Fitbit 10-K 1 Mar 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2017	Feb. 20, 2018	Jun. 30, 2017
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-K
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 1
Common Class A [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		208,207,282
Common Class B [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		31,289,730

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 341,966	$ 301,320
Marketable securities	337,334	404,693
Accounts receivable, net	406,019	477,825
Inventories	123,895	230,387
Income tax receivable	77,882	481
Prepaid expenses and other current assets	97,269	65,865
Total current assets	1,384,365	1,480,571
Property and equipment, net	104,908	76,553
Goodwill	51,036	51,036
Intangible assets, net	22,356	27,521
Deferred tax assets	3,990	175,797
Other assets	15,420	10,448
Total assets	1,582,075	1,821,926
Current liabilities:
Accounts payable	212,731	313,773
Accrued liabilities	452,137	390,561
Deferred revenue	35,504	42,612
Income taxes payable	928	9,394
Total current liabilities	701,300	756,340
Long-term deferred revenue	6,928	7,292
Other liabilities	49,884	59,762
Total liabilities	758,112	823,394
Commitments and contingencies (Note 7)
Stockholders’ equity:
Preferred stock, $0.0001 par value, 10,000,000 shares authorized	0	0
Additional paid-in capital	956,060	859,345
Accumulated other comprehensive loss	(9)	(978)
Retained earnings (accumulated deficit)	(132,112)	140,142
Total stockholders’ equity	823,963	998,532
Total liabilities and stockholders’ equity	1,582,075	1,821,926
Common Class A [Member]
Stockholders’ equity:
Common stock	21	18
Common Class B [Member]
Stockholders’ equity:
Common stock	$ 3	$ 5

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, authorized (in shares)	10,000,000	10,000,000
Common Class A [Member]
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock authorized (in shares)	600,000,000	600,000,000
Common stock issued (in shares)	207,453,624	177,212,531
Common stock outstanding (in shares)	207,453,624	177,212,531
Common Class B [Member]
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock authorized (in shares)	350,000,000	350,000,000
Common stock issued (in shares)	31,302,898	48,450,746
Common stock outstanding (in shares)	31,302,898	48,450,746

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Revenue	$ 1,615,519	$ 2,169,461	$ 1,857,998
Cost of revenue	924,618	1,323,577	956,935
Gross profit	690,901	845,884	901,063
Operating expenses:
Research and development	343,012	320,191	150,035
Sales and marketing	415,042	491,255	332,741
General and administrative	133,934	146,903	77,793
Change in contingent consideration	0	0	(7,704)
Total operating expenses	891,988	958,349	552,865
Operating income (loss)	(201,087)	(112,465)	348,198
Interest income (expense), net	3,647	3,156	(1,019)
Other income (expense), net	2,796	14	(59,230)
Income (loss) before income taxes	(194,644)	(109,295)	287,949
Income tax expense (benefit)	82,548	(6,518)	112,272
Net income (loss)	(277,192)	(102,777)	175,677
Less: noncumulative dividends to preferred stockholders	0	0	(2,526)
Less: undistributed earnings to participating securities	0	0	(59,133)
Net income (loss) attributable to common stockholders—basic	(277,192)	(102,777)	114,018
Add: adjustments for undistributed earnings to participating securities	0	0	8,821
Net income (loss) attributable to common stockholders—diluted	$ (277,192)	$ (102,777)	$ 122,839
Net income (loss) per share attributable to common stockholders:
Basic (in dollars per share)	$ (1.19)	$ (0.47)	$ 0.88
Diluted (in dollars per share)	$ (1.19)	$ (0.47)	$ 0.75
Shares used to compute net income (loss) per share attributable to common stockholders:
Basic (in shares)	232,032	220,405	129,886
Diluted (in shares)	232,032	220,405	164,213

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ (277,192)	$ (102,777)	$ 175,677
Cash flow hedges:
Change in unrealized gain on cash flow hedges, net of tax expense (benefit) of ($1), ($1,251), and $1,509, respectively	(19,422)	9,422	1,276
Less reclassification for realized net gains included in net income (loss), net of tax expense of $74, $509, and $759, respectively	19,965	(10,650)	(525)
Net change, net of tax	543	(1,228)	751
Available-for-sale investments:
Change in unrealized loss on investments	125	(126)	(63)
Less reclassification for realized net gains included in net income (loss)	(13)	(6)	8
Net change, net of tax	112	(132)	(55)
Change in foreign currency translation adjustment, net of tax	314	(309)	(42)
Comprehensive income (loss)	$ (276,223)	$ (104,446)	$ 176,331

Consolidated Statements of Com6

Consolidated Statements of Comprehensive Income (Loss) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Change in unrealized gain on cash flow hedges, tax	$ (1)	$ (1,251)	$ 1,509
Reclassification for realized net gains included in net income, tax	$ 74	$ 509	$ 759

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity (Deficit) Statement - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings (Accumulated Deficit) [Member]
Beginning balance (in shares) at Dec. 31, 2014	139,851,483
Beginning balance at Dec. 31, 2014	$ 67,814
Increase (Decrease) in Temporary Equity [Roll Forward]
Issuance of redeemable convertible preferred stock upon net exercise of redeemable convertible preferred stock warrants (unaudited) (in shares)	1,485,583
Issuance of redeemable convertible preferred stock upon exercise of redeemable convertible preferred stock warrants	$ 56,678
Conversion of redeemable convertible preferred stock to common stock upon public offering (unaudited) (in shares)	(141,337,066)
Conversion of redeemable convertible preferred stock to common stock upon initial public offering	$ (124,492)
Ending balance (in shares) at Dec. 31, 2015	0
Ending balance at Dec. 31, 2015	$ 0
Beginning balance (in shares) at Dec. 31, 2014		40,875,583
Beginning balance at Dec. 31, 2014	75,262	$ 4	$ 7,979	$ 37	$ 67,242
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon public offering, net of offering costs (in shares)		25,387,500
Issuance of common stock upon public offerings, net of offering costs	499,105	$ 3	499,102
Conversion of redeemable convertible preferred stock to common stock upon public offering (in shares)		141,337,066
Conversion of redeemable convertible preferred stock to common stock upon initial public offering	124,492	$ 14	124,478
Reclassification of redeemable convertible preferred stock warrant liability into additional paid in capital upon initial public offering	15,774		15,774
Issuance of common stock upon exercise of stock options (in shares)		5,396,591
Issuance of common stock upon exercise of stock options	4,018		4,018
Issuance of common stock in connection with acquisition (in shares)		1,059,688
Issuance of common stock in connection with acquisition	13,317		13,317
Issuance of common stock subject to vesting in connection with acquisition (in shares)		308,216
Issuance of common stock upon net exercise of common stock warrants (in shares)		416,929
Stock-based compensation expense	41,052		41,052
Cumulative effect adjustment related to recognition of previously unrecognized excess tax benefits from adoption of ASU 2016-09	32,100		32,100
Net income (loss)	175,677				175,677
Other comprehensive Income (loss)	654			654
Ending balance (in shares) at Dec. 31, 2015		214,781,573
Ending balance at Dec. 31, 2015	$ 981,451	$ 21	737,820	691	242,919
Ending balance (in shares) at Dec. 31, 2016	0
Ending balance at Dec. 31, 2016	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)		10,881,704
Issuance of common stock upon exercise of stock options	25,814	$ 2	25,812
Stock-based compensation expense	79,107		79,107
Taxes related to net share settlement of restricted stock units	(4,939)		(4,939)
Cumulative effect adjustment related to recognition of previously unrecognized excess tax benefits from adoption of ASU 2016-09	21,545		21,545
Net income (loss)	(102,777)				(102,777)
Other comprehensive Income (loss)	(1,669)			(1,669)
Ending balance (in shares) at Dec. 31, 2016		225,663,277
Ending balance at Dec. 31, 2016	$ 998,532	$ 23	859,345	(978)	140,142
Ending balance (in shares) at Dec. 31, 2017	0
Ending balance at Dec. 31, 2017	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)		13,093,245
Issuance of common stock upon exercise of stock options	19,011	$ 1	19,010
Stock-based compensation expense	92,081		92,081
Taxes related to net share settlement of restricted stock units	(14,376)		(14,376)
Cumulative effect adjustment related to recognition of previously unrecognized excess tax benefits from adoption of ASU 2016-09	4,938		0		4,938
Net income (loss)	(277,192)
Other comprehensive Income (loss)	969			969
Ending balance (in shares) at Dec. 31, 2017		238,756,522
Ending balance at Dec. 31, 2017	$ 823,963	$ 24	$ 956,060	$ (9)	$ (132,112)

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows from Operating Activities
Net income (loss)	$ (277,192)	$ (102,777)	$ 175,677
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Provision for doubtful accounts	7,893	339	1,115
Provision for excess and obsolete inventory	14,833	4,993	5,060
Depreciation	39,971	36,046	19,405
Amortization of intangible assets	5,722	2,087	1,702
Accelerated depreciation of property and equipment	5,250	19,805	1,206
Amortization of issuance costs and discount on debt	951	466	961
Stock-based compensation	91,581	79,432	41,024
Deferred income taxes	173,906	(100,434)	(42,538)
Revaluation of redeemable convertible preferred stock warrant liability	0	0	56,655
Change in contingent consideration	0	0	(7,704)
Other	216	(423)	(263)
Changes in operating assets and liabilities, net of acquisitions:
Accounts receivable	63,784	(8,701)	(231,100)
Inventories	92,129	(61,975)	(68,108)
Prepaid expenses and other assets	(113,111)	(37,876)	(29,215)
Fitbit Force recall reserve	(789)	(3,869)	(17,354)
Accounts payable	(86,115)	45,654	56,759
Accrued liabilities and other liabilities	56,172	213,361	138,748
Deferred revenue	(7,472)	5,456	34,891
Income taxes payable	(3,488)	47,136	4,336
Net cash provided by operating activities	64,241	138,720	141,257
Cash Flows from Investing Activities
Purchase of property and equipment	(89,160)	(78,640)	(30,566)
Purchase of marketable securities	(597,933)	(638,055)	(230,935)
Sales of marketable securities	42,406	46,511	58,011
Maturities of marketable securities	622,525	315,774	44,500
Acquisitions, net of cash acquired	(556)	(38,256)	(11,037)
Equity investment	(6,000)	0	0
Net cash used in investing activities	(28,718)	(392,666)	(170,027)
Cash Flows from Financing Activities
Payment of offering costs	0	(1,236)	(5,089)
Proceeds from issuance of common stock	19,011	25,969	4,018
Taxes paid related to net share settlement of restricted stock units	(14,376)	(4,939)	0
Proceeds from public offerings, net of underwriting discounts and commissions	0	0	505,275
Proceeds from issuance of debt and revolving credit facility	0	0	160,000
Repayment of debt	0	0	(294,503)
Payment of issuance costs	0	0	(748)
Net cash provided by financing activities	4,635	19,794	368,953
Net increase (decrease) in cash and cash equivalents	40,158	(234,152)	340,183
Effect of exchange rates on cash and cash equivalents	488	(374)	37
Cash and cash equivalents at beginning of period	301,320	535,846	195,626
Cash and cash equivalents at end of period	341,966	301,320	535,846
Supplemental Disclosure
Cash paid for interest	1,019	624	1,157
Cash paid for income taxes	382	34,014	150,923
Supplemental Disclosure of Non-Cash Investing and Financing Activity
Purchase of property and equipment included in accounts payable and accrued liabilities	4,197	19,778	10,534
Conversion of redeemable convertible preferred stock into Class B common stock	0	0	124,492
Reclassification of redeemable convertible preferred stock warrant liability to additional paid in capital	0	0	15,774
Issuance of redeemable convertible preferred stock upon net exercise of redeemable convertible preferred stock warrants	0	0	56,678
Deferred offering costs included in accounts payable and accruals	0	0	1,080
Issuance of common stock in connection with acquisitions	0	0	13,317
Contingent consideration related to acquisitions	$ 0	$ 0	$ (7,704)

