumulative effect of adopting this guidance as an adjustment to its opening accumulated deficit balance. The new standard may, in certain circumstances, impact the timing of when revenue is recognized for products shipped, and the timing and classification of certain sales incentives, which are expected to generally be recognized earlier than historical guidance. The Company believes the new guidance is materially consistent with its historical revenue recognition policy. In addition, ASU 2014-09 requires the presentation of sales returns reserve as a current liability. The Company’s sales return reserve was $75.0 million as of June 30, 2018 , presented within “Accrued liabilities” and was $109.9 million as of December 31, 2017, presented within “Accounts receivable, net.” The impact to revenue, accounts receivable, deferred revenue, and accrued liabilities as a result of applying ASU 2014-09 for the six months ended or as of June 30, 2018 was as follows (in thousands): Three Months Ended June 30, 2018 Six Months Ended June 30, 2018 Under ASC 605 Impact Under ASC 606 Under ASC 605 Impact Under ASC 606 Revenue $ 298,783 $ 561 $ 299,344 $ 545,834 $ 1,375 $ 547,209 Accounts receivable, net 167,042 74,996 242,038 167,042 74,996 242,038 Deferred revenue 33,789 (954 ) 32,835 33,789 (954 ) 32,835 Accrued liabilities 302,734 74,324 377,058 302,734 74,324 377,058 Accumulated deficit (332,945 ) 561 (332,384 ) (333,759 ) 1,375 (332,384 ) The impact to other financial statement line items was immaterial. Adoption of the standard had no impact to net cash from or used in operating, investing, or financing activities in the Company’s condensed consolidated statement of cash flows. In January 2016, the FASB issued ASU 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, which updates certain aspects of recognition, measurement, presentation and disclosure of financial instruments. The Company has elected to measure equity investments that do not have readily determinable fair values at cost minus impairment, if any, plus or minus changes resulting from observable price changes in orderly transactions for the identical or a similar investment of the same issuer. The adoption of this standard did not have a material impact on the Company’s consolidated financial statements. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230). ASU 2016-15 provides guidance intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. ASU 2016-15 provides guidance in a number of situations including, among others, contingent consideration payments made after a business combination, proceeds from the settlement of insurance claims, distributions received from equity method investees, and classifying cash receipts and payments that have aspects of more than one class of cash flows. ASU 2016-15 became effective for the Company on January 1, 2018. The adoption of this standard did not have a material impact on the Company’s consolidated financial statements. In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business. The purpose of ASU 2017-01 is to change the definition of a business to assist entities with evaluating when a set of transferred assets and activities is a business. ASU 2017-01 became effective for the Company on January 1, 2018. The adoption of this standard did not have a material impact on the Company’s consolidated financial statements. In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting. ASU 2017-09 was issued to clarify and reduce both (i) diversity in practice and (ii) cost and complexity when applying the guidance in Topic 718 to a change to the terms and conditions of a share-based payment award. ASU 2017-09 became effective for the Company on January 1, 2018. The adoption of this standard did not have a material impact on the Company’s consolidated financial statements. In August 2017, the FASB issued ASU 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities. ASU 2017-12 amends the hedge accounting rules to simplify the application of hedge accounting standard and better portray the economic results of risk management activities in the financial statements. The standard expands the ability to hedge non-financial and financial risk components, reduces complexity in fair value hedges of interest rate risk, eliminates the requirement to separately measure and report hedge ineffectiveness, as well as eases certain hedge effectiveness assessment requirements. ASU 2017-12 becomes effective for the Company on January 1, 2019 with early adoption permitted. The Company early adopted this new standard in the first quarter of 2018. The adoption of this new standard did not have a material impact on the Company’s consolidated financial statements.

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The carrying values of the Company’s accounts receivable, accounts payable, and accrued liabilities approximated their fair values due to the short period of time to maturity or repayment. The following tables set forth the Company’s financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): June 30, 2018 Level 1 Level 2 Level 3 Total Assets: Money market funds $ 153,241 $ — $ — $ 153,241 U.S. government agencies — 74,897 — 74,897 Corporate debt securities — 217,806 — 217,806 Derivative assets — 5,464 — 5,464 Total $ 153,241 $ 298,167 $ — $ 451,408 Liabilities: Derivative liabilities $ — $ 736 $ — $ 736 Stock warrant liability — — 486 486 Total $ — $ 736 $ 486 $ 1,222 December 31, 2017 Level 1 Level 2 Level 3 Total Assets: Money market funds $ 193,066 $ — $ — $ 193,066 U.S. government agencies — 79,624 — 79,624 Corporate debt securities — 291,582 — 291,582 Total $ 193,066 $ 371,206 $ — $ 564,272 Liabilities: Derivative liabilities $ — $ 2,138 $ — $ 2,138 Stock warrant liability — — 208 208 Total $ — $ 2,138 $ 208 $ 2,346 The fair value of the Company’s Level 1 financial instruments is based on quoted market prices in active markets for identical instruments. The fair value of the Company’s Level 2 financial instruments is based on observable inputs other than quoted prices in active markets for identical assets and liabilities, quoted prices for identical or similar assets or liabilities in inactive markets, or other inputs that are observable or can be corroborated by observable market data. In addition, Level 2 assets and liabilities include derivative financial instruments associated with hedging activity, which are further discussed in Note 3. Derivative financial instruments are initially measured at fair value on the contract date and are subsequently remeasured to fair value at each reporting date using inputs such as spot rates, forward rates, and discount rates. There is not an active market for each hedge contract, but the inputs used to calculate the value of the instruments are tied to active markets. There were no Level 3 assets as of June 30, 2018 and December 31, 2017. There were Level 3 liabilities as of June 30, 2018 and December 31, 2017. There have been no transfers between fair value measurement levels during the three and six months ended June 30, 2018 and July 1, 2017 . In 2017, the Company acquired an equity ownership interest in a privately-held company in exchange for $6.0 million in cash. This investment is accounted for using the cost method of accounting since the Company is unable to exercise any significant influence. Upon adoption of ASU 2016-01 on January 1, 2018, the Company elected to measure equity investments that do not have readily determinable fair values at cost minus impairment, if any, plus or minus changes resulting from observable price changes in orderly transactions for the identical or a similar investment of the same issuer. This investment has been recorded at historical cost, classified within “Other assets” on the Company’s consolidated balance sheet as of June 30, 2018 and December 31, 2017.

Financial Instruments

Financial Instruments	6 Months Ended
	Jun. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Financial Instruments	Financial Instruments Cash, Cash Equivalents and Marketable Securities The Company’s marketable securities are classified as available-for-sale as of the balance sheet date and are reported at fair value with unrealized gains and losses reported, net of tax, as a separate component of accumulated other comprehensive income (loss) in stockholders’ equity. Because the Company views marketable securities as available to support current operations as needed, it has classified all available-for-sale securities as current assets. Realized gains or losses and other-than-temporary impairments, if any, on available-for-sale securities are reported in other income (expense), net, as incurred. Investments are reviewed periodically to identify potential other-than-temporary impairments. No impairment loss has been recorded on the securities included in the tables below because the Company believes that the decrease in fair value of these securities is temporary and expects to recover up to, or beyond, the initial cost of investment for these securities. The following table sets forth cash, cash equivalents and marketable securities as of June 30, 2018 (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Cash and Cash Equivalents Marketable Securities Cash $ 134,519 $ — $ — $ 134,519 $ 134,519 $ — Money market funds 153,241 — — 153,241 153,241 — U.S. government agencies 75,032 — (135 ) 74,897 10,376 64,521 Corporate debt securities 217,942 4 (140 ) 217,806 29,980 187,826 Total $ 580,734 $ 4 $ (275 ) $ 580,463 $ 328,116 $ 252,347 The following table sets forth cash, cash equivalents and marketable securities as of December 31, 2017 (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Cash and Cash Equivalents Marketable Securities Cash $ 115,028 $ — $ — $ 115,028 $ 115,028 $ — Money market funds 193,066 — — 193,066 193,066 — U.S. government agencies 79,722 1 (99 ) 79,624 6,595 73,029 Corporate debt securities 291,738 15 (171 ) 291,582 27,277 264,305 Total $ 679,554 $ 16 $ (270 ) $ 679,300 $ 341,966 $ 337,334 The gross unrealized gains or losses on marketable securities as of June 30, 2018 and December 31, 2017 were not material. There were no available-for-sale investments as of June 30, 2018 and December 31, 2017 that have been in a continuous unrealized loss position for greater than 12 months on a material basis. The following table classifies marketable securities by contractual maturities (in thousands): June 30, 2018 December 31, 2017 Due in one year $ 252,347 $ 319,112 Due in one to two years — 18,222 Total $ 252,347 $ 337,334 Derivative Financial Instruments The Company operates in foreign countries, which exposes it to market risk associated with foreign currency exchange rate fluctuations between the U.S. dollar and various foreign currencies. In order to manage this risk, the Company may hedge a portion of its foreign currency exposures related to outstanding monetary assets and liabilities as well as forecasted revenues and expenses, using foreign currency exchange forward or option contracts. In general, the market risk related to these contracts is offset by corresponding gains and losses on the hedged transactions. The Company does not enter into derivative contracts for trading or speculative purposes. Cash Flow Hedges The Company has entered into foreign currency derivative contracts designated as cash flow hedges to hedge certain forecasted revenue and expense transactions denominated in currencies other than the U.S. dollar. The Company’s cash flow hedges consist of forward contracts with maturities of 12 months or less. The Company periodically assesses the effectiveness of its cash flow hedges. Effectiveness represents a derivative instrument’s ability to generate offsetting changes in cash flows related to the hedged risk. The Company records the gains or losses, net of tax, related to its cash flow hedges as a component of accumulated other comprehensive income (loss) in stockholders’ equity and subsequently reclassifies the gains or losses into revenue and operating expenses when the underlying hedged transactions are recognized. If the hedged transaction becomes probable of not occurring, the corresponding amounts in accumulated other comprehensive income (loss) would immediately be reclassified to other income (expense), net. Cash flows related to the Company’s cash flow hedging program are recognized as cash flows from operating activities in its statements of cash flows. Prior to the adoption of ASU 2017-12, the Company recorded the gains or losses related to the ineffective portion of its cash flow hedges, if any, immediately in other income (expense), net. For the period ended July 1, 2017 , the ineffective portion of its cash flow hedges were not material. The Company had outstanding contracts with a total notional amount of $99.7 million in cash flow hedges for forecasted revenue as of June 30, 2018 , and no outstanding contracts that were designated in cash flow hedges for forecasted revenue as of December 31, 2017. Balance Sheet Hedges The Company enters into foreign exchange contracts to hedge certain monetary assets and liabilities that are denominated in currencies other than the functional currency of its subsidiaries. These foreign exchange contracts are carried at fair value, do not qualify for hedge accounting treatment, and are not designated as hedging instruments. Changes in the value of the foreign exchange contracts are recognized in other income (expense), net, and offset the foreign currency gain or loss on the underlying net monetary assets or liabilities. The Company had outstanding balance sheet hedges with a total notional amount of $110.5 million and $141.2 million as of June 30, 2018 and December 31, 2017, respectively. Fair Value of Foreign Currency Derivatives The foreign currency derivative contracts that were not settled at the end of the period are recorded at fair value, on a gross basis, in the condensed consolidated balance sheets. The following table presents the fair value of the Company’s foreign currency derivative contracts as of the periods presented (in thousands): June 30, 2018 December 31, 2017 Balance Sheet Location Fair Value Derivative Assets Fair Value Derivative Liabilities Fair Value Derivative Assets Fair Value Derivative Liabilities Cash flow designated hedges Prepaid expenses and other current assets $ 5,420 $ — $ — $ — Cash flow designated hedges Accrued liabilities — — — — Hedges not designated Prepaid expenses and other current assets 44 — — — Hedges not designated Accrued liabilities — 736 — 2,138 Total fair value of derivative instruments $ 5,464 $ 736 $ — $ 2,138 Financial Statement Effect of Foreign Currency Derivative Contracts The following table presents the pre-tax impact of the Company’s foreign currency derivative contracts on other comprehensive income (“OCI”) and the condensed consolidated statements of operations for the periods presented (in thousands): Three Months Ended Six Months Ended Income Statement Location June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 Foreign exchange cash flow hedges: Gain (loss) recognized in OCI – effective portion $ 5,914 $ (13,029 ) $ 6,578 $ (13,987 ) Gain (loss) reclassified from OCI into income – effective portion Revenue 1,157 (1,035 ) 1,157 (280 ) Loss reclassified from OCI into income – effective portion Operating expenses — (440 ) — (1,405 ) Gain recognized in income – ineffective portion Other income, net — — — 21 Gain recognized in income – excluded time value portion Other income, net — 660 — 843 Foreign exchange balance sheet hedges: Gain (loss) recognized in income Other income, net 4,793 (3,547 ) 2,299 (6,776 ) As of June 30, 2018 , all net derivative gains related to the Company’s cash flow hedges will be reclassified from OCI into revenue within the next 12 months. Effect of Derivative Contracts on Condensed Consolidated Statements of Operations The following table provides the location in the condensed consolidated statements of operations and amount of the recognized gains or losses to the Company’s derivative instruments designated as hedging instruments (in thousands): Three Months Ended Six Months Ended June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 Total amounts presented in the condensed consolidated statement of operations in which the effects of cash flow hedges are recorded in revenue $ 299,344 $ 353,299 $ 547,209 $ 652,241 Total amounts presented in the condensed consolidated statement of operations in which the effects of cash flow hedges are recorded in operating expenses 218,103 212,654 415,579 422,332 Gains (losses) on foreign exchange contracts designated as cash flow hedges reclassified from OCI into revenue 1,157 (1,035 ) 1,157 (280 ) Gains (losses) on foreign exchange contracts designated as cash flow hedges reclassified from OCI into operating expenses — (440 ) — (1,405 ) Offsetting of Foreign Currency Derivative Contracts The Company presents its derivative assets and derivative liabilities at gross fair values in the condensed consolidated balance sheets. The Company generally enters into master netting arrangements, which mitigate credit risk by permitting net settlement of transactions with the same counterparty. The Company is not required to pledge, and is not entitled to receive, cash collateral related to these derivative instruments. The following tables set forth the available offsetting of net derivative assets under the master netting arrangements as of June 30, 2018 and December 31, 2017 (in thousands): Gross Amounts Offset in the Condensed Consolidated Balance Sheets Gross Amounts Not Offset in Condensed Consolidated Balance Sheets June 30, 2018 Gross Amounts Recognized Gross Amounts Offset Net Amounts Presented Financial Instruments Cash Collateral Received Net Amount Foreign exchange contracts assets $ 5,464 $ — $ 5,464 $ 736 $ — $ 4,728 Foreign exchange contracts liabilities 736 — 736 736 — — Gross Amounts Offset in the Condensed Consolidated Balance Sheets Gross Amounts Not Offset in Condensed Consolidated Balance Sheets December 31, 2017 Gross Amounts Recognized Gross Amounts Offset Net Amounts Presented Financial Instruments Cash Collateral Received Net Amount Foreign exchange contracts assets $ — $ — $ — $ — $ — $ — Foreign exchange contracts liabilities 2,138 — 2,138 — — 2,138

Balance Sheet Components

Balance Sheet Components	6 Months Ended
	Jun. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Balance Sheet Components	Balance Sheet Components Deferred Revenue Deferred revenue consists of deferred shipments in transit for which control has not yet transferred, deferred software, or amounts allocated to mobile dashboard and on-line apps and unspecified upgrade rights, and deferred subscription-based services for which payments have been received by the customer prior to revenue recognition of these performance obligations. The deferred shipments performance obligation is anticipated to be recognized within the next quarter. The deferred software and deferred subscription-based services performance obligations are anticipated to be recognized over the useful life or service periods of twelve to seventeen months. Changes in the total short-term and long-term deferred revenue balance were as follows (in thousands): Three Months Ended Six Months Ended June 30, 2018 June 30, 2018 Beginning balances $ 36,448 $ 42,432 Deferral of revenue 8,328 15,708 Recognition of deferred revenue (11,941 ) (25,305 ) Ending balances $ 32,835 $ 32,835 Revenue Returns Reserve Revenue returns reserve activities were as follows (in thousands): Three Months Ended Six Months Ended June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 Beginning balances $ 82,612 $ 68,317 $ 109,872 $ 98,851 Increases 36,929 48,285 63,001 89,911 Write-offs/Returns taken (44,545 ) (65,082 ) (97,877 ) (137,242 ) Ending balances $ 74,996 $ 51,520 $ 74,996 $ 51,520 Increases in the revenue returns reserve include provisions for open box returns and stock rotations. Inventories Inventories consisted of the following (in thousands): June 30, 2018 December 31, 2017 Components 11,687 3,825 Finished goods 128,743 120,070 Total inventories $ 140,430 $ 123,895 Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following (in thousands): June 30, 2018 December 31, 2017 POP displays, net $ 10,345 $ 14,750 Prepaid marketing 2,768 6,074 Derivative assets 5,464 — Prepaid expenses 14,482 24,204 Insurance receivable — 37,300 Other 30,168 14,941 Total prepaid expenses and other current assets $ 63,227 $ 97,269 Property and Equipment, Net Property and equipment, net, consisted of the following (in thousands): June 30, 2018 December 31, 2017 Tooling and manufacturing equipment $ 76,119 $ 66,854 Furniture and office equipment 22,509 20,942 Purchased and internally-developed software 20,346 18,112 Leasehold improvements 63,787 58,431 Total property and equipment 182,761 164,339 Less: Accumulated depreciation and amortization (72,576 ) (59,431 ) Property and equipment, net $ 110,185 $ 104,908 Total depreciation and amortization expense related to property and equipment, net was $13.1 million and $10.1 million for the three months ended June 30, 2018 and July 1, 2017 , respectively, and $23.6 million and $19.2 million for the six months ended June 30, 2018 and July 1, 2017 , respectively. Goodwill and Intangible Assets The changes in the carrying amount of goodwill were as follows (in thousands): Goodwill Balance at December 31, 2017 $ 51,036 Goodwill acquired 10,022 Balance at June 30, 2018 $ 61,058 The increase in the carrying amount of goodwill during the six months ended June 30, 2018 was attributable to an acquisition in February 2018. See Note 11 for additional information. The carrying amounts of the intangible assets as of June 30, 2018 and December 31, 2017 were as follows (in thousands, except useful life): June 30, 2018 December 31, 2017 Weighted Average Remaining Useful Life (years) Gross Accumulated Amortization Net Gross Accumulated Amortization Net Developed technology $ 35,988 $ (12,249 ) $ 23,739 $ 30,588 $ (8,738 ) $ 21,850 3.4 Customer relationships 3,918 (308 ) 3,610 — — — 6.4 Trademarks and other 1,150 (759 ) 391 1,278 (772 ) 506 0.6 Total intangible assets, net $ 41,056 $ (13,316 ) $ 27,740 $ 31,866 $ (9,510 ) $ 22,356 The increase in the carrying amount of goodwill during the six months ended June 30, 2018 was attributable to an acquisition in February 2018. See Note 11 for additional information. Total amortization expense related to intangible assets was $2.1 million and $1.4 million for the three months ended June 30, 2018 and July 1, 2017 , respectively, and $3.8 million and 2.8 million for the six months ended June 30, 2018 and July 1, 2017 , respectively. The estimated future amortization expense of acquired finite-lived intangible assets to be charged to cost of revenue and operating expenses after June 30, 2018 was as follows (in thousands): Cost of Revenue Operating Expenses Total Remaining 2018 $ 3,735 $ 386 $ 4,121 2019 6,690 771 7,461 2020 5,910 588 6,498 2021 5,910 541 6,451 2022 1,236 541 1,777 Thereafter 258 1,174 1,432 Total finite-lived intangible assets, net $ 23,739 $ 4,001 $ 27,740 Accrued Liabilities Accrued liabilities consisted of the following (in thousands): June 30, 2018 December 31, 2017 Product warranty $ 50,468 $ 87,882 Accrued revenue reserve from returns 74,996 — Accrued manufacturing expense and freight 34,493 41,901 Accrued sales incentives 87,756 111,592 Accrued sales and marketing 24,693 44,401 Accrued research and development 4,819 8,983 Accrued co-op advertising and marketing development funds 19,521 30,408 Employee-related liabilities 23,054 33,266 Sales taxes and VAT payable 26,127 21,340 Inventory received but not billed 4,615 10,526 Accrued legal settlements and fees 4,819 36,693 Derivative liabilities 736 2,138 Other 20,961 23,007 Accrued liabilities $ 377,058 $ 452,137 Product warranty reserve activities were as follows (in thousands) (1) : Three Months Ended Six Months Ended June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 Beginning balances $ 71,975 $ 90,459 $ 87,882 $ 99,923 Charged to cost of revenue (8,486 ) (1,595 ) (10,967 ) 17,342 Changes related to pre-existing warranties (4,469 ) 4,913 (7,872 ) 2,473 Settlement of claims (8,552 ) (21,016 ) (18,575 ) (46,977 ) Ending balances $ 50,468 $ 72,761 $ 50,468 $ 72,761 (1) Does not include reserves established as a result of the recall of the Fitbit Force. See the section titled “ — Fitbit Force Recall Reserve” in the Company’s Annual Report on Form 10-K for additional information regarding such reserves. Restructuring In January 2017, the Company announced cost-efficiency measures to be implemented in 2017 that include realigning sales and marketing spend and improved optimization of research and development investments. In addition, the Company announced a reorganization, including a reduction in workforce. This reorganization impacted approximately 110 employees, or approximately 6% of the Company’s global workforce. The Company recorded $6.4 million in total restructuring expenses, substantially all of which were severance and related costs, in the first quarter of 2017. The Company completed the reorganization in the fourth quarter of 2017. Accumulated Other Comprehensive Income (Loss) The components and activity of accumulated other comprehensive income (“AOCI”), net of tax, were as follows (in thousands): Unrealized Gains (Losses) on Cash Flow Hedges Currency Translation Adjustments Unrealized Gains (Losses) on Available-for-Sale Investments Total Balance at December 31, 2017 $ 66 $ — $ (75 ) $ (9 ) Other comprehensive income (loss) before reclassifications 5,912 — (208 ) 5,704 Amounts reclassified from AOCI (1,028 ) — 12 (1,016 ) Other comprehensive income (loss) 4,884 — (196 ) 4,688 Balance at June 30, 2018 $ 4,950 $ — $ (271 ) $ 4,679

Long-Term Debt

Long-Term Debt	6 Months Ended
	Jun. 30, 2018
Debt Disclosure [Abstract]
Long-Term Debt	Long-Term Debt 2015 Credit Agreement In December 2015, the Company entered into a second amended and restated credit agreement (the “Senior Facility”) with Silicon Valley Bank (“SVB”), as administrative agent, collateral agent, and lender, SunTrust Bank as syndication agent, SunTrust Robinson Humphrey, Inc. and several other lenders to replace the existing asset-based credit facility and cash flow facility. The Senior Facility allowed the Company to borrow up to $250.0 million , including up to $ 50.0 million for the issuance of letters of credit and up to $25.0 million for swing line loans, subject to certain financial covenants and ratios. The Company has the option to repay its borrowings under the Senior Facility without penalty prior to maturity. The Senior Facility requires the Company to comply with certain financial and non-financial covenants. The Senior Facility contains customary covenants that restrict the Company’s ability to, among other things, incur additional indebtedness, sell certain assets, guarantee certain obligations of third parties, declare dividends or make certain distributions, and undergo a merger or consolidation or certain other transactions. Obligations under the Senior Facility are collateralized by substantially all of the Company’s assets, excluding its intellectual property. In May 2017, the Company entered into a first amendment to the Senior Facility (the “First Amendment”), pursuant to which the aggregate amount the Company can borrow under the Senior Facility was reduced from $250.0 million to $100.0 million , with up to $50.0 million available for the issuance of letters of credit and up to $25.0 million available for swing line loans. In addition, pursuant to the First Amendment, the applicable margin in respect of the interest rates under the Senior Facility was amended to be based on the Company’s level of liquidity (defined as the sum of the Company’s aggregate cash holdings and the amount available under its revolving commitments) and range from, with respect to Alternate Base Rate loans, 0.5% to 1.0% , and, with respect to LIBOR loans, 1.5% to 2.0% . Among other changes, the First Amendment also removed the fixed charge coverage ratio covenant and the consolidated leverage ratio covenant, and added a general liquidity covenant requiring the Company to maintain liquidity of at least $200.0 million in unrestricted cash, of which $100.0 million in cash or cash equivalents must be held in accounts subject to control agreements with, and maintained by, SVB or its affiliates. The Company was in compliance with the financial covenants under the Senior Facility as of June 30, 2018 . As of June 30, 2018 , the Company had no outstanding borrowings under the Senior Facility and had outstanding letters of credit totaling $36.8 million , issued to cover various security deposits on its facility leases. Letters of Credit As of June 30, 2018 and December 31, 2017, the Company had outstanding letters of credit of $36.8 million and $36.9 million , respectively, in each period issued to cover the security deposit on the lease of its office headquarters in San Francisco, California, and other facility leases.