Business Overview and Basis of

Business Overview and Basis of Presentation	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Business Overview and Basis of Presentation	Business Overview and Basis of Presentation Description of Business Fitbit, Inc. (the “Company”) is a technology company focused on driving health solutions and positively impacting health outcomes. The Fitbit platform combines wearable devices with software and services to give its users tools to help them reach their health and fitness goals. The Company’s wearable devices, which include health and fitness trackers and smartwatches, enable its users to view data about their daily activity, exercise and sleep in real-time. The Company’s software and services, which include an online dashboard and mobile app, provide its users with data analytics, motivational and social tools, and virtual coaching through customized fitness plans and interactive workouts, drive engagement and can be leveraged to provide personalized insights. The Company sells devices through diversified sales channels that include distributors, retailers, a corporate wellness offering, and Fitbit.com. The Company has established wholly-owned subsidiaries globally and its corporate headquarters are located in San Francisco, California. Basis of Presentation and Principles of Consolidation The consolidated financial statements are prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”). The consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany transactions and balances have been eliminated. The Company’s fiscal year ends on December 31 of each year. In the first quarter of 2016, the Company adopted a 4-4-5 week quarterly calendar, which, for the 2016 fiscal year, was comprised of four fiscal quarters ending on April 2, 2016, July 2, 2016, October 1, 2016, and December 31, 2016. Use of Estimates The preparation of 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 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 consolidated financial statements. Comprehensive Income (Loss) Comprehensive income (loss) consists of two components, net income (loss) and other comprehensive income (loss), net of tax. Other comprehensive income refers to revenue, expenses, and gains and losses that are recorded as an element of stockholders’ equity but are excluded from net income (loss). The Company’s other comprehensive income (loss) consists of net unrealized gains and losses on derivative instruments accounted for as cash flow hedges, foreign currency translation adjustments from those subsidiaries not using the U.S. dollar as their functional currency, and unrealized gains and losses on available-for-sale securities. Out-of-Period Adjustments During the preparation of the consolidated financial statements as of and for the year ended December 31, 2017, the Company identified errors within the Company’s consolidated balance sheet for the year ended December 31, 2016, which financial statements were revised to correct the errors. The Company determined that within the consolidated balance sheet previously disclosed as of December 31, 2016, deferred revenue was overstated by $7.3 million and other liabilities was understated by the same amount, and deferred tax assets and income taxes payable were understated by $1.7 million . The Company evaluated the error and concluded that it was not material to the financial statements previously issued. 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. 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 year ended December 31, 2017 , $0.8 million of credits were utilized. The Company’s prepaid and other assets related to unused advertising credits as of December 31, 2017 and December 31, 2016 were $12.2 million and $13.0 million , respectively.

Significant Accounting Policies

Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Significant Accounting Policies	Significant Accounting Policies Cash, Cash Equivalents and Marketable Securities Cash and cash equivalents include all cash balances and highly liquid investments with original maturities of three months or less from the date of purchase. Cash equivalents and marketable securities consist of money market funds, U.S. government and agency securities, commercial paper, and corporate notes and bonds. 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 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 expense, net as incurred. Realized gains and losses on the sale of securities are determined by specific identification of each security’s cost basis. Investments are reviewed periodically to identify possible other-than-temporary impairments. No impairment loss has been recorded on the securities as the Company believes that any 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. Fair Value of Financial Instruments Assets and liabilities recorded at fair value on a recurring basis are categorized based upon the level of judgment associated with inputs used to measure their fair values. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the reporting date. The Company estimates fair value by applying the following hierarchy, which prioritizes the inputs used to measure fair value into three levels and bases the categorization within the hierarchy upon the lowest level of input that is available and significant to the fair value measurement: Level 1 —Quoted prices in active markets for identical assets or liabilities; Level 2 —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 for substantially the full term of the assets or liabilities; and Level 3 —Inputs that are generally unobservable and typically reflect management’s estimate of assumptions that market participants would use in pricing the asset or liability. Foreign Currencies The Company and all of its wholly-owned subsidiaries use the U.S. dollar as their functional currency. The Company’s subsidiaries that use the U.S. dollar as their functional currency remeasure local currency denominated monetary assets and liabilities at exchange rates in effect at the end of each period, and inventories, property, plant and equipment and other nonmonetary assets and liabilities at historical rates. Gains and losses from these remeasurements have been included in the Company’s operating results within other income (expense), net. Local currency transactions of these international operations are remeasured into U.S. dollars at the rates of exchange in effect at the date of the transaction. Foreign currency transaction gains (losses) were $2.6 million , $ 11.7 million , and $(1.3) million for 2017 , 2016 , and 2015, respectively. Derivative Instruments The Company accounts for its derivative instruments as either assets or liabilities and carries them at fair value. Derivatives held by the Company that are not designated as hedges are adjusted to fair value through earnings at each reporting date. In addition, the Company enters into derivatives that are accounted for as cash flow hedges. The Company records the gains or losses, net of tax, related to the effective portion of its cash flow hedges as a component of accumulated other comprehensive income in stockholders’ equity and subsequently reclassifies the gains or losses into revenue and operating expenses when the underlying hedged transactions are recognized. The Company periodically assesses the effectiveness of its cash flow hedges. The fair value of derivative assets and liabilities are included in prepaid expenses and other current assets and accrued liabilities on the consolidated balance sheets. Concentration of Credit Risk Financial instruments that potentially subject the Company to concentrations of credit risk consist primarily of cash, cash equivalents, marketable securities, accounts receivables, derivative instruments and cost method investments. Cash and cash equivalents are deposited with high quality financial institutions and may, at times, exceed federally insured limits. Management believes that the financial institutions that hold the Company’s deposits are financially credit worthy and, accordingly, minimal credit risk exists with respect to those balances. Generally, these deposits may be redeemed upon demand and, therefore, bear minimal interest rate risk. The Company’s accounts receivable is derived from customers located principally in the United States. The Company maintains credit insurance for the majority of its customer balances, performs ongoing credit evaluations of its customers, and maintains allowances for potential credit losses on customers’ accounts when deemed necessary. Credit losses historically have not been significant. The Company continuously monitors customer payments and maintains an allowance for doubtful accounts based on its assessment of various factors including historical experience, age of the receivable balances, and other current economic conditions or other factors that may affect customers’ ability to pay. The Company’s derivative instruments expose it to credit risk to the extent that its counterparties may be unable to meet the terms of the agreements. The Company seeks to mitigate this risk by limiting counterparties to major financial institutions and by spreading the risk across several major financial institutions. In addition, the potential risk of loss with any one counterparty resulting from this type of credit risk is monitored on an ongoing basis. Supplier Concentration The Company relies on third parties for the supply and manufacture of its products, as well as third-party logistics providers. In instances where these parties fail to perform their obligations, the Company may be unable to find alternative suppliers or satisfactorily deliver its products to its customers on time, if at all. Inventories Inventories consist of finished goods and component parts, which are purchased from contract manufacturers and component suppliers. Inventories are stated at the lower of cost or net realizable value. The Company assesses the valuation of inventory and periodically writes down the value for estimated excess and obsolete inventory based upon estimates of future demand and market conditions. Property and Equipment, Net Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization of property and equipment is calculated using the straight-line method over the estimated useful lives of the assets. Cost of maintenance and repairs that do not improve or extend the lives of the respective assets are expensed as incurred. The useful lives of the property and equipment are as follows: Tooling and manufacturing equipment One to three years Furniture and office equipment Three years Purchased software Three years Capitalized internally-developed software Two to eight years Leasehold improvements Shorter of remaining lease term or ten years Internally-Developed Software Costs The Company capitalizes eligible costs to acquire, develop, or modify internal-use software that are incurred subsequent to the preliminary project stage. Capitalized internally-developed software costs, net, were $11.2 million as of December 31, 2017 and $6.3 million as of December 31, 2016. Research and Development Research and development expenses consist primarily of personnel-related expenses, consulting and contractor expenses, tooling and prototype materials, and allocated overhead costs. Substantially all of the Company’s research and development expenses are related to developing new products and services and improving existing products and services. To date, research and development expenses have been expensed as incurred, because the release of products and services for sale has been short and development costs qualifying for capitalization have been insignificant. Business Combinations, Goodwill, and Intangible Assets The Company allocates the fair value of purchase consideration to tangible assets, liabilities assumed, and intangible assets acquired based on their estimated fair values. The excess of the fair value of purchase consideration over the fair values of these identifiable assets and liabilities is allocated to goodwill. The allocation of the purchase consideration requires management to make significant estimates and assumptions, especially with respect to intangible assets. These estimates can include, but are not limited to, future expected cash flows from acquired customers, acquired technology, and trade names from a market participant perspective, useful lives, and discount rates. Management’s estimates of fair value are based upon assumptions believed to be reasonable, but which are inherently uncertain and unpredictable and, as a result, actual results may differ from estimates. During the measurement period, which is up to one year from the acquisition date, the Company may record adjustments to the assets acquired and liabilities assumed, with the corresponding offset to goodwill. Upon the conclusion of the measurement period, any subsequent adjustments are recorded to earnings. The Company assesses goodwill for impairment at least annually during the fourth quarter and whenever events or changes in circumstances indicate that the carrying value of the asset may not be recoverable. Consistent with the determination that the Company has one operating segment, the Company has determined that there is one reporting unit and tests goodwill for impairment at the entity level. Goodwill is tested using the two-step process in accordance with ASC 350, Intangibles—Goodwill and Other . In the first step, the carrying amount of the reporting unit is compared to the fair value based on the fair value of the Company’s common stock. If the fair value of the reporting unit exceeds the carrying value, goodwill is not considered impaired and no further testing is required. If the carrying value of the reporting unit exceeds the fair value, goodwill is potentially impaired and the second step of the impairment test must be performed. In the second step, the implied fair value of the goodwill, as defined by ASC 350, is compared to its carrying amount to determine the amount of impairment loss, if any. The Company tested goodwill for impairment as of October 31, 2017 and 2016, and the fair value of the reporting unit exceeded the carrying value. The Company considered other factors in the performance of the annual goodwill impairment test in the fourth quarter of 2017, including assumptions about expected future revenue forecasts, changes in the overall economy, trends in its stock price, and other operating conditions. It is reasonably possible that the Company could perform significantly below its expectations or a deterioration of market and economic conditions could occur. This would adversely impact the Company's ability to meet its projected results, which could cause its goodwill to become impaired. If the