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Leases The Company’s principal facility is located in San Francisco, California. The Company also leases office space in various locations with expiration dates between 2019 and 2024. The lease agreements often include leasehold improvement incentives, escalating lease payments, renewal provisions and other provisions which require the Company to pay taxes, insurance, maintenance costs or defined rent increases. All of Company’s leases are accounted for as operating leases. During the three months ended June 30, 2018, the Company notified the lessor of its intent to sublease a portion of one of its San Francisco offices. Under the terms of the lease, the lessor has the right to recapture this space. The lessor elected to exercise their recapture right effective August 1, 2018, which resulted in a reduction of approximately $81.4 million in the Company’s future lease obligations associated with this lease. Future minimum payments under the Company’s noncancelable lease agreements as of the date of this filing, reduced by the recaptured amount, was as follow (in thousands): Remaining 2018 $ 17,328 2019 33,571 2020 29,398 2021 27,136 2022 26,855 Thereafter 37,555 Total future minimum lease payments $ 171,843 Rent expense is recorded over the lease terms on a straight-line basis. Rent expense was $7.2 million and $10.0 million for the three months ended June 30, 2018 and July 1, 2017 , respectively, and $ 16.8 million and $19.2 million for the six months ended June 30, 2018 and July 1, 2017 , respectively. Purchase Commitments The aggregate amount of open purchase orders as of June 30, 2018 was approximately $387.6 million , of which $185.0 million related to the Company’s migration to a third-party hosting provider. The Company cannot determine the aggregate amount of such purchase orders that represent contractual obligations because purchase orders may represent authorizations to purchase rather than binding agreements. The Company’s purchase orders are based on its current needs and are fulfilled by its suppliers, contract manufacturers, and logistics providers within short periods of time. During the normal course of business, the Company and its contract manufacturers procure components based upon a forecasted production plan. If the Company cancels all or part of the orders, or materially reduces forecasted orders, it may be liable to its suppliers and contract manufacturers for the cost of the excess components purchased by its contract manufacturers. As of June 30, 2018 , $14.3 million was accrued for such liabilities to contract manufacturers. Legal Proceedings Jawbone. On May 27, 2015, Aliphcom, Inc. d/b/a Jawbone (“Jawbone”), filed a lawsuit in the Superior Court of California in the County of San Francisco against the Company and five of its employees who were formerly employed by Jawbone, alleging trade secret misappropriation and unfair and unlawful business practices against all defendants, and alleging breach of contract and breach of implied covenant of good faith and fair dealing against the employee defendants. The complaint sought unspecified damages, including punitive damages and injunctive relief. On June 23, 2016, Jawbone filed a Second Amended Complaint, adding a sixth employee defendant and related allegations. On June 10, 2015, Jawbone and BodyMedia, Inc., a wholly-owned subsidiary of Jawbone (“BodyMedia”), filed a lawsuit against the Company in the U.S. District Court for the Northern District of California, alleging that the Company infringes certain U.S. patents. The complaint sought unspecified compensatory damages and attorneys’ fees from the Company and to permanently enjoin the Company from making, manufacturing, using, selling, importing, or offering the Company’s products for sale. The lawsuit was stayed pending resolution the investigation in the U.S. International Trade Commission (the “ITC”). On July 7, 2015, Jawbone and BodyMedia filed a complaint with the ITC requesting an investigation into purported violations of the Tariff Act of 1930 by the Company and Flextronics International Ltd. and Flextronics Sales and Marketing (A-P) Ltd. The complaint makes the same patent infringement and trade secret misappropriation claims as the two earlier cases. The complaint seeks a limited exclusion order and a cease and desist order halting the importation and sale of the infringing products. The ITC instituted the investigation on August 17, 2015. As a result of motions, all of the patent infringement claims were dismissed from the case. A trial on the trade secrets allegations took place from May 9 to 17, 2016. On August 23, 2016, the administrative law judge concluded that the Company did not misappropriate any Jawbone trade secrets. On October 20, 2016, the ITC terminated the investigation in the ITC. Jawbone appealed the dismissal of the patent infringement claims to the Federal Circuit. Oral argument was scheduled for November 9, 2017. On September 3, 2015, the Company filed a lawsuit against Jawbone in the U.S. District Court for the District of Delaware, alleging that Jawbone’s activity trackers infringe certain U.S. patents. This case was transferred to the U.S. District Court for the Northern District of California. The trial was scheduled for July 13, 2020. On September 8, 2015, the Company filed a complaint for patent infringement against Jawbone in the U.S. District Court for the Northern District of California, asserting that Jawbone’s activity trackers infringe certain U.S. patents. No trial date was set. On October 29, 2015, the Company filed a complaint for patent infringement against Jawbone in the U.S. District Court for the District of Delaware, asserting that Jawbone’s activity trackers infringe certain U.S. patents. That case was also transferred to the U.S. District Court for the Northern District of California. No trial date was set. On November 2, 2015, the Company filed a complaint with the ITC requesting an investigation into violations of the Tariff Act of 1930 by Jawbone and Body Media. The complaint asserted that Jawbone’s products infringe certain U.S. patents. The complaint sought a limited exclusion order and a cease and desist order halting the importation and sale of infringing products. The ITC instituted the investigation on December 1, 2015. On December 23, 2016, the Company filed a motion to terminate the investigation, and the ITC terminated the investigation on February 1, 2017. On December 8, 2017, the parties announced the global settlement of all of the outstanding civil litigation on confidential terms. Each of the pending cases has been dismissed with prejudice. On August 12, 2016, the Company was notified by Jawbone that Jawbone had received a confidential subpoena from the U.S. Attorney’s Office for the Northern District of California requesting certain of the Company’s confidential business information that appeared to be related to Jawbone’s allegations of trade secret misappropriation. On February 17, 2017, the Company received a subpoena for documents from the same office. On February 1, 2018, the Company received a second subpoena for documents. The Company is cooperating with the U.S. Attorney’s Office. On June 14, 2018, the six former Jawbone employees who were named as individual defendants in the state trade secret case were charged in a federal indictment with being in possession of certain Jawbone trade secrets. Sleep Tracking . On May 8, 2015, a purported class action lawsuit was filed against the Company in the U.S. District Court for the Northern District of California, alleging that the sleep tracking function available in certain trackers does not perform as advertised. Plaintiffs seek class certification, restitution, an award of unspecified compensatory and punitive damages, an award of reasonable costs and expenses, including attorneys’ fees, and other further relief as the Court may deem just and proper. On January 31, 2017, plaintiffs filed a motion for class certification. Plaintiffs’ motion for class certification was granted on November 20, 2017. On April 20, 2017, the Company filed a motion for summary judgment. The Company’s motion for summary judgment was denied on December 8, 2017. During the three months ended June 30, 2018, the parties agreed to a settlement and on August 1, 2018, the plaintiffs filed a motion for preliminary approval of the class action. Heart Rate Tracking. On January 6, 2016 and February 16, 2016, two purported class action lawsuits were filed against the Company in the U.S. District Court for the Northern District of California, alleging that the PurePulse® heart rate tracking technology does not consistently and accurately record users’ heart rates. Plaintiffs allege common law claims, as well as violations of various states’ false advertising, unfair competition, and consumer protection statutes, and seek class certification, injunctive and declaratory relief, restitution, an award of unspecified compensatory damages, exemplary damages, punitive damages, and statutory penalties and damages, an award of reasonable costs and expenses, including attorneys’ fees, and other further relief as the Court may deem just and proper. On April 15, 2016, the plaintiffs filed a Consolidated Master Class Action Complaint and, on May 19, 2016, filed an Amended Consolidated Master Class Action Complaint. On January 9, 2017, the Company filed a motion to compel arbitration. On October 11, 2017, the Court granted the motion to compel arbitration. Plaintiffs filed a motion for reconsideration, and that motion was denied on January 24, 2018. On February 20, 2018, plaintiffs filed a Second Amended Consolidated Master Class Action Complaint (“SAC”) on behalf of plaintiff Rob Dunn, the only plaintiff not ordered to arbitration, as a purported class action. The SAC alleges the same common law claims, as well as violations of false advertising, unfair competition, and consumer protection statutes of California and Arizona, and also seeks class certification, injunctive and declaratory relief, restitution, an award of unspecified compensatory damages, exemplary damages, punitive damages, and statutory penalties and damages, an award of reasonable costs and expenses, including attorneys’ fees, and other further relief as the Court may deem just and proper. On March 13, 2018, the Company filed a motion to dismiss for failure to state a claim and separately moved to strike the class allegations. The Court dismissed the claims for revocation of acceptance, violation of California’s Song-Beverly Consumer Warranty Act, and unjust enrichment, and allowed the remaining claims pending amendment to the complaint with further details. Plaintiff filed a Third Amended Complaint on June 19, 2018. The Court granted the motion to strike and ordered the plaintiff to amend to make clear that he is seeking to represent a class of opt-outs only, but that plaintiff would be free to amend in the event Fitbit’s arbitration agreement was found to be unenforceable. In response to an April 3, 2018 arbitration demand from Kate McLellan, one of the original plaintiffs who was compelled to arbitration, the Company attempted to resolve the individual claim with Ms. McLellan. At the May 31, 2018 hearing, the Court expressed concern that the Company was “picking off” McLellan and thereby undermining the arbitration option and the Court’s prior order on arbitration, and ordered additional briefing The Company believes that the plaintiffs’ allegations are without merit, and intends to vigorously defend against the claims. Because the Company is in the early stages of this litigation matter, the Company is unable to estimate a reasonably possible loss or range of loss, if any, that may result from this matter. Securities Litigation. On January 11, 2016, a putative securities class action was filed in the U.S. District Court for the Northern District of California naming as defendants the Company, certain of its officers and directors, and the underwriters of the Company’s initial public offering (the “IPO”). On May 10, 2016, the Court appointed the Fitbit Investor Group (consisting of five individual investors) as lead plaintiff, and an Amended Complaint was filed on July 1, 2016. Plaintiffs allege violations of the Securities Act of 1933, as amended (the “Securities Act”), and the Securities Exchange Act of 1934, as amended, based on alleged materially false and misleading statements about the Company’s products between October 27, 2014 and November 23, 2015. Plaintiffs seek to represent a class of persons who purchased or otherwise acquired the Company’s securities (i) on the open market between June 18, 2015 and May 19, 2016; and/or (ii) pursuant to or traceable to the IPO. Plaintiffs seek class certification, an award of unspecified compensatory damages, an award of reasonable costs and expenses, including attorneys’ fees, and other further relief as the Court may deem just and proper. On July 29, 2016, the Company filed a motion to dismiss. The court denied the motion on October 26, 2016. On April 26, 2017, the Company filed a motion for summary judgment, which is still pending. On April 28, 2016, a putative class action lawsuit alleging violations of the Securities Act was filed in the Superior Court of California, County of San Mateo, naming as defendants the Company, certain of its officers and directors, the underwriters of the IPO, and a number of its investors. Plaintiffs allege that the IPO registration statement contained material misstatements about the Company’s products. Plaintiffs seek to represent a class of persons who purchased the Company’s common stock in and/or traceable to the IPO and/or the November 2015 follow-on public offering (the “Secondary Offering”). Plaintiffs seek class certification, an award of unspecified compensatory damages, an award of reasonable costs and expenses, including attorneys’ fees, and other further relief as the Court may deem just and proper. On May 17, 2016, a similar class action lawsuit was filed in the Superior Court of California, County of San Francisco. The cases have now been consolidated in the County of San Francisco. On April 7, 2017, the Court granted a motion to dismiss the Section 11 claim based on the Secondary Offering and stayed the cases. On January 8, 2018, the plaintiffs in the federal and class action cases filed their motion for preliminary approval of settlement of the putative federal and state class actions for $33.3 million , which the Company accrued for as of December 31, 2017. On January 19, 2018, the court entered an order preliminarily approving the proposed settlement, and on April 20, 2018, the court approved the final settlement. The federal and class action cases have been dismissed with prejudice. On November 11, 2016, a derivative lawsuit was filed in the U.S. District Court for the Northern District of California derivatively on behalf of the Company naming as defendants certain of its officers and directors and as a nominal plaintiff the Company. The plaintiffs allege breach of fiduciary duty, unjust enrichment, section 14(a), and misappropriation based on the same set of alleged facts in the federal and state securities class action litigation. On February 2, 2017, a second derivative lawsuit was filed in the U.S. District Court for the District of Delaware on the same allegations and also including claims for abuse of control, gross mismanagement, and waste. On June 27, 2017, another derivative law suit was filed in the U.S. District Court for the Northern District of California on the same allegations. The Courts have ordered a stay in all three cases. On June 1, 2017 and June 9, 2017, two additional derivative lawsuits were filed in the Delaware Court of Chancery. Plaintiffs allege breach of fiduciary duty and insider trading against certain defendants who sold shares in the IPO and/or the Secondary Offering. On August 3, 2017, another derivative lawsuit was filed in the Delaware Court of Chancery on the same allegations. There is temporary stay in all three cases. On March 15, 2018, the three derivative lawsuits were consolidated and a Second Amended Complaint was filed on the same allegations of the individual complaints, alleging the same claims, and seeking the same remedy. On April 26, 2017, the Company filed a motion to dismiss for failure to state a claim. On October 31, 2017, a seventh derivative lawsuit was filed in the Superior Court of California, Country of San Francisco, on the same allegations. The Company has not yet been served in that case. On June 27, 2017, an individual investor lawsuit alleging violations of the Securities Act and state law claims for statutory fraud and unfair business practice was filed in the Superior Court of California, County of Alameda, naming as defendants the Company and certain of its officers. The allegations are based on the same set of alleged facts in the federal and state securities class action litigation. The individual case was resolved at a June 13, 2018 mediation. The Company believes that the plaintiffs’ allegations in the derivative actions and individual action are without merit, and intends to vigorously defend against the claims. Because the Company is in the early stages of these litigation matters, the Company is unable to estimate a reasonably possible loss or range of loss, if any, that may result from these matters. Immersion. On July 10, 2017, Immersion Corporation filed a lawsuit against the Company in the U.S. District Court for the Northern District of California, alleging that certain Fitbit devices infringe on U.S. Patent Nos. 8,059,105, 8,351,299, and 8,638,301. On October 5, 2017, the Company filed a motion to dismiss on grounds the patents are not eligible subject matter for patents. On March 5, 2018, the Court granted in part and denied in part, granting as to the ‘301 patent, but denying as to the other two patents. On July 10, 2017, Immersion Corporation also filed a lawsuit against the Company in the Shanghai Intellectual Property Court, alleging infringement of three Immersion Chinese patents. In addition to Fitbit, Inc., Immersion named Runtong, one of the Company’s former distributors in China. On August 23, 2017, two additional defendants were added, Fitbit Shanghai and Rkylin, a current distributor in China. In December 2017, the Company filed petitions to invalidate the patents. The invalidation proceedings as to all three patents were instituted. Hearings on two of the patents were held on April 16, 2018 and April 26, 2018; the third is scheduled for May 30, 2018. On July 8, 2018, the parties entered into a settlement agreement resolving the litigation globally. The cases have been dismissed with prejudice. Other. The Company is and, from time to time, may in the future become, involved in other legal proceedings in the ordinary course of business. The Company currently believes that the outcome of any of these existing legal proceedings, including the aforementioned cases, either individually or in the aggregate, will not have a material impact on the operating results, financial condition or cash flows of the Company. With respect to existing legal proceedings, the Company has either determined that the existence of a material loss is not reasonably possible or that it is unable to estimate a reasonably possible loss or range of loss. The Company may incur substantial legal fees, which are expensed as incurred, in defending against these legal proceedings. Indemnifications In the ordinary course of business, the Company enters into commercial agreements that may include indemnification provisions. Pursuant to such agreements, the Company may indemnify, hold harmless and defend an indemnified party for losses suffered or incurred by the indemnified party. Some of the provisions will limit losses to those arising from third-party actions. In some cases, the indemnification will continue after the termination of the agreement. The maximum potential amount of future payments the Company could be required to make under these provisions is not determinable. To date, the Company has not incurred material costs to defend lawsuits or settle claims related to these indemnification provisions. The Company has also entered into indemnification agreements with its directors and officers that may require the Company to indemnify its directors and officers against liabilities that may arise by reason of their status or service as directors or officers to the fullest extent permitted by Delaware corporate law. The Company also currently has directors’ and officers’ insurance.

Stockholders' Equity

Stockholders' Equity	6 Months Ended
	Jun. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stockholders' Equity	Stockholders’ Equity Stock Option Exchange On April 13, 2017, the Company filed its definitive proxy statement and submitted to stockholders a proposal for a stock option exchange program (the “Program”). The Program allowed the Company employees, including its executive officers other than its President, Chief Executive Officer, and Chairman, Chief Technology Officer, and Chief Financial Officer (“Eligible Employees”), to exchange out-of-the-money or “underwater” options to purchase shares of the Company’s Class A common stock or Class B common stock currently held by such Eligible Employees for a lesser number of restricted stock units (“RSUs”) that may be settled for shares of its Class A common stock, (“New RSUs”), under the Company’s 2015 Equity Incentive Plan (the “2015 Plan”). Each New RSU represented an unfunded right to receive one share of the Company’s Class A common stock on a date in the future, which generally is the date on which the New RSU will vest. Eligible Employees participating in the Program received one New RSU for every two “out-of-the-money” options that they exchange. The New RSUs would generally vest over the remaining vesting period of the exchanged option (subject to a one -year minimum vesting period). None of the members of the Company’s board of directors were eligible to participate in the Program. On May 25, 2017, the Company’s stockholders approved the Program at the 2017 Annual Meeting of Stockholders. The Company subsequently commenced the Program by filing a tender offer statement on Schedule TO with the SEC on June 21, 2017. The Program expired on July 19, 2017. A total of 3.7 million “underwater” stock options were tendered by the Eligible Employees, representing approximately 85% of the stock options eligible for exchange. On July 20, 2017, the Company granted an aggregate of 1.8 million New RSUs under the 2015 Plan in exchange for the “underwater” stock options tendered. The completion of the Program resulted in total incremental unrecognized stock-based compensation expense of $8.5 million , to be recognized over the greater of one year or the remaining vesting service period of the tendered stock options. Equity Incentive Plans In May 2015, the Company’s board of directors and stockholders adopted and approved the 2015 Plan. The 2015 Plan became effective on June 16, 2015 and serves as the successor to the Amended and Restated 2007 Stock Plan (the “2007 Plan”). The Company ceased granting awards under the 2007 Plan, and any outstanding stock options and RSUs granted under the 2007 Plan would remain subject to the terms of the 2007 Plan. As of June 30, 2018 , 17.3 million shares of Class A common stock were reserved and available for future issuance under the 2015 Plan. Stock Options Stock option activity under the equity incentive plans was as follows (in thousands, except per share amounts): Stock Options Outstanding Number of Shares Subject to Stock Options Weighted– Average Exercise Price Aggregate Intrinsic Value (1) Balance—December 31, 2017 21,386 $ 3.01 Granted — — Exercised (1,768 ) 2.55 Forfeited or canceled (561 ) 7.02 Balance—June 30, 2018 19,057 2.93 $ 58,535 Stock options vested and expected to vest—June 30, 2018 19,057 2.93 58,535 Stock options exercisable—June 30, 2018 16,654 2.61 55,317 (1) The aggregate intrinsic values of stock options outstanding, exercisable, vested and expected to vest as of June 30, 2018 were calculated as the difference between the exercise price of the stock options and the fair value of the Class A common stock of $6.53 as of June 30, 2018 . Restricted Stock Units RSU activity under the equity incentive plans was as follows (in thousands, except per share amounts): RSUs Outstanding Weighted- Average Grant Date Fair Value Unvested balance—December 31, 2017 19,188 $ 9.13 Granted 12,557 5.02 Vested (4,810 ) 8.80 Forfeited or canceled (3,135 ) 8.24 Unvested balance—June 30, 2018 23,800 $ 7.15 On May 4, 2018, the Company issued 0.8 million shares of market-based awards that vest based upon the achievement of a specified stock price. Market conditions were factored into the grant date fair value using a Monte Carlo valuation model, which utilized multiple input variables to determine the probability of the Company achieving the specified stock price targets. Stock-based compensation expense related to these awards will be recognized over the requisite service period regardless of whether the market condition is satisfied, provided that the requisite service period has been completed. Employee Stock Purchase Plan In May 2015, the Company’s board of directors adopted the 2015 Employee Stock Purchase Plan (the “2015 ESPP”), which became effective on June 17, 2015. A total of 3.8 million shares of Class A common stock were initially reserved for issuance under the 2015 ESPP. The 2015 ESPP allows eligible employees to purchase shares of the Company’s Class A common stock through payroll deductions at a price per share equal to 85% of the lesser of the fair market value of the Company’s Class A common stock (i) on the first trading day of the applicable offering period and (2) the last trading day of each purchase period in the applicable offering period. Except for the initial offering period, the 2015 ESPP provides for 6-month offering periods beginning in May and November of each year. The initial offering period began June 17, 2015 and ended in May 2016. Warrant On July 10, 2017, the Company issued a warrant to purchase 0.5 million shares of Class A common stock. The warrant is exercisable based on service and performance-based conditions and has an exercise price of $5.23 per share and a contractual term of ten years. As of June 30, 2018 , 0.5 million warrants were outstanding. Stock-Based Compensation Expense Total stock-based compensation expense recognized was as follows (in thousands): Three Months Ended Six Months Ended June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 Cost of revenue $ 2,032 $ 1,492 $ 3,130 $ 1,510 Research and development 15,090 12,648 29,761 27,333 Sales and marketing 3,911 3,987 7,358 7,622 General and administrative 4,824 3,839 9,249 7,994 Total stock-based compensation expense $ 25,857 $ 21,966 $ 49,498 $ 44,459 As of June 30, 2018 , the total unrecognized stock-based compensation expense related to unvested stock options and RSUs was $159.2 million , which the Company expects to recognize over an estimated weighted average period of 2.1 years.

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The Company is subject to income tax in the United States as well as other tax jurisdictions in which it conducts business. Earnings from non-U.S. activities are subject to local country income tax. For the three and six months ended June 30, 2018 , the Company recorded an expense for income taxes of $23.6 million and $23.0 million , respectively, for an effective tax rate of (24.9)% and (13.1)% , respectively. The effective tax rate for the six months ended June 30, 2018 , was different than the statutory federal tax rate primarily due to the impact of a full valuation allowance on the Company’s U.S. deferred tax assets, the mix of income/losses between its foreign jurisdictions, and pretax losses in jurisdictions for which no tax benefit will be recognized. On December 22, 2017, the U.S. Tax Cuts and Jobs Act of 2017 (“2017 Tax Act”) was signed into law and includes several key tax provisions that affected the Company, including a reduction of the statutory corporate tax rate from 35% to 21% effective for tax years beginning after December 31, 2017, elimination of the carryback of net operating losses generated after December 31, 2017, and changes to how the United States imposes income tax on multinational corporations, among others. In December 2017, Staff Accounting Bulletin No. 118 (“SAB 118”) was issued to address U.S. GAAP application when a registrant does not have the necessary information available, prepared, or analyzed (including computations) in reasonable detail to complete the accounting for certain income tax effects by the 2017 Tax Act. For the six months ended June 30, 2018 , no changes have been made to the provisional amounts previously recorded. The Company will complete its analysis within the measurement period in accordance with SAB 118. On July 24, 2018, the Ninth Circuit Court of Appeals issued an opinion in Altera Corp. v. Commissioner requiring related parties in an intercompany cost-sharing arrangement to share expenses related to share-based compensation. This opinion reversed a prior decision of the United States Tax Court. The Company is currently evaluating the impact of this decision and such impact could be material to its consolidated financial statements. For the three and six months ended July 1, 2017 , the Company recorded a benefit for income taxes of $4.7 million and $34.3 million respectively, for an effective tax rate of 7.4% and 22.5% , respectively. The effective tax rate for the six months ended July 1, 2017 was different than the statutory federal tax rate, primarily due to research and development credits, non-deductible stock-based compensation expense, unrecognized tax benefits, the reduction in the domestic production activities deduction in prior periods on account of current year operating losses carried back to refund prior period taxes, the impact of a valuation allowance of approximately $10.5 million recorded against certain of the Company’s deferred tax assets, and the mix of income between United States and foreign jurisdictions. The Company accounts for deferred taxes under ASC Topic 740, “Income Taxes” (“ASC 740”), which involves weighing positive and negative evidence concerning the realizability of the Company’s deferred tax assets in each jurisdiction. In evaluating its ability to realize the net deferred tax assets, the Company considered all available positive and negative evidence, including its past operating results and the forecast of future market growth, forecasted earnings, future taxable income, and prudent and feasible tax planning strategies. As of June 30, 2018 , the Company maintained a full valuation allowance against all its U.S. deferred tax assets. No valuation allowance has been recorded against the Company’s foreign deferred tax assets. The Company will continue to assess the realizability of its deferred tax assets in each of the applicable jurisdictions going forward. As of June 30, 2018 , the total amount of gross unrecognized tax benefits was $36.0 million , of which $25.4 million would affect the effective tax rate if recognized. The Company does not have any tax positions as of June 30, 2018 for which it is reasonably possible that the total amount of gross unrecognized tax benefits will increase or decrease within the following 12 months .

Net Loss per Share

Net Loss per Share	6 Months Ended
	Jun. 30, 2018
Earnings Per Share [Abstract]
Net Loss per Share	Net Loss per Share The following table sets forth the computation of the Company’s basic and diluted net loss per share (in thousands, except per share amounts): Three Months Ended Six Months Ended June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 Numerator: Net loss $ (118,268 ) $ (58,240 ) $ (199,145 ) $ (118,319 ) Denominator: Weighted-average shares of common stock—basic for Class A and Class B 242,898 230,322 241,227 228,788 Effect of dilutive securities — — — — Weighted-average shares of common stock—diluted for Class A and Class B 242,898 230,322 241,227 228,788 Net loss per share: Basic $ (0.49 ) $ (0.25 ) $ (0.83 ) $ (0.52 ) Diluted $ (0.49 ) $ (0.25 ) $ (0.83 ) $ (0.52 ) The following potentially dilutive common shares were excluded from the computation of diluted net loss per share for the periods presented because including them would have been anti-dilutive (in thousands): Three Months Ended Six Months Ended June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 Stock options to purchase common stock 12,201 19,074 12,203 20,924 RSUs 8,035 9,929 8,865 10,066 Diluted impact of ESPP 189 70 170 152 Diluted common stock subject to vesting — 110 — 120 Total 20,425 29,183 21,238 31,262

Significant Customer Informatio

Significant Customer Information and Other Information	6 Months Ended
	Jun. 30, 2018
Risks and Uncertainties [Abstract]
Significant Customer Information and Other Information	Significant Customer Information and Other Information Retailer and Distributor Concentration Retailers and distributors with revenue equal to or greater than 10% of total revenue for the three and six months ended June 30, 2018 and July 1, 2017 were as follows: Three Months Ended Six Months Ended June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 C 11 % 14 % * 12 % B * * 10 % * A * 11 * 11 * Represents less than 10%. Retailers and distributors that accounted for equal to or greater than 10% of accounts receivable at June 30, 2018 and December 31, 2017 were as follows: June 30, 2018 December 31, 2017 C 17 % 17 % B 15 13 D 11 * E * 11 * Represents less than 10%. Geographic and Other Information Revenue by geographic region, based on ship-to destinations, was as follows (in thousands): Three Months Ended Six Months Ended June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 United States $ 182,451 $ 199,201 $ 321,947 $ 369,621 Americas excluding United States 15,838 24,412 31,938 44,380 Europe, Middle East, and Africa 65,969 108,601 130,507 196,373 APAC 35,086 21,085 62,817 41,867 Total $ 299,344 $ 353,299 $ 547,209 $ 652,241 As of June 30, 2018 and December 31, 2017, long-lived assets, which represent property and equipment, located outside the United States were $36.3 million and $30.0 million , respectively.

Acquisitions

Acquisitions	6 Months Ended
	Jun. 30, 2018
Business Combinations [Abstract]
Acquisitions	Acquisitions 2018 Acquisition In February 2018, the Company completed a purchase of Twine Health, Inc., a privately-held company, which was accounted for as a business combination, for total purchase price consideration of $16.7 million , of which $5.4 million was allocated to developed technology intangible assets, $3.8 million to customer relationships intangible asset, $10.0 million to goodwill, $1.8 million to deferred tax liabilities, $0.2 million to deferred revenue, and $0.6 million to net assumed liabilities. The allocation of the purchase price consideration is provisional and the Company will complete its analysis within the measurement period pursuant to Accounting Standards Codification Topic 805, with any adjustments being recorded to goodwill. Approximately $2.6 million of the consideration payable to Twine Health, Inc. was held as partial security for certain indemnification obligations, and will be held back for payment until August 2019. The acquisition is expected to extend the Company’s reach into healthcare and lay the foundation to expand its offerings to health plans, health systems and self-insured employers, while creating opportunities to increase subscription-based revenue. The amortization period of the acquired developed technology and customer relationships are approximately four and seven years, respectively. Goodwill is not deductible for tax purposes. 2016 Acquisitions In December 2016, the Company completed a purchase of certain assets from Pebble Industries, Inc., a privately-held company (“Pebble”), which was accounted for as a business combination, for total cash consideration of $23.4 million , of which $9.6 million was allocated to developed technology intangible assets, $14.4 million to goodwill, and $0.6 million to assumed liabilities. Approximately $3.5 million of the consideration payable to Pebble was held as partial security for certain indemnification obligations, and will be held back for payment until March 2018. The acquisition is expected to enhance the features and functionality of the Company’s devices. The amortization period of the acquired developed technology is approximately 5 years . Goodwill is deductible for tax purposes. In December 2016, the Company completed a purchase of certain assets from Vector Watch S.R.L., a privately-held company (“Vector Watch”), which was accounted for as a business combination, for total cash consideration of $15.0 million , of which $3.9 million was allocated to developed technology intangible assets, $11.4 million to goodwill, and $0.3 million to assumed liabilities. Approximately $2.3 million of the consideration payable to Vector Watch was held as partial security for certain indemnification obligations, and will be held back for payment until December 2018. The acquisition is expected to enhance the features and functionality of the Company’s devices. The amortization period of the acquired developed technology is approximately 2.5 years . Goodwill is deductible for tax purposes. In May 2016, the Company completed a purchase of certain assets from Coin, Inc., a privately-held company, which was accounted for as a business combination, for total cash consideration of $7.0 million , of which $3.9 million was allocated to in-process research and development intangible assets, and $3.1 million to goodwill. The acquisition is expected to enhance the features and functionality of the Company’s devices. In-process research and development is not amortized until the completion or abandonment of the related development. Goodwill is deductible for tax purposes.