Company determines that its goodwill is impaired, it would be required to record a non-cash charge that could have a material adverse effect on its results of operations and financial position. Acquired finite-lived intangible assets are amortized over their estimated useful lives. The Company evaluates the recoverability of intangible assets for possible impairment whenever events or circumstances indicate that the carrying amount of such assets may not be recoverable. Recoverability of these assets is measured by a comparison of the carrying amounts to the future undiscounted cash flows the assets are expected to generate. If such review indicates that the carrying amount of intangible assets is not recoverable, the carrying amount of such assets is reduced to fair value. The Company has not recorded any such impairment charge during the years presented. Impairment of Long-Lived Assets The Company evaluates its long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparison of the carrying amounts to the expected future undiscounted cash flows attributable to these assets. If it is determined that an asset is not recoverable, an impairment loss is recorded in the amount by which the carrying amount of the assets exceeds the expected discounted future cash flows arising from those assets. Revenue Recognition 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 recognizes revenue when persuasive evidence of an arrangement exists, delivery has occurred, the sales price is fixed or determinable, and collection is reasonably assured. The Company considers delivery of its products to have occurred once title and risk of loss has been transferred. For customers where transfer of risk of loss is at the customer’s destination, the Company uses estimates to defer sales at the end of the reporting period based on historical experience of average transit time. The Company recognizes revenue, net of estimated sales returns, sales incentives, discounts, and sales tax. The Company generally recognizes revenue for products sold through retailers and distributors on a sell-in basis. The Company enters into multiple element arrangements that include hardware, software, and services. The first deliverable 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 deliverable 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 deliverable 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. Commencing in the first quarter of 2015, the Company began accounting for the embedded right as a separate unit of accounting, which is when it believes, through public announcements, it had created an implied obligation to, from time to time, provide future unspecified firmware upgrades and features to the firmware to improve and add new functionality to the health and fitness devices. In addition, the Company occasionally offers a fourth deliverable in bundled arrangements that allows access to subscription-based services related to the Company’s Fitbit Coach offering. The Company allocates revenue to all deliverables based on their relative selling prices. The Company uses a hierarchy to determine the selling price to be used for allocating revenue to deliverables: (i) vendor-specific objective evidence (“VSOE”) of fair value, (ii) third-party evidence (“TPE”), and (iii) best estimate of the selling price (“BESP”). The Company’s process for determining its BESP considers multiple factors including consumer behaviors and the Company’s internal pricing model and may vary depending upon the facts and circumstances related to each deliverable. BESP for the health and fitness devices and smartwatches and unspecified upgrade rights 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. BESP for upgrade rights currently ranges from $1 to $3 . TPE for the online dashboard and mobile apps is currently estimated at $0.99 . VSOE 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 access to software and services to certain customers in the corporate wellness program, which includes distribution capabilities, a real-time dashboard, and support services. The Company is currently unable to establish VSOE or TPE for the corporate wellness software and services. BESP for the corporate wellness software and services 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 software and services 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 accounts for shipping and handling fees billed to customers as revenue. Sales taxes and value added taxes (“VAT”) collected from customers 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 accounts receivable. Through December 31, 2017 , actual returns have primarily been open-box returns. In addition, through 2017, 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 an identifiable benefit in exchange for credits claimed by the customer and can reasonably estimate the fair value of the identifiable 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. Cost of Revenue Cost of revenue consists of product costs, including costs of contract manufacturers for production, shipping and handling costs, packaging, warranty replacement costs, fulfillment costs, manufacturing and tooling equipment depreciation, warehousing costs, excess and obsolete inventory write-downs, costs related to the Fitbit Force product recall, and certain allocated costs related to management, facilities, and personnel-related expenses and other expenses associated with supply chain logistics. Advertising Costs and Point of Purchase (“POP”) Displays Costs related to advertising and promotions, excluding cooperative advertising costs, are expensed to sales and marketing as incurred. Advertising and promotion expenses, including expenses for POP displays, for 2017 , 2016 , and 2015 were $226.3 million , $316.8 million , and $237.0 million , respectively. Co-op advertising costs are recorded as a reduction to revenue, and for 2017 , 2016 , and 2015 were $45.0 million , $52.9 million , and $38.3 million , respectively. The Company provides retailers with POP displays, generally free of charge, in order to facilitate the marketing of the Company’s products within retail stores. Any amounts capitalized related to the costs of the POP displays are recorded as prepaid expenses and other current assets on the consolidated balance sheets and recognized as expense over the expected period of the benefit provided by these assets, which is generally 12 months. The related expenses are included in sales and marketing expenses on the consolidated statements of operations. Product Warranty The Company offers a standard product warranty that its products will operate under normal use for a period of one -year from the date of original purchase, except in the European Union and certain Asia Pacific countries where the Company provides a two -year warranty. The Company has the obligation, at its option, to either repair or replace a defective product. At the time revenue is recognized, an estimate of future warranty costs is recorded as a component of cost of revenues. The estimate of future warranty costs is based on historical and projected warranty claim rates, historical and projected cost-per-claim and knowledge of specific product failures, if any, that are outside of the Company’s typical experience. The Company regularly review these estimates to assess the appropriateness of its recorded warranty liabilities and adjust the amounts as necessary. Factors that affect the warranty obligation include product failure rates, service delivery costs incurred in correcting the product failures, and warranty policies. The Company’s products are manufactured by contract manufacturers, and in certain cases, the Company may have recourse against such contract manufacturers. Should actual product failure rates, use of materials or other costs differ from the Company’s estimates, additional warranty liabilities could be incurred, which could materially affect its results of operations. The estimates and assumptions used to reserve for product warranty have been accurate in all material respects and have not materially changed in the past. Fitbit Force Product Recall The Company established reserves for the Fitbit Force recall when circumstances giving rise to the recall became known. It considered various factors in estimating the product recall exposure. These include estimates for: • refunds and product returns from retailer and distributor customers and end-users, which were charged to revenue and cost of revenue on the consolidated statements of operations; • logistics and handling fees for managing product returns and processing refunds, obsolescence of on-hand inventory, cancellation charges for existing purchase commitments and rework of component inventory by the Company’s contract manufacturers, accelerated depreciation of tooling and manufacturing equipment, which were charged to cost of revenue on the consolidated statements of operations; and • legal fees and settlement costs, which were charged to general and administrative expenses on the consolidated statements of operations. These factors above are updated and reevaluated each period and the related reserves are adjusted when factors indicate that the recall reserves are either insufficient to cover or exceed the estimated product recall expenses. Stock-Based Compensation Stock-based compensation is measured at the grant date based on the fair value of the award and is recognized as expense over the requisite service period, which is generally the vesting period of the respective award. Determining the fair value of stock-based awards at the grant date requires judgment. The fair value of restricted stock units, or RSUs, is the fair value of the Company’s common stock on the grant date. The Company uses the Black-Scholes option-pricing model to determine the fair value of stock options, warrants and shares issued under the 2015 Employee Stock Purchase Plan (the “2015 ESPP”). The Company recognizes tax benefits related to stock-based compensation to the extent that the total reduction to its income tax liability from stock-based compensation is greater than the amount of the deferred tax assets previously recorded in anticipation of these benefits. Segment Information The Company operates as one operating segment as it only reports financial information on an aggregate and consolidated basis to its Chief Executive Officer, who is the Company’s chief operating decision maker. Income Taxes The Company utilizes the asset and liability method of accounting for income taxes, which requires the recognition of deferred tax assets and liabilities for expected future consequences of temporary differences between the financial reporting and income tax bases of assets and liabilities using enacted tax rates. The Company makes estimates, assumptions, and judgments to determine our expense (benefit) for income taxes and also for deferred tax assets and liabilities and any valuation allowances recorded against its deferred tax assets. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes that recovery is not likely, the Company establishes a valuation allowance. The calculation of the Company’s income tax expense involves the use of estimates, assumptions, and judgments while taking into account current tax laws, its interpretation of current tax laws, and possible outcomes of future tax audits. The Company has established reserves to address potential exposures related to tax positions that could be challenged by tax authorities. Although the Company believes its estimates, assumptions, and judgments to be reasonable, any changes in tax law or its interpretation of tax laws and the resolutions of potential tax audits could significantly impact the amounts provided for income taxes in its consolidated financial statements. The U.S. Tax Cuts and Jobs Act of 2017 (the “2017 Tax Act”) was enacted on December 22, 2017, 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. The 2017 Tax Act requires complex computations to be performed that were not previously required in U.S. tax law, and preparation and analysis of information not previously required or regularly produced, which require significant judgment in interpreting accounting guidance for such items that is currently uncertain. In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”), which allows the Company to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. The Company has provided its best estimate of the impact of the 2017 Tax Act in its year-end income tax provision in accordance with its understanding of the 2017 Tax Act and guidance available as of the date of issuance of these consolidated financial statements. As the Company completes its analysis of the 2017 Tax Act, any subsequent adjustments to provisional amounts that it has recorded may or may not impact its provision for income taxes in the period in which the adjustments are made due to a full valuation allowance on its U.S. deferred tax assets. The Company expects to complete its analysis within the measurement period in accordance with SAB 118. The calculation of the Company’s deferred tax asset balance involves the use of estimates, assumptions, and judgments while taking into account estimates of the amounts and type of future taxable income. Actual future operating results and the underlying amount and type of income could differ materially from its estimates, assumptions, and judgments, thereby impacting its financial position and operating results. The Company includes interest and penalties related to unrecognized tax benefits within income tax expense. Interest and penalties related to unrecognized tax benefits have been recognized in the appropriate periods presented. Net Income (Loss) per Share Attributable to Common Stockholders Basic and diluted net income (loss) per share attributable to common stockholders is presented in conformity with the two-class method required for participating securities. The Company considers its redeemable convertible preferred stock to be participating securities. The holders of the redeemable convertible preferred stock did not have a contractual obligation to share in losses. In accordance with the two-class method, earnings allocated to these participating securities and the related number of outstanding shares of the participating securities, which include contractual participation rights in undistributed earnings, have been excluded from the computation of basic and diluted net income per share attributable to common stockholders. For the calculation of diluted net income per share, net income attributable to common stockholders for basic net income per share is adjusted by the effect of dilutive securities. Diluted net income per share attributable to common stockholders is computed by dividing the net income attrib

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements Fair Value Measurement of Financial Assets and Liabilities The carrying values of the Company’s accounts receivable and accounts payable, 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): 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 $ — $ 2,138 $ 208 $ 2,346 December 31, 2016 Level 1 Level 2 Level 3 Total Assets: Money market funds $ 50,125 $ — $ — $ 50,125 U.S. government agencies — 86,526 — 86,526 Corporate debt securities — 390,286 — 390,286 Derivative assets — 10,625 — 10,625 Total $ 50,125 $ 487,437 $ — $ 537,562 Liabilities: Derivative liabilities $ — $ 3,780 $ — $ 3,780 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 4. 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 was no activity relating to Level 3 financial assets or liabilities during 2017 and 2016, and there were no Level 3 financial assets or liabilities as of December 31, 2017 and 2016. There have been no transfers between fair value measurement levels during 2017 , 2016 and 2015 . 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. This investment has been recorded at historical cost, classified within “other assets” on the Company’s consolidated balance sheet as of December 31, 2017, and is reviewed for events or changes in circumstances that may have a significant adverse effect on its carrying value. There have been no changes in circumstances or identified events that may have a significant adverse effect on its carrying value.

Financial Instruments

Financial Instruments	12 Months Ended
	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [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 the 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 sets forth the cash, cash equivalents, and marketable securities as of December 31, 2016 (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Cash and Cash Equivalents Marketable Securities Cash $ 179,076 $ — $ — $ 179,076 $ 179,076 $ — Money market funds 50,125 — — 50,125 50,125 — U.S. government agencies 86,533 8 (15 ) 86,526 — 86,526 Corporate debt securities 390,466 24 (204 ) 390,286 72,119 318,167 Total $ 706,200 $ 32 $ (219 ) $ 706,013 $ 301,320 $ 404,693 The gross unrealized gains or losses on marketable securities as of December 31, 2017 and December 31, 2016 were not material. There were no available-for-sale investments as of December 31, 2017 and December 31, 2016 that have been in a continuous unrealized loss position for greater than twelve months on a material basis. The following table classifies marketable securities by contractual maturities (in thousands): December 31, 2017 December 31, 2016 Due in one year $ 319,112 $ 355,152 Due in one to two years 18,222 49,541 Total $ 337,334 $ 404,693 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 the effective portion of 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. The Company records the gains or losses related to the ineffective and excluded time value portion of the cash flow hedges, if any, immediately in other income (expense), net. 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. The Company had no outstanding contracts that were designated in cash flow hedges for forecasted revenue and expense transactions as of December 31, 2017 , and $20.0 million and $20.9 million of cash flow hedges for forecasted revenue and expense transactions, respectively, as of December 31, 2016 . Balance Sheet Hedges The Company enters into foreign exchange contracts to hedge 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 expense, net and offset the foreign currency gain or loss on the underlying net monetary assets or liabilities. The net notional amount of foreign currency contracts open in U.S. dollar equivalents was $141.2 million and $177.0 million as of December 31, 2017 and December 31, 2016 , 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 consolidated balance sheets. The following table presents the fair value of the Company’s foreign currency derivative contracts as of the dates presented (in thousands): December 31, 2017 December 31, 2016 Balance Sheet Location Fair Value Derivative Assets Fair Value Derivative Liabilities Fair Value Derivative Assets Fair Value Derivative Liabilities Cash flow designated hedges Prepaid expense and other current assets $ — $ — $ 813 $ — Cash flow designated hedges Accrued liabilities — — — 1,428 Hedges not designated Prepaid expense and other current assets — — 9,812 — Hedges not designated Accrued liabilities — 2,138 — 2,352 Total fair value of derivative instruments $ — $ 2,138 $ 10,625 $ 3,780 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 consolidated statement of operations for the periods presented (in thousands): Year Ended December 31, Income Statement Location 2017 2016 2015 Foreign exchange cash flow hedges: Gain (loss) recognized in OCI—effective portion $ (19,436 ) $ 8,171 $ 2,785 Gain (loss) reclassified from OCI into income—effective portion Revenue (18,532 ) 10,153 2,183 Gain (loss) reclassified from OCI into income—effective portion Operating expenses (1,405 ) 17 (899 ) Gain (loss) recognized in income—ineffective portion Other income (expense), net 21 (1,026 ) 202 Gain (loss) recognized in income—excluded time value portion Other income (expense), net 1,771 — — Foreign exchange balance sheet hedges: Gain (loss) recognized in income Other income (expense), net $ (10,516 ) $ 10,916 $ 5,861 As of December 31, 2017 , all net derivative gains related to the Company’s cash flow hedges will be reclassified from OCI into net income (loss) within the next 12 months. Offsetting of Foreign Currency Derivative Contracts The Company presents its derivative assets and derivative liabilities at gross fair values in the 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 table sets forth the available offsetting of net derivative assets and net derivative liabilities under the master netting arrangements as of December 31, 2017 and December 31, 2016 (in thousands): December 31, 2017 Gross Amounts Offset in the Consolidated Balance Sheets Gross Amounts Not Offset in Consolidated Balance Sheets Gross Amount Recognized Gross Amount Offset Net Amount Presented Financial Instruments Cash Collateral Received Net Amount Foreign exchange contracts assets $ — $ — $ — $ — $ — $ — Foreign exchange contracts liabilities 2,138 — 2,138 — — 2,138 December 31, 2016 Gross Amounts Offset in the Consolidated Balance Sheets Gross Amounts Not Offset in Consolidated Balance Sheets Gross Amounts Recognized Gross Amounts Offset Net Amount Presented Financial Instruments Cash Collateral Received Net Amount Foreign exchange contracts assets $ 10,625 $ — $ 10,625 $ 3,780 $ — $ 6,845 Foreign exchange contracts liabilities 3,780 — 3,780 3,780 — —

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Balance Sheet Components	Balance Sheet Components Allowance for Doubtful Accounts and Revenue Returns Reserve Changes in the allowance for doubtful accounts and revenue returns reserve were as follows (in thousands): Allowance for Doubtful Accounts Revenue Returns Reserve (1) Balance at December 31, 2014 838 26,559 Increases 1,115 169,677 Write-offs/returns taken (128 ) (122,191 ) Balance at December 31, 2015 1,825 74,045 Increases 339 275,815 Write-offs/returns taken (1,882 ) (251,009 ) Balance at December 31, 2016 282 98,851 Increases (2) 30,551 229,610 Write-offs/returns taken (2) (21,604 ) (218,589 ) Balance at December 31, 2017 $ 9,229 $ 109,872 (1) Increases in the revenue returns reserve include provisions for open box returns and stock rotations. (2) Change primarily related to the Wynit bankruptcy. See Note 1 for additional information. Inventories Inventories consisted of the following (in thousands): December 31, 2017 2016 Components $ 3,825 $ 1,035 Finished goods 120,070 229,352 Total inventories $ 123,895 $ 230,387 Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following (in thousands): December 31, 2017 2016 POP displays, net $ 14,750 $ 22,804 Prepaid expenses 24,204 17,161 Derivative assets — 10,625 Prepaid marketing 6,074 5,764 Insurance receivable 37,300 — Other 14,941 9,511 Total prepaid expenses and other current assets $ 97,269 $ 65,865 Property and Equipment, Net Property and equipment, net, consisted of the following (in thousands): December 31, 2017 2016 Tooling and manufacturing equipment $ 66,854 $ 60,944 Furniture and office equipment 20,942 14,424 Purchased and internally-developed software 18,112 12,032 Leasehold improvements 58,431 28,489 Total property and equipment 164,339 115,889 Less: Accumulated depreciation and amortization (59,431 ) (39,336 ) Property and equipment, net $ 104,908 $ 76,553 Total depreciation and amortization expense related to property and equipment, net was $40.0 million and $36.0 million for 2017 and 2016 , respectively. Goodwill and Intangible Assets The changes in the carrying amount of goodwill were as follows (in thousands). See Note 12 for additional information. Goodwill Balance at December 31, 2015 $ 22,157 Goodwill acquired 28,879 Balance at December 31, 2016 $ 51,036 Goodwill acquired — Balance at December 31, 2017 $ 51,036 The carrying amounts of the intangible assets as of December 31, 2017 and December 31, 2016 were as follows (in thousands, except useful life). See Note 12 for additional information. During 2017, the development was completed for certain technology that was in-process at the time of its acquisition. Accordingly, $3.9 million of in-process research and development recorded in the initial purchase price allocation has been reclassified to developed technology as of December 31, 2017. December 31, 2017 December 31, 2016 Weighted Average Remaining Useful Life (years) Gross Accumulated Amortization Net Gross Accumulated Amortization Net Developed technology $ 30,588 $ (8,738 ) $ 21,850 $ 26,092 $ (3,247 ) $ 22,845 4.0 Trademarks and other 1,278 (772 ) 506 1,278 (542 ) 736 0.9 Total finite-lived intangible assets subject to amortization, net 31,866 (9,510 ) 22,356 27,370 (3,789 ) 23,581 In-process research and development — — — 3,940 — 3,940 Total intangible assets, net $ 31,866 $ (9,510 ) $ 22,356 $ 31,310 $ (3,789 ) $ 27,521 Total amortization expense related to intangible assets was $5.7 million and $2.1 million for 2017 and 2016 , respectively. The estimated future amortization expense of acquired finite-lived intangible assets to be charged to cost of revenue and operating expenses after 2017 , is as follows (in thousands): Cost of Revenue Operating Expenses Total 2018 $ 6,120 $ 230 $ 6,350 2019 5,340 230 5,570 2020 4,560 46 4,606 2021 4,560 — 4,560 2022 1,270 — 1,270 Total intangible assets, net $ 21,850 $ 506 $ 22,356 Accrued Liabilities Accrued liabilities consisted of the following (in thousands): December 31, 2017 2016 Product warranty $ 87,882 $ 99,923 Accrued manufacturing expense and freight 41,901 75,579 Accrued sales incentives 111,592 74,181 Accrued sales and marketing 44,401 41,948 Accrued research and development 8,983 5,989 Accrued co-op advertising and marketing development funds 30,408 40,002 Employee-related liabilities 33,266 13,934 Sales taxes and VAT payable 21,340 8,891 Inventory received but not billed 10,526 7,363 Accrued legal settlements and fees 36,693 3,963 Derivative liabilities 2,138 3,780 Other 23,007 15,008 Accrued liabilities $ 452,137 $ 390,561 Product warranty reserve activities were as follows (in thousands): Reserve For Product Warranty (1) Balance at December 31, 2014 $ 20,098 Charged to cost of revenue 84,184 Changes in estimate related to pre-existing warranties (8,968 ) Settlement of claims (55,102 ) Balance at December 31, 2015 $ 40,212 Charged to cost of revenue 185,434 Changes in estimate related to pre-existing warranties 4,072 Settlement of claims (129,795 ) Balance at December 31, 2016 $ 99,923 Charged to cost of revenue 53,840 Changes in estimate related to pre-existing warranties 11,788 Settlement of claims (77,669 ) Balance at December 31, 2017 $ 87,882 (1) Does not include reserves established as a result of the recall of the Fitbit Force. See the section titled “—Fitbit Force Recall Reserve” for additional information regarding such reserves. The Company corrected the allocation of customer support costs and freight and fulfillment to the amounts in “Charged to cost of revenue” and “Settlement of claims” for the year ended December 31, 2016, which resulted in an increase in “Charged to cost of revenue” and “Settlement of claims” of $50.7 million . These costs are included in the warranty reserve beginning and ending balances. The Company does not consider this correction to be material and there was no impact to its consolidated balance sheets, statement of operations, and statement of cash flows. During 2017 and 2016, changes related to pre-existing warranties resulted primarily from an increase in the estimated cost of replacement units. During 2015, changes in estimate related to pre-existing warranties resulted from a reduction in the estimated number of units to be replaced and in the estimated cost of replacement units based on additional historical experience. Fitbit Force Recall Reserve In March 2014, the Company announced a recall for one of its products, the Fitbit Force (“Fitbit Force Recall”). The product recall, which is regulated by the U.S. Consumer Product Safety Commission, covered all Fitbit Force units sold since the product was first introduced in October 2013. The product recall program has no expiration date. As a result of the Fitbit Force recall, the Company established reserves that include cost estimates for customer refunds, logistics and handling fees for managing product returns and processing refunds, obsolescence of on-hand inventory, cancellation charges for existing purchase commitments and rework of component inventory with the contract manufacturer, accelerated depreciation of tooling and manufacturing equipment, and legal settlement costs. Fitbit Force recall reserve activities were as follows (in thousands): Reserve For Fitbit Force Recall Balance at December 31, 2014 $ 22,476 Charged to cost of revenue (5,755 ) Charged to general and administrative (1,174 ) Settlement of claims (10,425 ) Balance at December 31, 2015 5,122 Settlement of claims (3,869 ) Balance at December 31, 2016 1,253 Settlement of claims (789 ) Balance at December 31, 2017 $ 464 During 2015, a benefit to cost of revenue of $5.8 million was recognized due to a change in estimate of costs to fulfill Fitbit Force returns. In addition, a benefit to legal expenses of $4.4 million was recognized in general and administrative costs, of which $1.2 million was previously included in the Fitbit Force recall reserve due to the settlement of Fitbit Force legal liabilities. 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. The restructuring reserve activities were as follows (in thousands): Restructuring Reserve Balance at December 31, 2016 $ — Restructuring charges 6,375 Cash paid (4,983 ) Other - noncash (1,392 ) Balance at December 31, 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 on Cash Flow Hedges Currency Translation Adjustments Unrealized Gains (Losses) on Available-for-Sale Investments Total Balance at December 31, 2015 $ 751 $ (5 ) $ (55 ) $ 691 Other comprehensive income (loss) before reclassifications 9,422 (309 ) (126 ) 8,987 Amounts reclassified from AOCI (10,650 ) — (6 ) (10,656 ) Other comprehensive income (loss) (1,228 ) (309 ) (132 ) (1,669 ) Balance at December 31, 2016 (477 ) (314 ) (187 ) (978 ) Other comprehensive income (loss) before reclassifications (19,422 ) 314 125 (18,983 ) Amounts reclassified from AOCI 19,965 — (13 ) 19,952 Other comprehensive income (loss) 543 314 112 969 Balance at December 31, 2017 $ 66 $ — $ (75 ) $ (9 )