Basis of Presentation and Sum18

Basis of Presentation and Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation and Summary of Significant Accounting Policies	The accompanying condensed consolidated financial statements of Fitbit, Inc. (the “Company”) are unaudited. The condensed consolidated balance sheet at December 31, 2017 has been derived from the audited financial statements of the Company. The accompanying condensed financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) for interim financial information, and in management’s opinion, includes all adjustments, consisting of only normal recurring adjustments, necessary for the fair statement of the Company’s financial position, its results of operations, and cash flows for the interim periods presented. The results of operations for the three and six months ended June 30, 2018 are not necessarily indicative of the results to be expected for the full fiscal year or any other period. The accompanying condensed consolidated financial statements should be read in conjunction with the consolidated financial statements and notes thereto contained in the Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2017, filed with the Securities and Exchange Commission (“SEC”) on March 1, 2018. The Company’s fiscal year ends on December 31 of each year. The Company is on a 4-4-5 week quarterly calendar. There were 91 days in each of the three months ended June 30, 2018 and July 1, 2017 , and 181 and 182 days in the six months ended June 30, 2018 and July 1, 2017 , respectively.
Principles of Consolidation	The condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany transactions and balances have been eliminated.
Use of Estimates	Use of Estimates The preparation of condensed consolidated financial statements in accordance with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported and disclosed in the condensed consolidated financial statements and accompanying notes. The primary estimates and assumptions made by management are related to revenue recognition, reserves for sales returns and incentives, reserves for warranty, valuation of stock options, fair value of derivative assets and liabilities, allowance for doubtful accounts, inventory valuation, fair value of goodwill and acquired tangible and intangible assets and liabilities assumed during acquisitions, the number of reportable segments, the recoverability of intangible assets and their useful lives, contingencies, and income taxes. Actual results could differ from those estimates, and such differences may be material to the condensed consolidated financial statements.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Accounting Pronouncements Not Yet Adopted In February 2016, the Financial Accounting Standards Board (the “FASB”) issued ASU 2016-02, Leases (Topic 842). ASU 2016-02 requires lessees to recognize right-of-use assets and lease liabilities for operating leases, initially measured at the present value of the lease payments, on the balance sheet. ASU 2016-02 will become effective for the Company on January 1, 2019, and requires adoption using a modified retrospective approach. The Company is currently evaluating the impact of this guidance on its consolidated financial statements. The Company anticipates that the adoption will have a material impact on its consolidated balance sheets, as it will now include a right of use asset and a lease liability for the obligation to make lease payments related to substantially all operating lease arrangements; however, the Company does not expect the adoption to have a material impact on its consolidated statements of operations. In June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. ASU 2016-13 provides for a new impairment model which requires measurement and recognition of expected credit losses for most financial assets and certain other instruments, including but not limited to accounts receivable and available for sale debt securities. ASU 2016-13 will become effective for the Company on January 1, 2020 and early adoption is permitted. The Company is currently evaluating the impact of this guidance on its consolidated financial statements. In January 2017, the FASB issued ASU 2017-04, Intangibles-Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment. ASU 2017-04 simplifies the subsequent measurement of goodwill by eliminating the second step of the goodwill impairment test. The second step measures a goodwill impairment loss by comparing the implied fair value of a reporting unit’s goodwill with the carrying amount of that goodwill. Under ASU 2017-04, a company will record an impairment charge based on the excess of a reporting unit’s carrying amount over its fair value. ASU 2017-04 will be applied prospectively and is effective for annual or interim goodwill impairment tests in fiscal years beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. The Company is currently evaluating the impact of this guidance on its consolidated financial statements. In June 2018, the FASB issued ASU 2018-07, Compensation-Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting. ASU 2018-07 expands the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from nonemployees. ASU 2018-07 will become effective for the Company on January 1, 2019 and early adoption is permitted. The Company is currently evaluating the impact of this guidance on its consolidated financial statements. Accounting Pronouncements Recently Adopted In May 2014, the FASB, issued ASU 2014-09, Revenue from Contracts with Customers (Topic 606), which affects any entity that either enters into contracts with customers to transfer goods and services or enters into contracts for the transfer of nonfinancial assets. The standard’s core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The Company adopted ASU 2014-09 effective January 1, 2018, utilizing the modified retrospective transition method. Prior periods were not retrospectively adjusted. Upon adoption, the Company recognized an immaterial cumulative effect of adopting this guidance as an adjustment to its opening accumulated deficit balance. The new standard may, in certain circumstances, impact the timing of when revenue is recognized for products shipped, and the timing and classification of certain sales incentives, which are expected to generally be recognized earlier than historical guidance. The Company believes the new guidance is materially consistent with its historical revenue recognition policy. In addition, ASU 2014-09 requires the presentation of sales returns reserve as a current liability. The Company’s sales return reserve was $75.0 million as of June 30, 2018 , presented within “Accrued liabilities” and was $109.9 million as of December 31, 2017, presented within “Accounts receivable, net.” The impact to revenue, accounts receivable, deferred revenue, and accrued liabilities as a result of applying ASU 2014-09 for the six months ended or as of June 30, 2018 was as follows (in thousands): Three Months Ended June 30, 2018 Six Months Ended June 30, 2018 Under ASC 605 Impact Under ASC 606 Under ASC 605 Impact Under ASC 606 Revenue $ 298,783 $ 561 $ 299,344 $ 545,834 $ 1,375 $ 547,209 Accounts receivable, net 167,042 74,996 242,038 167,042 74,996 242,038 Deferred revenue 33,789 (954 ) 32,835 33,789 (954 ) 32,835 Accrued liabilities 302,734 74,324 377,058 302,734 74,324 377,058 Accumulated deficit (332,945 ) 561 (332,384 ) (333,759 ) 1,375 (332,384 ) The impact to other financial statement line items was immaterial. Adoption of the standard had no impact to net cash from or used in operating, investing, or financing activities in the Company’s condensed consolidated statement of cash flows. In January 2016, the FASB issued ASU 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, which updates certain aspects of recognition, measurement, presentation and disclosure of financial instruments. The Company has elected to measure equity investments that do not have readily determinable fair values at cost minus impairment, if any, plus or minus changes resulting from observable price changes in orderly transactions for the identical or a similar investment of the same issuer. The adoption of this standard did not have a material impact on the Company’s consolidated financial statements. In August 2016, the FASB issued ASU 2016-15, Statement of Cash Flows (Topic 230). ASU 2016-15 provides guidance intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. ASU 2016-15 provides guidance in a number of situations including, among others, contingent consideration payments made after a business combination, proceeds from the settlement of insurance claims, distributions received from equity method investees, and classifying cash receipts and payments that have aspects of more than one class of cash flows. ASU 2016-15 became effective for the Company on January 1, 2018. The adoption of this standard did not have a material impact on the Company’s consolidated financial statements. In January 2017, the FASB issued ASU 2017-01, Business Combinations (Topic 805): Clarifying the Definition of a Business. The purpose of ASU 2017-01 is to change the definition of a business to assist entities with evaluating when a set of transferred assets and activities is a business. ASU 2017-01 became effective for the Company on January 1, 2018. The adoption of this standard did not have a material impact on the Company’s consolidated financial statements. In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting. ASU 2017-09 was issued to clarify and reduce both (i) diversity in practice and (ii) cost and complexity when applying the guidance in Topic 718 to a change to the terms and conditions of a share-based payment award. ASU 2017-09 became effective for the Company on January 1, 2018. The adoption of this standard did not have a material impact on the Company’s consolidated financial statements. In August 2017, the FASB issued ASU 2017-12, Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities. ASU 2017-12 amends the hedge accounting rules to simplify the application of hedge accounting standard and better portray the economic results of risk management activities in the financial statements. The standard expands the ability to hedge non-financial and financial risk components, reduces complexity in fair value hedges of interest rate risk, eliminates the requirement to separately measure and report hedge ineffectiveness, as well as eases certain hedge effectiveness assessment requirements. ASU 2017-12 becomes effective for the Company on January 1, 2019 with early adoption permitted. The Company early adopted this new standard in the first quarter of 2018. The adoption of this new standard did not have a material impact on the Company’s consolidated financial statements.

Basis of Presentation and Sum19

Basis of Presentation and Summary of Significant Accounting Policies (Tables)	6 Months Ended
	Jun. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of New Accounting Pronouncements and Changes in Accounting Principles	The impact to revenue, accounts receivable, deferred revenue, and accrued liabilities as a result of applying ASU 2014-09 for the six months ended or as of June 30, 2018 was as follows (in thousands): Three Months Ended June 30, 2018 Six Months Ended June 30, 2018 Under ASC 605 Impact Under ASC 606 Under ASC 605 Impact Under ASC 606 Revenue $ 298,783 $ 561 $ 299,344 $ 545,834 $ 1,375 $ 547,209 Accounts receivable, net 167,042 74,996 242,038 167,042 74,996 242,038 Deferred revenue 33,789 (954 ) 32,835 33,789 (954 ) 32,835 Accrued liabilities 302,734 74,324 377,058 302,734 74,324 377,058 Accumulated deficit (332,945 ) 561 (332,384 ) (333,759 ) 1,375 (332,384 )

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Schedule of fair value of assets and liabilities measured on recurring basis	The following tables set forth the Company’s financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands): June 30, 2018 Level 1 Level 2 Level 3 Total Assets: Money market funds $ 153,241 $ — $ — $ 153,241 U.S. government agencies — 74,897 — 74,897 Corporate debt securities — 217,806 — 217,806 Derivative assets — 5,464 — 5,464 Total $ 153,241 $ 298,167 $ — $ 451,408 Liabilities: Derivative liabilities $ — $ 736 $ — $ 736 Stock warrant liability — — 486 486 Total $ — $ 736 $ 486 $ 1,222 December 31, 2017 Level 1 Level 2 Level 3 Total Assets: Money market funds $ 193,066 $ — $ — $ 193,066 U.S. government agencies — 79,624 — 79,624 Corporate debt securities — 291,582 — 291,582 Total $ 193,066 $ 371,206 $ — $ 564,272 Liabilities: Derivative liabilities $ — $ 2,138 $ — $ 2,138 Stock warrant liability — — 208 208 Total $ — $ 2,138 $ 208 $ 2,346

Financial Instruments (Tables)

Financial Instruments (Tables)	6 Months Ended
	Jun. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Schedule of available for sale securities including fair value and gross unrealized losses	The following table sets forth cash, cash equivalents and marketable securities as of June 30, 2018 (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Cash and Cash Equivalents Marketable Securities Cash $ 134,519 $ — $ — $ 134,519 $ 134,519 $ — Money market funds 153,241 — — 153,241 153,241 — U.S. government agencies 75,032 — (135 ) 74,897 10,376 64,521 Corporate debt securities 217,942 4 (140 ) 217,806 29,980 187,826 Total $ 580,734 $ 4 $ (275 ) $ 580,463 $ 328,116 $ 252,347 The following table sets forth cash, cash equivalents and marketable securities as of December 31, 2017 (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Cash and Cash Equivalents Marketable Securities Cash $ 115,028 $ — $ — $ 115,028 $ 115,028 $ — Money market funds 193,066 — — 193,066 193,066 — U.S. government agencies 79,722 1 (99 ) 79,624 6,595 73,029 Corporate debt securities 291,738 15 (171 ) 291,582 27,277 264,305 Total $ 679,554 $ 16 $ (270 ) $ 679,300 $ 341,966 $ 337,334 The following table classifies marketable securities by contractual maturities (in thousands): June 30, 2018 December 31, 2017 Due in one year $ 252,347 $ 319,112 Due in one to two years — 18,222 Total $ 252,347 $ 337,334
Schedule of derivative instruments in statement of financial position, fair value	The following table presents the fair value of the Company’s foreign currency derivative contracts as of the periods presented (in thousands): June 30, 2018 December 31, 2017 Balance Sheet Location Fair Value Derivative Assets Fair Value Derivative Liabilities Fair Value Derivative Assets Fair Value Derivative Liabilities Cash flow designated hedges Prepaid expenses and other current assets $ 5,420 $ — $ — $ — Cash flow designated hedges Accrued liabilities — — — — Hedges not designated Prepaid expenses and other current assets 44 — — — Hedges not designated Accrued liabilities — 736 — 2,138 Total fair value of derivative instruments $ 5,464 $ 736 $ — $ 2,138
Schedule of cash flow hedging instruments, statement of operations	The following table presents the pre-tax impact of the Company’s foreign currency derivative contracts on other comprehensive income (“OCI”) and the condensed consolidated statements of operations for the periods presented (in thousands): Three Months Ended Six Months Ended Income Statement Location June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 Foreign exchange cash flow hedges: Gain (loss) recognized in OCI – effective portion $ 5,914 $ (13,029 ) $ 6,578 $ (13,987 ) Gain (loss) reclassified from OCI into income – effective portion Revenue 1,157 (1,035 ) 1,157 (280 ) Loss reclassified from OCI into income – effective portion Operating expenses — (440 ) — (1,405 ) Gain recognized in income – ineffective portion Other income, net — — — 21 Gain recognized in income – excluded time value portion Other income, net — 660 — 843 Foreign exchange balance sheet hedges: Gain (loss) recognized in income Other income, net 4,793 (3,547 ) 2,299 (6,776 )
Schedule of derivative contracts on consolidated statement of operations	The following table provides the location in the condensed consolidated statements of operations and amount of the recognized gains or losses to the Company’s derivative instruments designated as hedging instruments (in thousands): Three Months Ended Six Months Ended June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 Total amounts presented in the condensed consolidated statement of operations in which the effects of cash flow hedges are recorded in revenue $ 299,344 $ 353,299 $ 547,209 $ 652,241 Total amounts presented in the condensed consolidated statement of operations in which the effects of cash flow hedges are recorded in operating expenses 218,103 212,654 415,579 422,332 Gains (losses) on foreign exchange contracts designated as cash flow hedges reclassified from OCI into revenue 1,157 (1,035 ) 1,157 (280 ) Gains (losses) on foreign exchange contracts designated as cash flow hedges reclassified from OCI into operating expenses — (440 ) — (1,405 )
Schedule of cash flow hedging instruments, comprehensive income	The following table presents the pre-tax impact of the Company’s foreign currency derivative contracts on other comprehensive income (“OCI”) and the condensed consolidated statements of operations for the periods presented (in thousands): Three Months Ended Six Months Ended Income Statement Location June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 Foreign exchange cash flow hedges: Gain (loss) recognized in OCI – effective portion $ 5,914 $ (13,029 ) $ 6,578 $ (13,987 ) Gain (loss) reclassified from OCI into income – effective portion Revenue 1,157 (1,035 ) 1,157 (280 ) Loss reclassified from OCI into income – effective portion Operating expenses — (440 ) — (1,405 ) Gain recognized in income – ineffective portion Other income, net — — — 21 Gain recognized in income – excluded time value portion Other income, net — 660 — 843 Foreign exchange balance sheet hedges: Gain (loss) recognized in income Other income, net 4,793 (3,547 ) 2,299 (6,776 )
Schedule of offsetting of foreign currency derivative contracts	The following tables set forth the available offsetting of net derivative assets under the master netting arrangements as of June 30, 2018 and December 31, 2017 (in thousands): Gross Amounts Offset in the Condensed Consolidated Balance Sheets Gross Amounts Not Offset in Condensed Consolidated Balance Sheets June 30, 2018 Gross Amounts Recognized Gross Amounts Offset Net Amounts Presented Financial Instruments Cash Collateral Received Net Amount Foreign exchange contracts assets $ 5,464 $ — $ 5,464 $ 736 $ — $ 4,728 Foreign exchange contracts liabilities 736 — 736 736 — — Gross Amounts Offset in the Condensed Consolidated Balance Sheets Gross Amounts Not Offset in Condensed Consolidated Balance Sheets December 31, 2017 Gross Amounts Recognized Gross Amounts Offset Net Amounts Presented Financial Instruments Cash Collateral Received Net Amount Foreign exchange contracts assets $ — $ — $ — $ — $ — $ — Foreign exchange contracts liabilities 2,138 — 2,138 — — 2,138