Long-Term Debt

Long-Term Debt	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Long-Term Debt	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 December 31, 2017 . As of December 31, 2017 , the Company had no outstanding borrowings under the Senior Facility and had outstanding letters of credit totaling $36.9 million , issued to cover various security deposits on its facility leases. Capitalized issuance costs are amortized to interest expense over the term of the related financing arrangement on a straight-line basis. Interest expense related to issuance costs for 2017 , 2016 , and 2015 was $1.1 million , $0.5 million , and $1.0 million , respectively. As of December 31, 2017, capitalized issuance costs were $0.6 million . Letters of Credit As of December 31, 2017 and 2016 , the Company had outstanding letters of credit of $36.9 million and $38.0 million , respectively, 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	12 Months Ended
	Dec. 31, 2017
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 2018 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. Rent expense is recorded over the lease terms on a straight-line basis. Rent expense was $40.0 million , $29.9 million , and $8.4 million for 2017 , 2016 , and 2015 , respectively. Future minimum payments under the leases as of December 31, 2017 were as follows (in thousands): Year ending December 31, Amounts 2018 $ 40,856 2019 46,713 2020 42,942 2021 41,331 2022 41,477 Thereafter 58,879 Total $ 272,198 Purchase Commitments The aggregate amount of open purchase orders as of December 31, 2017 was approximately $157.7 million . 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 reduce 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 December 31, 2017 , $21.8 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 certain 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 an additional employee defendant and related allegations. The trial was currently scheduled for April 30, 2018. 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 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. 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. 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. Trial has been scheduled for April 30, 2018. 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. 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 and unfair competition 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. 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 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 . The settlement remains subject to further court approval. On January 19, 2018, the court entered an order preliminarily approving the proposed settlement. A hearing is currently scheduled for April 20, 2018 to determine whether the settlement is fair, reasonable, and accurate. Refer to Note 14. Subsequent Events for more information. 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. Plaintiffs alleges 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 October 31, 2017, a seventh derivative lawsuit was filed in the Superior Court of California, Country of San Francisco, on the same allegations. We have 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 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. 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 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	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stockholders' Equity	Stockholders’ Equity Initial public offering and follow-on offering In June 2015, the Company completed its IPO of Class A common stock, in which the Company issued and sold 22.4 million shares and certain of its stockholders sold 19.7 million shares, for which the Company did not receive any proceeds, including 5.5 million shares pursuant to the underwriters’ option to purchase additional shares. The shares were sold at an initial public offering price of $20.00 per share for net proceeds of $420.9 million to the Company, after deducting underwriting discounts and commissions of $26.9 million . Offering costs incurred by the Company were approximately $5.0 million . In November 2015, the Company completed a follow-on offering of Class A common stock, in which the Company sold 3.0 million shares and certain of its stockholders sold 16.6 million shares, for which the Company did not receive any proceeds, including 2.6 million shares pursuant to the underwriters’ option to purchase additional shares. The shares were sold at a public offering price of $29.00 per share for net proceeds of $84.4 million to the Company, after deducting underwriting discounts and commissions of $2.6 million . Offering costs incurred by the Company were approximately $1.2 million . Redeemable convertible preferred stock and redeemable convertible preferred stock warrants In connection with the Company’s IPO, the then-outstanding 141.3 million shares of redeemable convertible preferred stock were converted into Class B common stock upon the closing of the IPO. In addition, the then-outstanding 0.4 million redeemable convertible preferred stock warrants automatically converted to Class B common stock warrants upon the closing of the IPO. These remaining outstanding Class B common stock warrants were exercised subsequent to the IPO during 2015. Preferred Stock Upon completion of its IPO on June 22, 2015, the Company filed a Restated Certificate of Incorporation, which authorized the issuance of preferred stock with rights and preferences, including voting rights, designated from time to time by the board of directors. As of December 31, 2017 , there were 10 million shares of preferred stock authorized with a par value of $0.0001 per share, and no shares of preferred stock issued or outstanding. Common Stock In connection with the IPO in 2015, the Company established two classes of authorized common stock, Class A common stock and Class B common stock. All shares of common stock outstanding immediately prior to the IPO were converted into an equivalent amount of shares of Class B common stock. As of December 31, 2017 , the Company had 600 million shares of Class A common stock authorized with a par value of $0.0001 per share and 350 million shares of Class B common stock authorized with a par value of $0.0001 per share. As of December 31, 2017 , the Company 207.5 million shares of Class A common stock were issued and outstanding and 31.3 million shares of Class B common stock were issued and outstanding. As of December 31, 2016 , 177.2 million shares of Class A common stock were issued and outstanding and 48.5 million shares of Class B common stock were issued and outstanding. Holders of Class A common stock are entitled to one vote for each share of Class A common stock held on all matters submitted to a vote of stockholders and holders of Class B common stock are entitled to ten votes for each share of Class B common stock held on all matters submitted to a vote of stockholders. Except with respect to voting, the rights of the holders of Class A and Class B common stock are identical. Shares of Class B common stock are voluntarily convertible into shares of Class A common stock at the option of the holder and generally automatically convert into shares of our Class A common stock upon a transfer. Stock Option Exchange On April 13, 2017, the Company filed its definitive proxy statement, submitting to stockholders a proposal for a stock option exchange program (the “Program”). The Program would allow 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 represents 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 would receive 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 Securities and Exchange Commission 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 Equity Incentive Plan (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. 2007 Equity Incentive Plan The Amended and Restated 2007 Stock Plan (the “2007 Plan”) provided for the grant of incentive and non-statutory stock options and RSUs to employees, directors, and consultants under terms and provisions established by the board of directors. Stock options granted under the 2007 Plan are generally subject to a four -year vesting period, with 25% vesting after a one -year period and monthly vesting thereafter. Stock options expire after ten years. RSUs granted under the 2007 Plan are generally subject to a three - or four -year vesting period with annual vesting. The 2015 Plan became effective in June 2015. As a result, the Company will not grant any additional stock options under the 2007 Plan and the 2007 Plan has been terminated. Any outstanding stock options and RSUs granted under the 2007 Plan will remain outstanding, subject to the terms of the 2007 Plan and applicable award agreements, until such shares are issued under those awards, by exercise of stock options or settlement of RSUs, or until the awards terminate or expire by their terms. 2015 Equity Incentive Plan In May 2015, the Company’s board of directors and stockholders adopted and approved the 2015 Plan. The 2015 Plan became effective in June 2015 and serves as the successor to the 2007 Plan. The remaining shares available for issuance under the 2007 Plan became reserved for issuance under the 2015 Plan. The number of shares reserved for issuance under the 2015 Plan will increase automatically on the first day of January of each year starting in 2016 through 2025 by the number of shares of Class A common stock equal to 5% of the total outstanding shares of common stock as of the immediately preceding December 31. The share reserve may also increase to the extent that outstanding awards expire or terminate un-exercised. As of December 31, 2017 , 12.5 million shares were available for grant under the 2015 Plan. The 2015 Plan authorizes the award of stock options, restricted stock awards, stock appreciation rights, RSUs, performance awards, and stock bonuses to employees, directors, consultants, independent contractors, and advisors. In general, stock options and RSUs will vest over a three - or four -year period, and have a maximum term of ten years. The exercise price of a stock option will be not less than 100% of the fair market value of the shares on the date of grant. 2015 Employee Stock Purchase Plan In May 2015, the Company’s board of directors adopted the 2015 ESPP, which became effective in June 2015. The number of shares reserved for issuance under the 2015 Plan will increase automatically on the first day of January of each year starting in 2016 through 2025 by the number of shares of Class A common stock equal to 1% of the total outstanding shares of common stock as of the immediately preceding December 31. The 2015 ESPP allows eligible employees to purchase shares of the Company’s Class A common stock at a discount through payroll deductions of up to 15% of eligible compensation, subject to any plan limitations. 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. As of December 31, 2017 , 4.5 million shares were available for grant under the 2015 Employee Stock Purchase Plan. On each purchase date, eligible employees will purchase Class A common stock 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 (ii) the last trading day of each purchase period in the applicable offering period. Stock Options Activity under the 2007 Plan and 2015 Plan is as follows (in thousands except per share amounts): Stock Options Outstanding Number of Shares Subject to Stock Options Weighted– Average Exercise Price Weighted– Average Remaining Contractual Term (in years) Aggregate Intrinsic Value Balance—December 31, 2016 34,454 $ 3.85 Granted 1,150 $ 5.63 Exercised (7,164 ) $ 1.22 Canceled (7,054 ) $ 9.35 Balance—December 31, 2017 21,386 $ 3.01 5.7 $ 64,582 Stock options exercisable—December 31, 2017 16,827 $ 2.52 5.3 $ 57,472 Stock options vested and expected to vest—December 31, 2017 21,386 $ 3.01 5.7 $ 64,582 The aggregate intrinsic values of stock options outstanding, exercisable, vested and expected to vest were calculated as the difference between the exercise price of the stock options and the fair value of the Class A common stock of $5.71 as of December 31, 2017 . Restricted Stock Units RSU activity under the equity incentive plans is as follows: RSUs Outstanding Weighted- Average Grant Date Fair Value (in thousands) Unvested balance—December 31, 2016 11,578 $ 16.85 Granted 18,912 6.47 Vested (6,191 ) 12.57 Forfeited or canceled (5,111 ) 12.61 Unvested balance—December 31, 2017 19,188 9.13 Warrants On July 10, 2017, the Company issued a warrant to a third party vendor 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 December 31, 2017, 0.1 million shares have vested. Stock-Based Compensation Expense Total stock-based compensation recognized was as follows (in thousands): Year Ended December 31, 2017 2016 2015 Cost of revenue $ 5,312 $ 4,797 $ 4,739 Research and development 54,123 47,207 18,251 Sales and marketing 14,959 11,575 7,419 General and administrative 17,187 15,853 10,615 Total stock-based compensation expense $ 91,581 $ 79,432 $ 41,024 The weighted-average grant date fair value of stock options granted during 2017 , 2016 , and 2015 was $5.63 , $14.06 , and $10.67 per share, respectively. The total grant date fair value of stock options that vested during 2017 , 2016 , and 2015 was $20.2 million , $32.9 million , and $18.6 million , respectively. As of December 31, 2017 , the total unrecognized compensation expense related to unvested stock options was $11.3 million , which the Company expects to recognize over an estimated weighted average period of 1.4 years . As of December 31, 2017, the total unrecognized compensation expense related to unvested RSUs was $159.0 million , which the Company expects to recognize over an estimated weighted average period of 2.2 years . Stock-based compensation is measured at the grant date based on the fair value of the award and is recognized as expense over the requisite service period, which is generally the vesting period of the respective award. The fair value of RSUs is the fair value of the Company’s Class A common stock on the grant date. In determining the fair value of the stock options, warrants and the equity awards issued under the 2015 ESPP, the Company uses the Black-Scholes option-pricing model and assumptions discussed below. Each of these inputs is subjective and generally requires significant judgment. Fair Value of Common Stock —The fair value of the shares of common stock underlying stock options had historically been established by the Company’s board of directors. Following the completion of the IPO, the Company began using the market closing price for the Company’s Class A common stock as reported on the New York Stock Exchange. Expected Term —The Company does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate expected term due to the limited period of time stock-based awards have been exercisable. As a result, for stock options, the Company used the simplified method to calculate the expected term, which is equal to the average of the stock-based award’s weighted average vesting period and its contractual term. The expected term of the 2015 ESPP is based on the contractual term. Volatility —The Company estimates the expected volatility of the common stock underlying its stock options at the grant date by taking the average historical volatility of the common stock of a group of comparable publicly traded companies over a period equal to the expected life of the stock options. Risk-Free Rate —The risk-free interest rate is estimated average interest rate based on U.S. Treasury zero-coupon notes with terms consistent with the expected term of the awards. Dividend Yield —The Company has never declared or paid any cash dividends and does not presently plan to pay cash dividends in the foreseeable future. Consequently, it used an expected dividend yield of zero. The assumptions used in calculating the fair value of the stock-based awards represent management judgment. As a result, if factors change and different assumptions are used, the stock-based compensation expense could be materially different in the future. The fair value of the stock option awards, warrants and awards issued under the 2015 ESPP granted to employees was estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions: Year Ended December 31, 2017 2016 2015 Employee stock options Expected term (in years) 6.25 6.25 6.25 Volatility 32.2% 40.7% 52.1% - 56.9% Risk-free interest rate 2.1% 1.6% 1.5% - 1.9% Dividend yield —% —% —% Warrants Expected term (in years) 9.5 — — Volatility 32.0% —% —% Risk-free interest rate 2.1% —% —% Dividend yield —% —% —% Employee stock purchase plan Expected term (in years) 0.5 0.5 0.5 – 0.9 Volatility 27.7% - 31.3% 30.1% - 39.0% 27.7% - 35.0% Risk-free interest rate 1.0% - 1.4% 0.4% - 0.6% 0.3% Dividend yield —% —% —%

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The following table presents domestic and foreign components of income (loss) before income taxes for the periods presented (in thousands): Year Ended December 31, 2017 2016 2015 United States $ (158,187 ) $ 5,577 $ 286,380 Foreign (36,457 ) (114,872 ) 1,569 Total $ (194,644 ) $ (109,295 ) $ 287,949 The income tax expense (benefit) is composed of the following (in thousands): Year Ended December 31, 2017 2016 2015 Current: Federal $ (87,961 ) $ 78,782 $ 140,396 State (8,429 ) 9,878 13,307 Foreign 5,032 5,256 1,107 Total current (91,358 ) 93,916 154,810 Deferred: Federal 154,817 (87,584 ) (33,421 ) State 18,902 (11,622 ) (8,941 ) Foreign 187 (1,228 ) (176 ) Total deferred 173,906 (100,434 ) (42,538 ) Total income tax expense (benefit) $ 82,548 $ (6,518 ) $ 112,272 The reconciliation of the Company’s effective tax rate to the statutory federal rate is as follows: Year Ended December 31, 2017 2016 2015 Tax at federal statutory rate 35.0 % 35.0 % 35.0 % State taxes, net of federal effect (5.4 ) 4.3 1.5 Foreign rate differential (9.3 ) (38.9 ) (0.8 ) Tax credits 4.1 9.0 (2.0 ) Domestic production activities deduction (3.5 ) 5.0 (3.3 ) Stock-based compensation (1) (5.3 ) (4.6 ) 1.7 Change in prior year reserves (2.0 ) 1.9 — Out-of-period adjustment — (2.8 ) — Warrant fair value adjustment — — 6.9 Change in valuation allowance (35.2 ) — — Effect of change in tax rate due to Tax Act (23.4 ) — — Other 2.6 (2.9 ) — Effective tax rate (42.4 )% 6.0 % 39.0 % (1) Starting in 2017, excess tax benefits from share-based award activity are reflected as reduction of the provision for income taxes, whereas they were previously recognized in equity. This will result in increased volatility in the Company’s effective tax rate. The amount of net stock compensation windfalls previously recognized by the Company in equity in 2015 and 2016 was $32.1 million and $22.0 million, respectively. For 2017, the Company recorded an expense for income taxes of $82.5 million , resulting in an effective tax rate of (42.4)% . The effective tax rate is different than the statutory federal tax rate primarily due to losses in certain foreign jurisdictions for which a tax benefit may not be realized and the establishment of a full valuation allowance on its U.S. deferred tax assets, partially offset by the anticipated carryback of losses incurred in 2017. On December 22, 2017, the U.S. Tax Cuts and Jobs Act of 2017 (the “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. The Company recorded a provisional tax expense for the impact of the 2017 Tax Act of approximately $45.5 million as a result of re-measurement of the federal portion of its deferred tax assets as of December 31, 2017 from 35% to the new 21% tax rate. As the Company completes its analysis of the 2017 Tax Act, any subsequent adjustments to provisional amounts that it has recorded may or may not impact its provision for income taxes in the period in which the adjustments are made due to a full valuation allowance on its U.S. deferred tax assets. The tax effects of temporary differences that give rise to significant portions of the deferred tax assets were as follows (in thousands): December 31, 2017 2016 Deferred tax assets: Net operating losses and credits $ 23,338 $ 9,446 Fixed assets and intangible assets 10,625 16,272 Accruals and reserves 49,886 112,915 Stock-based compensation 12,154 17,864 Inventory 4,345 8,513 Other 3,325 11,134 Total deferred tax assets 103,673 176,144 Less: valuation allowance (99,570 ) — Deferred tax assets, net of valuation allowance 4,103 176,144 Deferred tax liabilities: Other (369 ) (353 ) Total deferred tax liabilities (369 ) (353 ) Net deferred tax assets $ 3,734 $ 175,791 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. The Company evaluated its ability to realize the benefit of its net deferred tax assets and weighed all available positive and negative evidence both objective and subjective in nature. In determining the need for a valuation allowance, the weight given to positive and negative evidence is commensurate with the extent to which the evidence may be objectively verified. Consideration was given to negative evidence such as: the duration and severity of losses in prior years, high seasonal revenue concentrations, increasing competitive pressures, and a challenging retail environment. Realization of the Company’s net deferred tax assets is dependent upon its generation of sufficient taxable income in future years in appropriate tax jurisdictions to obtain benefit from the reversal of temporary differences, net operating loss carryforwards and tax credit carryforwards. The amount of net deferred tax assets considered realizable is subject to adjustment in future periods if estimates of future taxable income change. In evaluating the need for a valuation allowance, and determining that a valuation allowance was appropriate, the Company considered the fact that it has had five consecutive quarters of losses through December 31, 2017 and the effect of the recently enacted 2017 Tax Act. As of December 31, 2017 , the Company established a valuation allowance of $99.6 million against 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 December 31, 2017, the Company has federal net operating loss carryforwards of $2.2 million which expire beginning after 2033, California net operating loss carryforwards of $25.8 million which expire beginning after 2034, and other states net operating loss carryforwards of $27.2 million which expire beginning after 2033. As of December 31, 2017, the Company has federal research tax credit carryforwards of approximately $1.6 million , which if not utilized, begin to expire after 2030, California research tax credit carryforwards of approximately $25.6 million , which do not expire, Massachusetts research tax credit carryforwards of approximately $1.2 million , which if not utilized, begin to expire after 2031, and California hiring tax credit carryforwards of approximately $0.3 million , which if not utilized, begin to expire after 2026. As of December 31, 2017, the Company has United Kingdom net operating loss carryforwards of $15.6 million , which do not expire. Utilization of the net operating loss and tax credit carry forwards are subject to an annual limitation due to the ownership percentage change limitations provided by the Internal Revenue Code of 1986 and similar state provisions. The annual limitation may result in the expiration of the net operating loss before utilization. The Company does not expect the limitation to result in a reduction in total amount utilizable. The Company is subject to income taxes in the U.S. (federal and state) and numerous foreign jurisdictions. Significant judgment is required in evaluating the Company’s tax positions and determining its provision for income taxes. During the ordinary course of business, there are many transactions and calculations for which the ultimate tax determination is uncertain. The Company establishes reserves for tax-related uncertainties based on estimates of whether, and the extent to which, additional taxes will be due. These reserves are established when the Company believes that certain positions might be challenged despite its belief that its tax return positions are fully supportable. The Company adjusts these reserves in light of changing facts and circumstances, such as the outcome of tax audits. The provision for income taxes includes the impact of reserve provisions and changes to reserves that are considered appropriate. As of December 31, 2017 and 2016, the Company has $29.9 million and $35.6 million of unrecognized tax benefits. A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands): December 31, 2017 2016 2015 Balance at beginning of year $ 35,584 $ 23,518 $ 10,594 Reductions based on tax positions related to prior year (6,335 ) (2,100 ) (18 ) Additions based on tax positions related to prior year 108 2,809 — Additions based on tax positions related to current year 9,289 11,357 12,942 Reductions due to tax authorities’ settlements (8,603 ) — — Reductions due to expiration of statutes of limitation (105 ) — — Balance at end of year $ 29,938 $ 35,584 $ 23,518 At December 31, 2017 , the total amount of gross unrecognized tax benefits was $29.9 million , of which $24.8 million would affect the Company’s effective tax rate if recognized. The Company does not have any tax positions as of December 31, 2017 for which it is reasonably possible the total amount of gross unrecognized tax benefits will increase or decrease within the following 12 months . The Company’s policy is to record interest and penalties related to unrecognized tax benefits as income tax expense. As of December 31, 2017 and 2016, the Company has accrued and $2.4 million and $2.2 million related to interest and penalties. The Company is subject to taxation in the United States and various states and foreign jurisdictions. The material jurisdictions in which the Company is subject to potential examination include the United States and Ireland. The Company believes that adequate amounts have been reserved for these jurisdictions. For federal, state and non-U.S. tax returns, the Company is generally no longer subject to tax examinations for years prior to 2015.

Net Income (Loss) per Share Att

Net Income (Loss) per Share Attributable to Common Stockholders	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Net Income (Loss) per Share Attributable to Common Stockholders	Net Income (Loss) per Share Attributable to Common Stockholders The following table sets forth the computation of the Company’s basic and diluted net income (loss) per share attributable to common stockholders (in thousands, except per share amounts): Year Ended December 31, 2017 2016 2015 Numerator: Net income (loss) $ (277,192 ) $ (102,777 ) $ 175,677 Less: noncumulative dividends to preferred stockholders — — (2,526 ) Less: undistributed earnings to participating securities — — (59,133 ) Net income (loss) attributable to common stockholders—basic (277,192 ) (102,777 ) 114,018 Add: adjustments to undistributed earnings to participating securities — — 8,821 Net income (loss) attributable to common stockholders—diluted $ (277,192 ) $ (102,777 ) $ 122,839 Denominator: Weighted-average shares of common stock—basic for Class A and Class B 232,032 220,405 129,886 Effect of dilutive securities — — 34,327 Weighted-average shares of common stock—diluted for Class A and Class B 232,032 220,405 164,213 Net income (loss) per share attributable to common stockholders: Basic $ (1.19 ) $ (0.47 ) $ 0.88 Diluted $ (1.19 ) $ (0.47 ) $ 0.75 The following potentially dilutive common shares on a weighted average basis were excluded from the computation of diluted net income (loss) per share for the periods presented because including them would have been antidilutive (in thousands): December 31, 2017 2016 2015 Stock options to purchase common stock 17,469 34,454 445 RSUs 10,030 11,578 692 Warrants 216 — — Diluted common stock subject to vesting 84 — — Diluted impact of ESPP 162 — — Redeemable stock unites — — 65,903 Redeemable convertible preferred stock warrants — — 921 Total 27,961 46,032 67,961

Significant Customer Informatio

Significant Customer Information and Other Information	12 Months Ended
	Dec. 31, 2017
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 were as follows: December 31, 2017 2016 2015 C 13 % 14 % 14 % A * 14 15 B * 10 14 * Revenue was less than 10%. Retailers and distributors that accounted for equal to or greater than 10% of accounts receivable at December 31, 2017 and 2016 were as follows: December 31, 2017 2016 C 17 % 19 % B 13 * E 11 * A * 16 D * 12 * Accounts receivable were less than 10%. Geographic and Other Information Revenue by geographic region, based on ship-to destinations, was as follows (in thousands): December 31, 2017 2016 2015 United States $ 944,052 $ 1,539,600 $ 1,381,152 Americas excluding United States 116,330 110,111 92,252 Europe, Middle East, and Africa 440,135 389,154 208,767 APAC 115,002 130,596 175,827 Total $ 1,615,519 $ 2,169,461 $ 1,857,998 As of December 31, 2017 and 2016 , long-lived assets, which represent property and equipment, located outside the United States were $30.0 million and $30.1 million , respectively.