Balance Sheet Components (Table

Balance Sheet Components (Tables)	6 Months Ended
	Jun. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of deferred revenue	Changes in the total short-term and long-term deferred revenue balance were as follows (in thousands): Three Months Ended Six Months Ended June 30, 2018 June 30, 2018 Beginning balances $ 36,448 $ 42,432 Deferral of revenue 8,328 15,708 Recognition of deferred revenue (11,941 ) (25,305 ) Ending balances $ 32,835 $ 32,835
Schedule of accounts receivable reserves	Revenue returns reserve activities were as follows (in thousands): Three Months Ended Six Months Ended June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 Beginning balances $ 82,612 $ 68,317 $ 109,872 $ 98,851 Increases 36,929 48,285 63,001 89,911 Write-offs/Returns taken (44,545 ) (65,082 ) (97,877 ) (137,242 ) Ending balances $ 74,996 $ 51,520 $ 74,996 $ 51,520
Schedule of inventories	Inventories consisted of the following (in thousands): June 30, 2018 December 31, 2017 Components 11,687 3,825 Finished goods 128,743 120,070 Total inventories $ 140,430 $ 123,895
Schedule of prepaid expenses and other current asset	Prepaid expenses and other current assets consisted of the following (in thousands): June 30, 2018 December 31, 2017 POP displays, net $ 10,345 $ 14,750 Prepaid marketing 2,768 6,074 Derivative assets 5,464 — Prepaid expenses 14,482 24,204 Insurance receivable — 37,300 Other 30,168 14,941 Total prepaid expenses and other current assets $ 63,227 $ 97,269
Schedule of property and equipment	Property and equipment, net, consisted of the following (in thousands): June 30, 2018 December 31, 2017 Tooling and manufacturing equipment $ 76,119 $ 66,854 Furniture and office equipment 22,509 20,942 Purchased and internally-developed software 20,346 18,112 Leasehold improvements 63,787 58,431 Total property and equipment 182,761 164,339 Less: Accumulated depreciation and amortization (72,576 ) (59,431 ) Property and equipment, net $ 110,185 $ 104,908
Schedule of goodwill	Goodwill Balance at December 31, 2017 $ 51,036 Goodwill acquired 10,022 Balance at June 30, 2018 $ 61,058
Schedule of intangible assets (excluding goodwill)	June 30, 2018 December 31, 2017 Weighted Average Remaining Useful Life (years) Gross Accumulated Amortization Net Gross Accumulated Amortization Net Developed technology $ 35,988 $ (12,249 ) $ 23,739 $ 30,588 $ (8,738 ) $ 21,850 3.4 Customer relationships 3,918 (308 ) 3,610 — — — 6.4 Trademarks and other 1,150 (759 ) 391 1,278 (772 ) 506 0.6 Total intangible assets, net $ 41,056 $ (13,316 ) $ 27,740 $ 31,866 $ (9,510 ) $ 22,356
Schedule of estimated future amortization expense	The estimated future amortization expense of acquired finite-lived intangible assets to be charged to cost of revenue and operating expenses after June 30, 2018 was as follows (in thousands): Cost of Revenue Operating Expenses Total Remaining 2018 $ 3,735 $ 386 $ 4,121 2019 6,690 771 7,461 2020 5,910 588 6,498 2021 5,910 541 6,451 2022 1,236 541 1,777 Thereafter 258 1,174 1,432 Total finite-lived intangible assets, net $ 23,739 $ 4,001 $ 27,740
Schedule of accrued liabilities	Accrued liabilities consisted of the following (in thousands): June 30, 2018 December 31, 2017 Product warranty $ 50,468 $ 87,882 Accrued revenue reserve from returns 74,996 — Accrued manufacturing expense and freight 34,493 41,901 Accrued sales incentives 87,756 111,592 Accrued sales and marketing 24,693 44,401 Accrued research and development 4,819 8,983 Accrued co-op advertising and marketing development funds 19,521 30,408 Employee-related liabilities 23,054 33,266 Sales taxes and VAT payable 26,127 21,340 Inventory received but not billed 4,615 10,526 Accrued legal settlements and fees 4,819 36,693 Derivative liabilities 736 2,138 Other 20,961 23,007 Accrued liabilities $ 377,058 $ 452,137
Schedule of product warranty reserves	Product warranty reserve activities were as follows (in thousands) (1) : Three Months Ended Six Months Ended June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 Beginning balances $ 71,975 $ 90,459 $ 87,882 $ 99,923 Charged to cost of revenue (8,486 ) (1,595 ) (10,967 ) 17,342 Changes related to pre-existing warranties (4,469 ) 4,913 (7,872 ) 2,473 Settlement of claims (8,552 ) (21,016 ) (18,575 ) (46,977 ) Ending balances $ 50,468 $ 72,761 $ 50,468 $ 72,761 (1) Does not include reserves established as a result of the recall of the Fitbit Force. See the section titled “ — Fitbit Force Recall Reserve” in the Company’s Annual Report on Form 10-K for additional information regarding such reserves.
Schedule of accumulated other comprehensive income	The components and activity of accumulated other comprehensive income (“AOCI”), net of tax, were as follows (in thousands): Unrealized Gains (Losses) on Cash Flow Hedges Currency Translation Adjustments Unrealized Gains (Losses) on Available-for-Sale Investments Total Balance at December 31, 2017 $ 66 $ — $ (75 ) $ (9 ) Other comprehensive income (loss) before reclassifications 5,912 — (208 ) 5,704 Amounts reclassified from AOCI (1,028 ) — 12 (1,016 ) Other comprehensive income (loss) 4,884 — (196 ) 4,688 Balance at June 30, 2018 $ 4,950 $ — $ (271 ) $ 4,679

Commitments and Contingencies (

Commitments and Contingencies (Tables)	6 Months Ended
	Jun. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Rental Payments for Operating Leases	Remaining 2018 $ 17,328 2019 33,571 2020 29,398 2021 27,136 2022 26,855 Thereafter 37,555 Total future minimum lease payments $ 171,843

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	6 Months Ended
	Jun. 30, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of stock option activity	Stock option activity under the equity incentive plans was as follows (in thousands, except per share amounts): Stock Options Outstanding Number of Shares Subject to Stock Options Weighted– Average Exercise Price Aggregate Intrinsic Value (1) Balance—December 31, 2017 21,386 $ 3.01 Granted — — Exercised (1,768 ) 2.55 Forfeited or canceled (561 ) 7.02 Balance—June 30, 2018 19,057 2.93 $ 58,535 Stock options vested and expected to vest—June 30, 2018 19,057 2.93 58,535 Stock options exercisable—June 30, 2018 16,654 2.61 55,317 (1) The aggregate intrinsic values of stock options outstanding, exercisable, vested and expected to vest as of June 30, 2018 were calculated as the difference between the exercise price of the stock options and the fair value of the Class A common stock of $6.53 as of June 30, 2018 .
Schedule of restricted stock unit activity	RSU activity under the equity incentive plans was as follows (in thousands, except per share amounts): RSUs Outstanding Weighted- Average Grant Date Fair Value Unvested balance—December 31, 2017 19,188 $ 9.13 Granted 12,557 5.02 Vested (4,810 ) 8.80 Forfeited or canceled (3,135 ) 8.24 Unvested balance—June 30, 2018 23,800 $ 7.15
Schedule of stock-based compensation expense	Total stock-based compensation expense recognized was as follows (in thousands): Three Months Ended Six Months Ended June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 Cost of revenue $ 2,032 $ 1,492 $ 3,130 $ 1,510 Research and development 15,090 12,648 29,761 27,333 Sales and marketing 3,911 3,987 7,358 7,622 General and administrative 4,824 3,839 9,249 7,994 Total stock-based compensation expense $ 25,857 $ 21,966 $ 49,498 $ 44,459

Net Loss per Share (Tables)

Net Loss per Share (Tables)	6 Months Ended
	Jun. 30, 2018
Earnings Per Share [Abstract]
Schedule of basic and diluted net income (loss) per share	The following table sets forth the computation of the Company’s basic and diluted net loss per share (in thousands, except per share amounts): Three Months Ended Six Months Ended June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 Numerator: Net loss $ (118,268 ) $ (58,240 ) $ (199,145 ) $ (118,319 ) Denominator: Weighted-average shares of common stock—basic for Class A and Class B 242,898 230,322 241,227 228,788 Effect of dilutive securities — — — — Weighted-average shares of common stock—diluted for Class A and Class B 242,898 230,322 241,227 228,788 Net loss per share: Basic $ (0.49 ) $ (0.25 ) $ (0.83 ) $ (0.52 ) Diluted $ (0.49 ) $ (0.25 ) $ (0.83 ) $ (0.52 )
Schedule of antidilutive securities excluded from earnings per share	The following potentially dilutive common shares were excluded from the computation of diluted net loss per share for the periods presented because including them would have been anti-dilutive (in thousands): Three Months Ended Six Months Ended June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 Stock options to purchase common stock 12,201 19,074 12,203 20,924 RSUs 8,035 9,929 8,865 10,066 Diluted impact of ESPP 189 70 170 152 Diluted common stock subject to vesting — 110 — 120 Total 20,425 29,183 21,238 31,262

Significant Customer Informat26

Significant Customer Information and Other Information (Tables)	6 Months Ended
	Jun. 30, 2018
Risks and Uncertainties [Abstract]
Schedules of concentration of risk	Revenue by geographic region, based on ship-to destinations, was as follows (in thousands): Three Months Ended Six Months Ended June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 United States $ 182,451 $ 199,201 $ 321,947 $ 369,621 Americas excluding United States 15,838 24,412 31,938 44,380 Europe, Middle East, and Africa 65,969 108,601 130,507 196,373 APAC 35,086 21,085 62,817 41,867 Total $ 299,344 $ 353,299 $ 547,209 $ 652,241 Retailers and distributors with revenue equal to or greater than 10% of total revenue for the three and six months ended June 30, 2018 and July 1, 2017 were as follows: Three Months Ended Six Months Ended June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 C 11 % 14 % * 12 % B * * 10 % * A * 11 * 11 * Represents less than 10%. Retailers and distributors with revenue equal to or greater than 10% of total revenue for the three and six months ended June 30, 2018 and July 1, 2017 were as follows: Three Months Ended Six Months Ended June 30, 2018 July 1, 2017 June 30, 2018 July 1, 2017 C 11 % 14 % * 12 % B * * 10 % * A * 11 * 11 * Represents less than 10%. Retailers and distributors that accounted for equal to or greater than 10% of accounts receivable at June 30, 2018 and December 31, 2017 were as follows: June 30, 2018 December 31, 2017 C 17 % 17 % B 15 13 D 11 * E * 11 * Represents less than 10%.

- Accounting Pronouncements Rec

- Accounting Pronouncements Recently Adopted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017	Mar. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash provided by financing activities			$ 4	$ 6,173
Net cash provided by operating activities			$ (57,217)	3,228
Best estimated selling price on upgrade rights			$ 0.99
Revenue	$ 299,344	$ 353,299	$ 547,209	652,241
Accounts receivable, net	242,038		242,038			$ 406,019
Deferred revenue	28,116		28,116			35,504
Deferred Revenue	32,835		32,835		$ 36,448	42,432
Accrued liabilities	377,058		377,058			452,137
Accumulated deficit	(332,384)		(332,384)			(132,112)
Accounting Standards Update 2014-09 [Member] | Accrued liabilities [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reserves and accruals, returns and allowances	75,000		$ 75,000
Accounting Standards Update 2014-09 [Member] | Accounts Receivable [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reserves and accruals, returns and allowances						$ 109,900
Accounting Standards Update 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash provided by operating activities				25,500
Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated selling price			$ 1
Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated selling price			$ 3
Subscription-based Premium Services [Member] | Customer Concentration Risk [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Concentration risk, percentage			1.00%
Advertising Barter Transactions [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Nonmonetary transaction, advertising credit utilized during the period	1,000	$ 200	$ 1,100	$ 300
Under ASC 605 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	298,783		545,834
Accounts receivable, net	167,042		167,042
Deferred revenue	33,789		33,789
Accrued liabilities	302,734		302,734
Impact [Member] | Accounting Standards Update 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue	561		1,375
Accounts receivable, net	74,996		74,996
Deferred revenue	(954)		(954)
Accrued liabilities	74,324		74,324
Three Months Ended Under ASC 605 [Member] | Under ASC 605 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit	(332,945)		(332,945)
Three Month Ended Impact under 606 [Member] | Impact [Member] | Accounting Standards Update 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit	561		561
Six Months Ended Under ASC 605 [Member] | Under ASC 605 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit	(333,759)		(333,759)
Six Month Ended Impact under 606 [Member] | Impact [Member] | Accounting Standards Update 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accumulated deficit	$ 1,375		$ 1,375

Basis of Presentation and Sum28

Basis of Presentation and Summary of Significant Accounting Policies - Customer Bankruptcy & Non-Monetary Transactions (Details) - USD ($) $ in Millions	Jul. 01, 2017	Jan. 31, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017	Sep. 30, 2017	Dec. 31, 2016
Concentration Risk [Line Items]
Release of product return and rebate reserves							$ 12.4
WynitDistribution [Member] | Customer Concentration Risk [Member]
Concentration Risk [Line Items]
Unrecognized revenue, net					$ 8.1
Allowance for Doubtful Accounts Receivable					35.8
Unrecoverable inventory					5.5
Accounts receivable for shipments made in the quarter					30.3				$ 30.3
Insurance proceeds receivable, accounts receivable					22.7				22.7
Probably insurance proceeds					26.8				26.8
Insurance proceeds receivable, inventory					4.1				4.1
Charges offset by insurance recorded									$ 9
Bad debt expense					7.6
Cost of revenue					$ 1.4
Proceeds from insurance settlement		$ 5.4		$ 21.4
WynitDistribution [Member] | Revenue [Member] | Customer Concentration Risk [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	11.00%
WynitDistribution [Member] | Accounts Receivable [Member] | Customer Concentration Risk [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	19.00%
Advertising Barter Transactions [Member]
Concentration Risk [Line Items]
Nonmonetary transaction, gross operating revenue recognized										$ 15
Nonmonetary transaction, cost of goods transferred										7
Nonmonetary transaction, advertising credit received										13
Nonmonetary transaction, cash received										$ 2
Nonmonetary transaction, remaining advertising credit			$ 11.1				$ 11.1
Nonmonetary transaction, advertising credit, expected period of use							4 years
Nonmonetary transaction, advertising credit utilized during the period			$ 1			$ 0.2	$ 1.1	$ 0.3