Acquisitions

Acquisitions	12 Months Ended
	Dec. 31, 2017
Business Combinations [Abstract]
Acquisitions	Acquisitions 2016 Acquisitions In December 2016, the Company completed a purchase of certain assets from Pebble Industries, Inc., a privately-held company, 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 Industries, Inc. 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, 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 S.R.L. 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. FitStar Acquisition — 2015 In March 2015, the Company acquired all of the outstanding securities of FitStar, Inc., a privately-held company, for aggregate acquisition consideration of $32.5 million , comprised of $13.3 million related to the issuance of 1,059,688 shares of the Company’s common stock, net of a repurchase of 24,949 shares, $11.5 million of cash, and $7.7 million of contingent consideration. FitStar is a provider of interactive video-based exercise experiences on mobile devices and computers that utilize proprietary algorithms to adjust and customize workouts for individual users. The acquisition is expected to enhance the Company’s software and services offerings. Under the acquisition agreement, the Company was obligated to issue additional common stock or pay cash to FitStar shareholders. The actual amount of this contingent consideration depended on market-based events that may occur in the future. The Company determined the fair market value of this contingent consideration to be $7.7 million as of the acquisition date using the Monte Carlo simulation method. The fair value of this liability was adjusted at each reporting period, and the change in fair value was included in other income (expense), net on the consolidated statement of operations. As a result of the Company’s IPO, the Company recorded a change in fair value of $7.7 million as a benefit and as of December, 31, 2015 the fair value of the contingent consideration liability was zero . The terms related to the contingent consideration have expired as of December 31, 2015 and no amounts were paid or shares issued for the contingent consideration. The following table summarizes the fair value of assets acquired and liabilities assumed (in thousands): Goodwill $ 22,157 Developed and core technology 12,640 Customer relationships 128 Trademarks 1,150 Assumed liabilities, net of assets (3,552 ) Total $ 32,523 The amortization periods of the acquired developed technology, customer relationships, and trademarks are 7.0 years , 1.3 years , and 5.0 years , respectively. Goodwill is not deductible for tax purposes. In addition, upon acquisition, the Company issued 308,216 shares of common stock net of a repurchase of 24,948 shares, valued at $4.2 million . The Company is also obligated to make cash payments up to $1.2 million . Both the common stock and the cash payments are additional consideration which is contingent upon former employees of FitStar continuing to be employed by the Company. As such, this additional consideration was not part of the purchase price and is recognized as post-acquisition compensation expense over the related requisite service period. The Company recorded acquisition-related transaction costs of $1.3 million and $0.3 million , which were included in general and administrative expenses in the consolidated statement of operations during 2016 and 2015, respectively. The results of operations for the acquired companies during 2016 and 2015 are included in the accompanying consolidated statements of operations from the date of acquisition. Pro forma and historical results of operations for the acquired companies have not been presented because they are not material, either individually or in the aggregate, to the Company’s consolidated financial statements.

Selected Unaudited Quarterly Fi

Selected Unaudited Quarterly Financial Data	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Selected Unaudited Quarterly Financial Data	Selected Unaudited Quarterly Financial Data The following tables show a summary of the Company’s unaudited quarterly financial information for each of the four quarters of 2017 and 2016 (in thousands, except per share amounts): Three Months Ended December 31, 2017 (3) September 30, 2017 (2) July 1, 2017 April 1, 2017 (1) Revenue $ 570,756 $ 392,522 $ 353,299 $ 298,942 Gross profit $ 248,597 $ 174,760 $ 149,245 $ 118,299 Net loss $ (45,470 ) $ (113,403 ) $ (58,240 ) $ (60,079 ) Net loss per share attributable to common stockholders—basic $ (0.19 ) $ (0.48 ) $ (0.25 ) $ (0.27 ) Net loss per share attributable to common stockholders—diluted $ (0.19 ) $ (0.48 ) $ (0.25 ) $ (0.27 ) Three Months Ended December 31, 2016 October 1, 2016 July 2, 2016 April 2, 2016 Revenue $ 573,775 $ 503,802 $ 586,528 $ 505,356 Gross profit $ 126,502 $ 240,658 $ 244,969 $ 233,755 Net income (loss) $ (146,273 ) $ 26,120 $ 6,341 $ 11,035 Net income (loss) per share attributable to common stockholders—basic $ (0.65 ) $ 0.12 $ 0.03 $ 0.05 Net income (loss) per share attributable to common stockholders—diluted $ (0.65 ) $ 0.11 $ 0.03 $ 0.05 (1) During the first quarter of 2017, the Company recorded restructuring expenses of $6.3 million . See Note 5. Balance Sheet Components for more information. In addition, the Company’s adoption of ASU 2016-09 on January 1, 2017 resulted in an increase to the provision for income taxes of $2.8 million . See Note 2. Significant Accounting Policies for more information. (2) During the third quarter of 2017, as a result of one of the Company’s customers filing for bankruptcy, the Company recorded a net charge of $9.0 million comprised of net bad debt expense of $7.6 million and net cost of revenue of $1.4 million . See Note 1. Customer Bankruptcy for more information. In addition, during the third quarter of 2017, the Company recorded a $111.4 million valuation allowance against a portion of its U.S. deferred taxes. (3) During the fourth quarter of 2017, as a result of the Tax Act, the Company recorded a provisional tax expense for the impact of the 2017 Tax Act of $45.5 million as a result of re-measurement of the federal portion of its deferred tax assets as of December 31, 2017 from 35% to the new 21% tax rate.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events On January 8, 2018, the plaintiffs in the federal and state class action cases in the securities litigation filed a motion for preliminary approval of a global settlement of all of the putative federal and state class actions for $33.3 million . On January 19, 2018, the court preliminary approved the settlement. The Company evaluated this subsequent development concluding that it provided additional evidence about a condition that existed as of December 31, 2017. The Company accrued the entire amount of this liability and recorded the $33.3 million settlement expense within general and administrative expenses in 2017 since it concluded that it was probable of payment and reasonably estimable. The Company maintains directors and officers liability insurance that covers the exposure related to the securities litigation. The Company recorded an insurance receivable based on an analysis of its insurance policies, including its exclusions, an assessment of the nature of the claim, and information from its insurance carriers. As of December 31, 2017, the Company has recorded an insurance receivable of $32.3 million , which is included in prepaid expenses and other current assets, associated with the amount it has concluded is probable related to the claim. The $32.3 million insurance receivable allowed the Company to recover the settlement expense, resulting in a net charge of $1.0 million in its consolidated statement of operations. The Company will continue to assess the probable amount of insurance proceeds expected to be received in future reporting periods until the final resolution, and make adjustments, if necessary, based on additional facts as they arise. In February 2018, the Company acquired Twine Health, Inc., a privately-held company with a health coaching platform for cash consideration of $17.5 million . This acquisition is to be accounted for as a business combination. Management is currently evaluating the purchase price allocation for this transaction.