Fair Value Measurements (Detail

Fair Value Measurements (Details) - USD ($)	3 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Dec. 31, 2017
Assets:
Available for sale securities	$ 252,347,000		$ 252,347,000	$ 337,334,000
Liabilities:
Transfers between fair value levels	0	$ 0	0
U.S. government agencies [Member]
Assets:
Available for sale securities	64,521,000		64,521,000	73,029,000
Corporate debt securities [Member]
Assets:
Available for sale securities	187,826,000		187,826,000	264,305,000
Level 3 [Member]
Assets:
Total	0		0	0
Other Assets [Member]
Liabilities:
Cash paid for acquisition				6,000,000
Fair Value, Measurements, Recurring [Member]
Assets:
Money market funds	153,241,000		153,241,000	193,066,000
Derivative assets	5,464,000		5,464,000
Total	451,408,000		451,408,000	564,272,000
Liabilities:
Derivative liabilities	736,000		736,000	2,138,000
Stock warrant liability	486,000		486,000	208,000
Total	1,222,000		1,222,000	2,346,000
Fair Value, Measurements, Recurring [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	74,897,000		74,897,000	79,624,000
Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	217,806,000		217,806,000	291,582,000
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Money market funds	153,241,000		153,241,000	193,066,000
Derivative assets	0		0
Total	153,241,000		153,241,000	193,066,000
Liabilities:
Derivative liabilities	0		0	0
Stock warrant liability	0		0	0
Total	0		0	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	0		0	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	0		0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets:
Money market funds	0		0	0
Derivative assets	5,464,000		5,464,000
Total	298,167,000		298,167,000	371,206,000
Liabilities:
Derivative liabilities	736,000		736,000	2,138,000
Stock warrant liability	0		0	0
Total	736,000		736,000	2,138,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	74,897,000		74,897,000	79,624,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	217,806,000		217,806,000	291,582,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Assets:
Money market funds	0		0	0
Derivative assets	0		0
Total	0		0	0
Liabilities:
Derivative liabilities	0		0	0
Stock warrant liability	486,000		486,000	208,000
Total	486,000		486,000	208,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	0		0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	$ 0		$ 0	$ 0

Financial Instruments - Amortiz

Financial Instruments - Amortized to fair value (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jul. 01, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	$ 328,116	$ 341,966	$ 318,708	$ 301,320
Available-for-sale debt securities, gross unrealized gains	4	16
Available-for-sale debt securities, gross unrealized losses	(275)	(270)
Cash, cash equivalents, and available-for-sale securities, amortized cost	580,734	679,554
Cash, cash equivalents, available-for-sale securities, fair value	580,463	679,300
Total	252,347	337,334
U.S. government agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	10,376	6,595
Available-for-sale debt securities, amortized cost	75,032	79,722
Available-for-sale debt securities, gross unrealized gains		1
Available-for-sale debt securities, gross unrealized losses	(135)	(99)
Cash, cash equivalents, available-for-sale securities, fair value	74,897	79,624
Total	64,521	73,029
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	29,980	27,277
Available-for-sale debt securities, amortized cost	217,942	291,738
Available-for-sale debt securities, gross unrealized gains	4	15
Available-for-sale debt securities, gross unrealized losses	(140)	(171)
Cash, cash equivalents, available-for-sale securities, fair value	217,806	291,582
Total	187,826	264,305
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	134,519	115,028
Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	$ 153,241	$ 193,066

Financial Instruments - Unreali

Financial Instruments - Unrealized Loss Position (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities in continuous loss position for one year or more, gross unrealized losses	$ 0	$ 0

Financial Instruments - Contrac

Financial Instruments - Contractual maturity dates (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Due in one year	$ 252,347	$ 319,112
Due in one to two years	0	18,222
Total	$ 252,347	$ 337,334

Financial Instruments - Additio

Financial Instruments - Additional Derivative Information (Details) - Foreign currency exchange contract [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jul. 01, 2017	Dec. 31, 2015	Dec. 31, 2017
Not designated as hedging instrument [Member]
Derivative [Line Items]
Derivative, notional amount	$ 110,500				$ 141,200
Cash flow hedges [Member] | Designated as hedging instrument [Member]
Derivative [Line Items]
Derivative, notional amount based on forecasted revenue	99,700				$ 0
Other Income, Net [Member] | Cash flow hedges [Member] | Designated as hedging instrument [Member]
Derivative [Line Items]
Gain (loss) recognized in income – ineffective portion	$ 0	$ 0	$ 21	$ 0

- Financial Position, Fair Valu

- Financial Position, Fair Value (Details) - Foreign currency exchange contract [Member] - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Fair Value Derivative Assets	$ 5,464	$ 0
Fair Value Derivative Liabilities	736	2,138
Designated as hedging instrument [Member] | Prepaid expenses and other current assets [Member]
Derivatives, Fair Value [Line Items]
Fair Value Derivative Assets	5,420	0
Fair Value Derivative Liabilities	0	0
Designated as hedging instrument [Member] | Accrued liabilities [Member]
Derivatives, Fair Value [Line Items]
Fair Value Derivative Assets	0	0
Fair Value Derivative Liabilities	0	0
Not designated as hedging instrument [Member] | Prepaid expenses and other current assets [Member]
Derivatives, Fair Value [Line Items]
Fair Value Derivative Assets	44	0
Fair Value Derivative Liabilities	0	0
Not designated as hedging instrument [Member] | Accrued liabilities [Member]
Derivatives, Fair Value [Line Items]
Fair Value Derivative Assets	0	0
Fair Value Derivative Liabilities	$ 736	$ 2,138

Financial Instruments - Stateme

Financial Instruments - Statement of Operations and Other Comprehensive Income (Details) - Foreign currency exchange contract [Member] - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017	Dec. 31, 2015
Designated as hedging instrument [Member] | Cash flow hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in OCI – effective portion	$ 5,914,000	$ (13,029,000)		$ (13,987,000)	$ 6,578,000
Revenue [Member] | Designated as hedging instrument [Member] | Cash flow hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from OCI into income – effective portion					1,157,000
Operating expenses [Member] | Designated as hedging instrument [Member] | Cash flow hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from OCI into income – effective portion					0
Other Income, Net [Member] | Designated as hedging instrument [Member] | Cash flow hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in income – ineffective portion	0	0		21,000	0
Gain recognized in income – excluded time value portion	0	660,000	$ 0	843,000
Other Income, Net [Member] | Not designated as hedging instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in income	$ 4,793,000	(3,547,000)		(6,776,000)	$ 2,299,000
Reclassification out of accumulated other comprehensive income [Member] | Accumulated net gain (loss) from cash flow hedges attributable to parent [Member] | Revenue [Member] | Designated as hedging instrument [Member] | Cash flow hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from OCI into income – effective portion		(1,035,000)	1,157,000	(280,000)
Reclassification out of accumulated other comprehensive income [Member] | Accumulated net gain (loss) from cash flow hedges attributable to parent [Member] | Operating expenses [Member] | Designated as hedging instrument [Member] | Cash flow hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from OCI into income – effective portion		$ (440,000)	$ 0	$ (1,405,000)

Financial Instruments - Schedul

Financial Instruments - Schedule of Derivative Contracts on Consolidated Statement of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Revenue	$ 299,344	$ 353,299	$ 547,209	$ 652,241
Cost of revenue	180,329	204,054	314,071	384,697
Accumulated net gain (loss) from cash flow hedges attributable to parent [Member] | Reclassification out of accumulated other comprehensive income [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Revenue	1,157	(1,035)	1,157	(280)
Cost of revenue	0	(440)	0	(1,405)
Cash flow hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Cost of revenue	$ 218,103	$ 212,654	$ 415,579	$ 422,332

Financial Instruments - Offsett

Financial Instruments - Offsetting of Foreign Currency Derivative Contracts (Details) - Foreign currency exchange contract [Member] - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Offsetting Liabilities [Line Items]
Gross Assets Recognized	$ 736	$ 2,138
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Net Amounts Presented in Condensed Consolidated Balance Sheets	736	2,138
Gross Amounts Not Offset in Condensed Consolidated Balance Sheets, Financial Instruments	736	0
Gross Amounts Not Offset in Condensed Consolidated Balance Sheets, Cash Collateral Pledged	0	0
Net Amount	0	2,138
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	5,464	0
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Net Amounts Presented in Condensed Consolidated Balance Sheets	5,464	0
Gross Amounts Not Offset in Condensed Consolidated Balance Sheets, Financial Instruments	736	0
Gross Amounts Not Offset in Condensed Consolidated Balance Sheets, Cash Collateral Received	0	0
Net Amount	$ 4,728	$ 0

Balance Sheet Components - Defe

Balance Sheet Components - Deferred Revenue (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Deferred Revenue [Roll Forward]
Beginning balances	$ 36,448	$ 42,432
Deferral of revenue	8,328	15,708
Recognition of deferred revenue	(11,941)	(25,305)
Ending balances	$ 32,835	$ 32,835

Balance Sheet Components - Reve

Balance Sheet Components - Revenue Returns Reserves (Details) - Revenue Reserve [Member] - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balance	$ 82,612	$ 68,317	$ 109,872	$ 98,851
Increases	36,929	48,285	63,001	89,911
Write-offs/Returns taken	(44,545)	(65,082)	97,877	137,242
Ending balances	$ 74,996	$ 51,520	$ 74,996	$ 51,520

Balance Sheet Components - Inve

Balance Sheet Components - Inventories (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Components	$ 11,687	$ 3,825
Finished goods	128,743	120,070
Total inventories	$ 140,430	$ 123,895

Balance Sheet Components - Prep

Balance Sheet Components - Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
POP displays, net	$ 10,345	$ 14,750
Prepaid marketing	2,768	6,074
Derivative assets	5,464	0
Prepaid expenses	14,482	24,204
Insurance receivable	0	37,300
Other	30,168	14,941
Total prepaid expenses and other current assets	$ 63,227	$ 97,269

Balance Sheet Components - Prop

Balance Sheet Components - Property and Equipment, Net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 182,761		$ 182,761		$ 164,339
Less: Accumulated depreciation and amortization	(72,576)		(72,576)		(59,431)
Property and equipment, net	110,185		110,185		104,908
Depreciation and amortization expense	13,100	$ 10,100	23,600	$ 19,199
Tooling and manufacturing equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	76,119		76,119		66,854
Furniture and office equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	22,509		22,509		20,942
Purchased and internally-developed software [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	20,346		20,346		18,112
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 63,787		$ 63,787		$ 58,431

Balance Sheet Components - Good

Balance Sheet Components - Goodwill (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018USD ($)
Goodwill [Roll Forward]
Balance at the beginning of the period	$ 51,036
Goodwill acquired	10,022
Balance at the end of the period	$ 61,058

Balance Sheet Components - Inta

Balance Sheet Components - Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross	$ 41,056		$ 41,056		$ 31,866
Accumulated Amortization	(13,316)		(13,316)		(9,510)
Net	27,740		27,740		22,356
Intangible assets, net	27,740		27,740		22,356
Amortization of intangible assets	2,100	$ 1,400	3,805	$ 2,753
Developed technology [Member]
Finite-Lived Intangible Assets [Line Items]
Gross	35,988		35,988		30,588
Accumulated Amortization	(12,249)		(12,249)		(8,738)
Net	23,739		$ 23,739		21,850
Weighted Average Remaining Useful Life (years)			3 years 4 months 24 days
Customer Relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross	3,918		$ 3,918		0
Accumulated Amortization	(308)		(308)		0
Net	3,610		$ 3,610		0
Weighted Average Remaining Useful Life (years)			6 years 4 months 24 days
Trademarks and other [Member]
Finite-Lived Intangible Assets [Line Items]
Gross	1,150		$ 1,150		1,278
Accumulated Amortization	(759)		(759)		(772)
Net	$ 391		$ 391		$ 506
Weighted Average Remaining Useful Life (years)			7 months 6 days

Balance Sheet Components - Esti

Balance Sheet Components - Estimated Future Amortization Expense (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Remaining 2,018	$ 4,121
2,019	7,461
2,020	6,498
2,021	6,451
2,022	1,777
Thereafter	1,432
Net	27,740	$ 22,356
Cost of revenue [Member]
Finite-Lived Intangible Assets [Line Items]
Remaining 2,018	3,735
2,019	6,690
2,020	5,910
2,021	5,910
2,022	1,236
Thereafter	258
Net	23,739
Operating expenses [Member]
Finite-Lived Intangible Assets [Line Items]
Remaining 2,018	386
2,019	771
2,020	588
2,021	541
2,022	541
Thereafter	1,174
Net	$ 4,001

Balance Sheet Components - Accr

Balance Sheet Components - Accrued Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Product warranty	$ 50,468	$ 87,882
Accrued revenue reserve from returns	74,996	0
Accrued manufacturing expense and freight	34,493	41,901
Accrued sales incentives	87,756	111,592
Accrued sales and marketing	24,693	44,401
Accrued research and development	4,819	8,983
Accrued co-op advertising and marketing development funds	19,521	30,408
Employee-related liabilities	23,054	33,266
Sales taxes and VAT payable	26,127	21,340
Inventory received but not billed	4,615	10,526
Accrued legal settlements and fees	4,819	36,693
Derivative liabilities	736	2,138
Other	20,961	23,007
Accrued liabilities	$ 377,058	$ 452,137

Balance Sheet Components - Prod

Balance Sheet Components - Product Warranty (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Movement in Standard and Extended Product Warranty, Increase (Decrease) [Roll Forward]
Beginning balances	$ 71,975	$ 90,459	$ 87,882	$ 99,923
Charged to cost of revenue	(8,486)	(1,595)	(10,967)	17,342
Changes related to pre-existing warranties	(4,469)	4,913	(7,872)	2,473
Settlement of claims	(8,552)	(21,016)	(18,575)	(46,977)
Ending balances	$ 50,468	$ 72,761	$ 50,468	$ 72,761

Balance Sheet Components - Rest

Balance Sheet Components - Restructuring (Details) - Employee Severance [Member] $ in Millions	1 Months Ended
	Jan. 31, 2017USD ($)employee
Restructuring Cost and Reserve [Line Items]
Expected number of positions eliminated | employee	110
Number of positions eliminated in the period, percent	6.00%
Severance costs | $	$ 6.4

Balance Sheet Components - Accu

Balance Sheet Components - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			$ 823,963
Net change, net of tax	$ 130	$ 6	(196)	$ 73
Balance, end of period	678,959		678,959
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(9)
Other comprehensive income (loss) before reclassifications			5,704
Amounts reclassified from AOCI			(1,016)
Net change, net of tax			4,688
Balance, end of period	4,679		4,679
Unrealized Gains (Losses) on Cash Flow Hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			66
Other comprehensive income (loss) before reclassifications			5,912
Amounts reclassified from AOCI			(1,028)
Net change, net of tax			4,884
Balance, end of period	4,950		4,950
Currency Translation Adjustments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			0
Other comprehensive income (loss) before reclassifications			0
Amounts reclassified from AOCI			0
Net change, net of tax			0
Balance, end of period	0		0
Unrealized Gains (Losses) on Available-for-Sale Investments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance, beginning of period			(75)
Other comprehensive income (loss) before reclassifications			(208)
Amounts reclassified from AOCI			12
Net change, net of tax			(196)
Balance, end of period	$ (271)		$ (271)

Long-Term Debt (Details)

Long-Term Debt (Details) - USD ($)	1 Months Ended
	May 31, 2017	Jun. 30, 2018	Dec. 31, 2017	May 03, 2017	Dec. 31, 2015
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Credit facility, amount outstanding		$ 36,800,000	$ 36,900,000
2015 Credit Agreement [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Credit facility, borrowing capacity				$ 100,000,000	$ 250,000,000
Covenant, minimum unrestricted cash	$ 200,000,000
Covenant, cash and cash equivalents held in accounts subject to control agreements	$ 100,000,000
Credit facility, amount outstanding		$ 0
2015 Credit Agreement [Member] | Letter of Credit [Member]
Line of Credit Facility [Line Items]
Credit facility, borrowing capacity				50,000,000	50,000,000
2015 Credit Agreement [Member] | Swing Line Loan [Member]
Line of Credit Facility [Line Items]
Credit facility, borrowing capacity				$ 25,000,000	$ 25,000,000
2015 Credit Agreement [Member] | Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Stated interest rate				1.50%
2015 Credit Agreement [Member] | Minimum [Member] | Line of Credit [Member] | Base Rate [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.50%
2015 Credit Agreement [Member] | Maximum [Member] | London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Stated interest rate				2.00%
2015 Credit Agreement [Member] | Maximum [Member] | Line of Credit [Member] | Base Rate [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.00%

Commitments and Contingencies -

Commitments and Contingencies - Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity (Details) $ in Thousands	Jun. 30, 2018USD ($)
Commitments and Contingencies Disclosure [Abstract]
Remaining 2,018	$ 17,328
2,019	33,571
2,020	29,398
2,021	27,136
2,022	26,855
Thereafter	37,555
Total future minimum lease payments	$ 171,843

Commitments and Contingencies52

Commitments and Contingencies - Narrative (Details) $ in Millions	Mar. 05, 2018patent	Jan. 19, 2018USD ($)	Jul. 10, 2017patent	Jun. 09, 2017lawsuit	May 31, 2016investor	Feb. 16, 2016lawsuit	Jun. 30, 2018USD ($)	Jul. 01, 2017USD ($)	Jun. 30, 2018USD ($)	Jul. 01, 2017USD ($)	Aug. 23, 2017defendant
Loss Contingencies [Line Items]
Rent expense							$ 7.2	$ 10	$ 16.8	$ 19.2
Settlement amount award to other party		$ 33.3
Loss contingency, individual investors named as lead plaintiffs | investor					5
Pending Litigation [Member]
Loss Contingencies [Line Items]
Loss contingency, number of lawsuits filed | lawsuit				2
PurePulse Class Action Lawsuit [Member] | Pending Litigation [Member]
Loss Contingencies [Line Items]
Loss contingency, number of lawsuits filed | lawsuit						2
Immersion [Member] | Pending Litigation [Member]
Loss Contingencies [Line Items]
Number of patents found not infringed | patent	2
Patents allegedly infringed, number | patent			3
Number of additional defendants | defendant											2
Purchase Commitment [Member]
Loss Contingencies [Line Items]
Remaining minimum amount committed							387.6		387.6
Purchase Commitment to Third Party Hosting Provider [Member]
Loss Contingencies [Line Items]
Remaining minimum amount committed							185		185
Accrued liabilities [Member] | Purchase Commitment [Member]
Loss Contingencies [Line Items]
Remaining minimum amount committed							$ 14.3		$ 14.3

Stockholders' Equity - Narrativ

Stockholders' Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	Jul. 20, 2017	Apr. 14, 2017	May 31, 2015	Jun. 30, 2018	Jul. 19, 2017	Jul. 10, 2017
Employee Stock Options And Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to unvested options				$ 159.2
Unrecognized compensation expense related to unvested options, estimated weighted average period				2 years 1 month 6 days
Common Class A [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of securities called by each warrant						500,000
Exercise price of warrant (in usd per share)						$ 5.23
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period				10 years
Warrants outstanding				500,000
Common Class A [Member] | 2015 Employee Stock Purchase Plan [Member] | Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance (in shares)			3,800,000
Percent of share price available to employees			85.00%
Eligible Employees [Member] | 2015 Equity Incentive Plan [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unfunded right to receive stock on a date in time in the future (in shares)		1
Out-of-the-money options		2
Minimum vesting		1 year
Underwater options tendered					3,700,000
Underwater options tendered, percent					85.00%
Stock issued in exchange (in shares)	1,800,000
Stock issued in exchange	$ 8.5

Stockholders' Equity - Stock Op

Stockholders' Equity - Stock Option Activity (Details) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended
	Jun. 30, 2018USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Balance, beginning of period (in shares) | shares	21,386
Granted (in shares) | shares	0
Exercised (in shares) | shares	(1,768)
Canceled (in shares) | shares	(561)
Balance, end of period (in shares) | shares	19,057
Options exercisable (in shares) | shares	19,057
Options vested and expected to vest (in shares) | shares	16,654
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Balance, weighted average exercise price, beginning of period (in dollars per share)	$ 3.01
Granted, weighted average exercise price (in dollars per share)	0
Exercised, weighted average exercise price (in dollars per share)	0.0255
Canceled, weighted average exercise price (in dollars per share)	0.0702
Balance, weighted average exercise price, end of period (in dollars per share)	0.0293
Options exercisable, weighted average exercise price (in dollars per share)	0.0293
Options vested and expected to vest, weighted average exercise price (in dollars per share)	$ 0.0261
Options outstanding, aggregate intrinsic value, end of period | $	$ 58,535
Options exercisable, aggregate intrinsic value | $	58,535
Options vested and expected to vest, aggregate intrinsic value | $	$ 55,317
Fair value of the Class A common stock (in dollars per share)	$ 6.53

Stockholders' Equity - Restrict

Stockholders' Equity - Restricted Stock Unit Activity (Details) - Restricted Stock Units (RSUs) [Member] shares in Thousands	6 Months Ended
	Jun. 30, 2018$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Beginning Unvested balance (in shares) | shares	19,188
Granted (in shares) | shares	12,557
Vested (in shares) | shares	(4,810)
Forfeited or canceled (in shares) | shares	(3,135)
Ending Unvested balance (in shares) | shares	23,800
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Beginning Unvested balance (in dollars per share) | $ / shares	$ 9.13
Granted (in dollars per share) | $ / shares	5.02
Vested (in dollars per share) | $ / shares	8.80
Forfeited or canceled (in dollars per share) | $ / shares	8.24
Ending Unvested balance (in dollars per share) | $ / shares	$ 7.15

Stockholders' Equity - Stock Co

Stockholders' Equity - Stock Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	$ 25,857	$ 21,966	$ 49,498	$ 44,459
Cost of revenue [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	2,032	1,492	3,130	1,510
Research and development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	15,090	12,648	29,761	27,333
Sales and marketing [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	3,911	3,987	7,358	7,622
General and administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation	$ 4,824	$ 3,839	$ 9,249	$ 7,994

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Income Tax Disclosure [Abstract]
Income tax benefit	$ (23,615)	$ 4,673	$ (23,006)	$ 34,344
Effective income tax rate	(24.90%)	7.40%	(13.10%)	22.50%
Unrecognized tax benefits	$ 36,000		$ 36,000
Unrecognized tax benefits that would impact effective tax rate	$ 25,400		$ 25,400

Net Loss per Share - Schedule o

Net Loss per Share - Schedule of Basic and Diluted EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Numerator:
Net loss	$ (118,268)	$ (58,240)	$ (199,145)	$ (118,319)
Denominator:
Weighted-average shares of common stock—basic (in shares)	242,898	230,322	241,227	228,788
Effect of dilutive securities (in shares)	0	0	0	0
Weighted-average shares of common stock—diluted (in shares)	242,898	230,322	241,227	228,788
Net loss per share:
Basic (in dollars per share)	$ (0.49)	$ (0.25)	$ (0.83)	$ (0.52)
Diluted (in dollars per share)	$ (0.49)	$ (0.25)	$ (0.83)	$ (0.52)

- Antidilutive Securities (Deta

- Antidilutive Securities (Details) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	20,425	29,183	21,238	31,262
Employee Stock Option [Member] | Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	12,201	19,074	12,203	20,924
Restricted Stock Units (RSUs) [Member] | Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	8,035	9,929	8,865	10,066
Diluted Impact of ESPP [Member] | Common Stock [Member] | 2015 Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	189	70	170	152
Diluted Common Stock Subject to Vesting [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	0	110	0	120

Significant Customer Informat60

Significant Customer Information and Other Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017	Dec. 31, 2017	Dec. 31, 2015
Concentration Risk [Line Items]
Revenue	$ 299,344	$ 353,299	$ 547,209	$ 652,241
Geographic Concentration Risk [Member] | Non-US [Member]
Concentration Risk [Line Items]
Long-lived assets including property and equipment	$ 36,300		$ 36,300		$ 30,000
Revenue [Member] | Customer Concentration Risk [Member] | Customer C [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	11.00%	14.00%		12.00%
Revenue [Member] | Customer Concentration Risk [Member] | Customer B [Member]
Concentration Risk [Line Items]
Concentration risk, percentage			10.00%
Revenue [Member] | Customer Concentration Risk [Member] | Customer A [Member]
Concentration Risk [Line Items]
Concentration risk, percentage		11.00%		11.00%
Revenue [Member] | Geographic Concentration Risk [Member]
Concentration Risk [Line Items]
Revenue		$ 353,299	$ 299,344	$ 652,241		$ 547,209
Revenue [Member] | Geographic Concentration Risk [Member] | United States [Member]
Concentration Risk [Line Items]
Revenue		199,201	182,451	369,621		321,947
Revenue [Member] | Geographic Concentration Risk [Member] | Americas excluding United States [Member]
Concentration Risk [Line Items]
Revenue		24,412	15,838	44,380		31,938
Revenue [Member] | Geographic Concentration Risk [Member] | Europe, Middle East, and Africa [Member]
Concentration Risk [Line Items]
Revenue		108,601	65,969	196,373		130,507
Revenue [Member] | Geographic Concentration Risk [Member] | APAC [Member]
Concentration Risk [Line Items]
Revenue		$ 21,085	$ 35,086	$ 41,867		$ 62,817
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Customer C [Member]
Concentration Risk [Line Items]
Concentration risk, percentage			17.00%		17.00%
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Customer B [Member]
Concentration Risk [Line Items]
Concentration risk, percentage			15.00%		13.00%
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Customer D [Member]
Concentration Risk [Line Items]
Concentration risk, percentage			11.00%
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Customer E [Member]
Concentration Risk [Line Items]
Concentration risk, percentage					11.00%

Acquisitions - Narrative (Detai

Acquisitions - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended			12 Months Ended
	Feb. 28, 2018	Dec. 31, 2016	May 31, 2016	Dec. 31, 2016	Jun. 30, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill					$ 61,058	$ 51,036
Twine Health, Inc. [Member]
Business Acquisition [Line Items]
Cash paid for acquisition	$ 16,700
Goodwill	10,000
Deferred tax liabilities	1,800
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Deferred Revenue	200
Assumed liabilities	600
Consideration held as security for indemnifications obligations	2,600
Twine Health, Inc. [Member] | In Process Research and Development [Member]
Business Acquisition [Line Items]
Intangibles acquired	$ 5,400
Amortization period	4 years
Twine Health, Inc. [Member] | Customer Relationships [Member]
Business Acquisition [Line Items]
Intangibles acquired	$ 3,800
Amortization period	7 years
Pebble Industries, Inc [Member]
Business Acquisition [Line Items]
Cash paid for acquisition		$ 23,400
Goodwill		14,400		$ 14,400
Assumed liabilities		600		600
Consideration held as security for indemnifications obligations		3,500
Pebble Industries, Inc [Member] | In Process Research and Development [Member]
Business Acquisition [Line Items]
Intangibles acquired		9,600		$ 9,600
Amortization period				5 years
Vector Watch S.R.L. [Member]
Business Acquisition [Line Items]
Cash paid for acquisition		15,000
Goodwill		11,400		$ 11,400
Assumed liabilities		300		300
Consideration held as security for indemnifications obligations		2,300
Vector Watch S.R.L. [Member] | In Process Research and Development [Member]
Business Acquisition [Line Items]
Intangibles acquired		$ 3,900		$ 3,900
Amortization period		2 years 6 months
Coin, Inc [Member]
Business Acquisition [Line Items]
Cash paid for acquisition			$ 7,000
Goodwill			3,100
Coin, Inc [Member] | In Process Research and Development [Member]
Business Acquisition [Line Items]
Intangibles acquired			$ 3,900