Significant Accounting Polici23

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation and Principles of Consolidation	Basis of Presentation and Principles of Consolidation The consolidated financial statements are prepared in accordance with U.S. generally accepted accounting principles (“U.S. GAAP”). The consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany transactions and balances have been eliminated. The Company’s fiscal year ends on December 31 of each year. In the first quarter of 2016, the Company adopted a 4-4-5 week quarterly calendar, which, for the 2016 fiscal year, was comprised of four fiscal quarters ending on April 2, 2016, July 2, 2016, October 1, 2016, and December 31, 2016.
Use of Estimates	Use of Estimates The preparation of 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 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 consolidated financial statements.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) consists of two components, net income (loss) and other comprehensive income (loss), net of tax. Other comprehensive income refers to revenue, expenses, and gains and losses that are recorded as an element of stockholders’ equity but are excluded from net income (loss). The Company’s other comprehensive income (loss) consists of net unrealized gains and losses on derivative instruments accounted for as cash flow hedges, foreign currency translation adjustments from those subsidiaries not using the U.S. dollar as their functional currency, and unrealized gains and losses on available-for-sale securities.
Cash and Cash Equivalents	Cash, Cash Equivalents and Marketable Securities Cash and cash equivalents include all cash balances and highly liquid investments with original maturities of three months or less from the date of purchase. Cash equivalents and marketable securities consist of money market funds, U.S. government and agency securities, commercial paper, and corporate notes and bonds.
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 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 expense, net as incurred. Realized gains and losses on the sale of securities are determined by specific identification of each security’s cost basis. Investments are reviewed periodically to identify possible other-than-temporary impairments. No impairment loss has been recorded on the securities as the Company believes that any 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.
Fair Value of Financial Instruments	Fair Value of Financial Instruments Assets and liabilities recorded at fair value on a recurring basis are categorized based upon the level of judgment associated with inputs used to measure their fair values. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the reporting date. The Company estimates fair value by applying the following hierarchy, which prioritizes the inputs used to measure fair value into three levels and bases the categorization within the hierarchy upon the lowest level of input that is available and significant to the fair value measurement: Level 1 —Quoted prices in active markets for identical assets or liabilities; Level 2 —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 for substantially the full term of the assets or liabilities; and Level 3 —Inputs that are generally unobservable and typically reflect management’s estimate of assumptions that market participants would use in pricing the asset or liability.
Foreign Currencies	Foreign Currencies The Company and all of its wholly-owned subsidiaries use the U.S. dollar as their functional currency. The Company’s subsidiaries that use the U.S. dollar as their functional currency remeasure local currency denominated monetary assets and liabilities at exchange rates in effect at the end of each period, and inventories, property, plant and equipment and other nonmonetary assets and liabilities at historical rates. Gains and losses from these remeasurements have been included in the Company’s operating results within other income (expense), net. Local currency transactions of these international operations are remeasured into U.S. dollars at the rates of exchange in effect at the date of the transaction. Foreign currency transaction gains (losses) were $2.6 million , $ 11.7 million , and $(1.3) million for 2017 , 2016 , and 2015, respectively.
Derivative Instruments	Derivative Instruments The Company accounts for its derivative instruments as either assets or liabilities and carries them at fair value. Derivatives held by the Company that are not designated as hedges are adjusted to fair value through earnings at each reporting date. In addition, the Company enters into derivatives that are accounted for as cash flow hedges. The Company records the gains or losses, net of tax, related to the effective portion of its cash flow hedges as a component of accumulated other comprehensive income in stockholders’ equity and subsequently reclassifies the gains or losses into revenue and operating expenses when the underlying hedged transactions are recognized. The Company periodically assesses the effectiveness of its cash flow hedges. The fair value of derivative assets and liabilities are included in prepaid expenses and other current assets and accrued liabilities on the consolidated balance sheets.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments that potentially subject the Company to concentrations of credit risk consist primarily of cash, cash equivalents, marketable securities, accounts receivables, derivative instruments and cost method investments. Cash and cash equivalents are deposited with high quality financial institutions and may, at times, exceed federally insured limits. Management believes that the financial institutions that hold the Company’s deposits are financially credit worthy and, accordingly, minimal credit risk exists with respect to those balances. Generally, these deposits may be redeemed upon demand and, therefore, bear minimal interest rate risk. The Company’s accounts receivable is derived from customers located principally in the United States. The Company maintains credit insurance for the majority of its customer balances, performs ongoing credit evaluations of its customers, and maintains allowances for potential credit losses on customers’ accounts when deemed necessary. Credit losses historically have not been significant. The Company continuously monitors customer payments and maintains an allowance for doubtful accounts based on its assessment of various factors including historical experience, age of the receivable balances, and other current economic conditions or other factors that may affect customers’ ability to pay. The Company’s derivative instruments expose it to credit risk to the extent that its counterparties may be unable to meet the terms of the agreements. The Company seeks to mitigate this risk by limiting counterparties to major financial institutions and by spreading the risk across several major financial institutions. In addition, the potential risk of loss with any one counterparty resulting from this type of credit risk is monitored on an ongoing basis.
Supplier Concentration	Supplier Concentration The Company relies on third parties for the supply and manufacture of its products, as well as third-party logistics providers. In instances where these parties fail to perform their obligations, the Company may be unable to find alternative suppliers or satisfactorily deliver its products to its customers on time, if at all.
Inventories	Inventories Inventories consist of finished goods and component parts, which are purchased from contract manufacturers and component suppliers. Inventories are stated at the lower of cost or net realizable value. The Company assesses the valuation of inventory and periodically writes down the value for estimated excess and obsolete inventory based upon estimates of future demand and market conditions.
Property and Equipment, Net	Property and Equipment, Net Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization of property and equipment is calculated using the straight-line method over the estimated useful lives of the assets. Cost of maintenance and repairs that do not improve or extend the lives of the respective assets are expensed as incurred. The useful lives of the property and equipment are as follows: Tooling and manufacturing equipment One to three years Furniture and office equipment Three years Purchased software Three years Capitalized internally-developed software Two to eight years Leasehold improvements Shorter of remaining lease term or ten years
Internally-Development Software Costs	Internally-Developed Software Costs The Company capitalizes eligible costs to acquire, develop, or modify internal-use software that are incurred subsequent to the preliminary project stage.
Research and Development	Research and Development Research and development expenses consist primarily of personnel-related expenses, consulting and contractor expenses, tooling and prototype materials, and allocated overhead costs. Substantially all of the Company’s research and development expenses are related to developing new products and services and improving existing products and services. To date, research and development expenses have been expensed as incurred, because the release of products and services for sale has been short and development costs qualifying for capitalization have been insignificant.
Business Combinations	Business Combinations, Goodwill, and Intangible Assets The Company allocates the fair value of purchase consideration to tangible assets, liabilities assumed, and intangible assets acquired based on their estimated fair values. The excess of the fair value of purchase consideration over the fair values of these identifiable assets and liabilities is allocated to goodwill. The allocation of the purchase consideration requires management to make significant estimates and assumptions, especially with respect to intangible assets. These estimates can include, but are not limited to, future expected cash flows from acquired customers, acquired technology, and trade names from a market participant perspective, useful lives, and discount rates. Management’s estimates of fair value are based upon assumptions believed to be reasonable, but which are inherently uncertain and unpredictable and, as a result, actual results may differ from estimates. During the measurement period, which is up to one year from the acquisition date, the Company may record adjustments to the assets acquired and liabilities assumed, with the corresponding offset to goodwill. Upon the conclusion of the measurement period, any subsequent adjustments are recorded to earnings.
Goodwill	The Company assesses goodwill for impairment at least annually during the fourth quarter and whenever events or changes in circumstances indicate that the carrying value of the asset may not be recoverable. Consistent with the determination that the Company has one operating segment, the Company has determined that there is one reporting unit and tests goodwill for impairment at the entity level. Goodwill is tested using the two-step process in accordance with ASC 350, Intangibles—Goodwill and Other . In the first step, the carrying amount of the reporting unit is compared to the fair value based on the fair value of the Company’s common stock. If the fair value of the reporting unit exceeds the carrying value, goodwill is not considered impaired and no further testing is required. If the carrying value of the reporting unit exceeds the fair value, goodwill is potentially impaired and the second step of the impairment test must be performed. In the second step, the implied fair value of the goodwill, as defined by ASC 350, is compared to its carrying amount to determine the amount of impairment loss, if any. The Company tested goodwill for impairment as of October 31, 2017 and 2016, and the fair value of the reporting unit exceeded the carrying value.
Intangible Assets	Acquired finite-lived intangible assets are amortized over their estimated useful lives. The Company evaluates the recoverability of intangible assets for possible impairment whenever events or circumstances indicate that the carrying amount of such assets may not be recoverable. Recoverability of these assets is measured by a comparison of the carrying amounts to the future undiscounted cash flows the assets are expected to generate. If such review indicates that the carrying amount of intangible assets is not recoverable, the carrying amount of such assets is reduced to fair value. The Company has not recorded any such impairment charge during the years presented.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company evaluates its long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparison of the carrying amounts to the expected future undiscounted cash flows attributable to these assets. If it is determined that an asset is not recoverable, an impairment loss is recorded in the amount by which the carrying amount of the assets exceeds the expected discounted future cash flows arising from those assets.
Revenue Recognition	Revenue Recognition 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 recognizes revenue when persuasive evidence of an arrangement exists, delivery has occurred, the sales price is fixed or determinable, and collection is reasonably assured. The Company considers delivery of its products to have occurred once title and risk of loss has been transferred. For customers where transfer of risk of loss is at the customer’s destination, the Company uses estimates to defer sales at the end of the reporting period based on historical experience of average transit time. The Company recognizes revenue, net of estimated sales returns, sales incentives, discounts, and sales tax. The Company generally recognizes revenue for products sold through retailers and distributors on a sell-in basis. The Company enters into multiple element arrangements that include hardware, software, and services. The first deliverable 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 deliverable 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 deliverable 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. Commencing in the first quarter of 2015, the Company began accounting for the embedded right as a separate unit of accounting, which is when it believes, through public announcements, it had created an implied obligation to, from time to time, provide future unspecified firmware upgrades and features to the firmware to improve and add new functionality to the health and fitness devices. In addition, the Company occasionally offers a fourth deliverable in bundled arrangements that allows access to subscription-based services related to the Company’s Fitbit Coach offering. The Company allocates revenue to all deliverables based on their relative selling prices. The Company uses a hierarchy to determine the selling price to be used for allocating revenue to deliverables: (i) vendor-specific objective evidence (“VSOE”) of fair value, (ii) third-party evidence (“TPE”), and (iii) best estimate of the selling price (“BESP”). The Company’s process for determining its BESP considers multiple factors including consumer behaviors and the Company’s internal pricing model and may vary depending upon the facts and circumstances related to each deliverable. BESP for the health and fitness devices and smartwatches and unspecified upgrade rights 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. BESP for upgrade rights currently ranges from $1 to $3 . TPE for the online dashboard and mobile apps is currently estimated at $0.99 . VSOE 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 access to software and services to certain customers in the corporate wellness program, which includes distribution capabilities, a real-time dashboard, and support services. The Company is currently unable to establish VSOE or TPE for the corporate wellness software and services. BESP for the corporate wellness software and services 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 software and services 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 accounts for shipping and handling fees billed to customers as revenue. Sales taxes and value added taxes (“VAT”) collected from customers 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 accounts receivable. Through December 31, 2017 , actual returns have primarily been open-box returns. In addition, through 2017, 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 an identifiable benefit in exchange for credits claimed by the customer and can reasonably estimate the fair value of the identifiable 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.
Cost of Revenue	Cost of Revenue Cost of revenue consists of product costs, including costs of contract manufacturers for production, shipping and handling costs, packaging, warranty replacement costs, fulfillment costs, manufacturing and tooling equipment depreciation, warehousing costs, excess and obsolete inventory write-downs, costs related to the Fitbit Force product recall, and certain allocated costs related to management, facilities, and personnel-related expenses and other expenses associated with supply chain logistics.
Advertising Costs	Advertising Costs and Point of Purchase (“POP”) Displays Costs related to advertising and promotions, excluding cooperative advertising costs, are expensed to sales and marketing as incurred. Advertising and promotion expenses, including expenses for POP displays, for 2017 , 2016 , and 2015 were $226.3 million , $316.8 million , and $237.0 million , respectively. Co-op advertising costs are recorded as a reduction to revenue, and for 2017 , 2016 , and 2015 were $45.0 million , $52.9 million , and $38.3 million , respectively. The Company provides retailers with POP displays, generally free of charge, in order to facilitate the marketing of the Company’s products within retail stores. Any amounts capitalized related to the costs of the POP displays are recorded as prepaid expenses and other current assets on the consolidated balance sheets and recognized as expense over the expected period of the benefit provided by these assets, which is generally 12 months. The related expenses are included in sales and marketing expenses on the consolidated statements of operations.
Product Warranty	Product Warranty The Company offers a standard product warranty that its products will operate under normal use for a period of one -year from the date of original purchase, except in the European Union and certain Asia Pacific countries where the Company provides a two -year warranty. The Company has the obligation, at its option, to either repair or replace a defective product. At the time revenue is recognized, an estimate of future warranty costs is recorded as a component of cost of revenues. The estimate of future warranty costs is based on historical and projected warranty claim rates, historical and projected cost-per-claim and knowledge of specific product failures, if any, that are outside of the Company’s typical experience. The Company regularly review these estimates to assess the appropriateness of its recorded warranty liabilities and adjust the amounts as necessary. Factors that affect the warranty obligation include product failure rates, service delivery costs incurred in correcting the product failures, and warranty policies. The Company’s products are manufactured by contract manufacturers, and in certain cases, the Company may have recourse against such contract manufacturers. Should actual product failure rates, use of materials or other costs differ from the Company’s estimates, additional warranty liabilities could be incurred, which could materially affect its results of operations. The estimates and assumptions used to reserve for product warranty have been accurate in all material respects and have not materially changed in the past.
Fitbit Force Product Recall	Fitbit Force Product Recall The Company established reserves for the Fitbit Force recall when circumstances giving rise to the recall became known. It considered various factors in estimating the product recall exposure. These include estimates for: • refunds and product returns from retailer and distributor customers and end-users, which were charged to revenue and cost of revenue on the consolidated statements of operations; • logistics and handling fees for managing product returns and processing refunds, obsolescence of on-hand inventory, cancellation charges for existing purchase commitments and rework of component inventory by the Company’s contract manufacturers, accelerated depreciation of tooling and manufacturing equipment, which were charged to cost of revenue on the consolidated statements of operations; and • legal fees and settlement costs, which were charged to general and administrative expenses on the consolidated statements of operations. These factors above are updated and reevaluated each period and the related reserves are adjusted when factors indicate that the recall reserves are either insufficient to cover or exceed the estimated product recall expenses.
Share-Based Compensation	Stock-Based Compensation Stock-based compensation is measured at the grant date based on the fair value of the award and is recognized as expense over the requisite service period, which is generally the vesting period of the respective award. Determining the fair value of stock-based awards at the grant date requires judgment. The fair value of restricted stock units, or RSUs, is the fair value of the Company’s common stock on the grant date. The Company uses the Black-Scholes option-pricing model to determine the fair value of stock options, warrants and shares issued under the 2015 Employee Stock Purchase Plan (the “2015 ESPP”). The Company recognizes tax benefits related to stock-based compensation to the extent that the total reduction to its income tax liability from stock-based compensation is greater than the amount of the deferred tax assets previously recorded in anticipation of these benefits.
Segment Information	Segment Information The Company operates as one operating segment as it only reports financial information on an aggregate and consolidated basis to its Chief Executive Officer, who is the Company’s chief operating decision maker.
Income Taxes	Income Taxes The Company utilizes the asset and liability method of accounting for income taxes, which requires the recognition of deferred tax assets and liabilities for expected future consequences of temporary differences between the financial reporting and income tax bases of assets and liabilities using enacted tax rates. The Company makes estimates, assumptions, and judgments to determine our expense (benefit) for income taxes and also for deferred tax assets and liabilities and any valuation allowances recorded against its deferred tax assets. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes that recovery is not likely, the Company establishes a valuation allowance. The calculation of the Company’s income tax expense involves the use of estimates, assumptions, and judgments while taking into account current tax laws, its interpretation of current tax laws, and possible outcomes of future tax audits. The Company has established reserves to address potential exposures related to tax positions that could be challenged by tax authorities. Although the Company believes its estimates, assumptions, and judgments to be reasonable, any changes in tax law or its interpretation of tax laws and the resolutions of potential tax audits could significantly impact the amounts provided for income taxes in its consolidated financial statements. The U.S. Tax Cuts and Jobs Act of 2017 (the “2017 Tax Act”) was enacted on December 22, 2017, 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. The 2017 Tax Act requires complex computations to be performed that were not previously required in U.S. tax law, and preparation and analysis of information not previously required or regularly produced, which require significant judgment in interpreting accounting guidance for such items that is currently uncertain. In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”), which allows the Company to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. The Company has provided its best estimate of the impact of the 2017 Tax Act in its year-end income tax provision in accordance with its understanding of the 2017 Tax Act and guidance available as of the date of issuance of these consolidated financial statements. As the Company completes its analysis of the 2017 Tax Act, any subsequent adjustments to provisional amounts that it has recorded may or may not impact its provision for income taxes in the period in which the adjustments are made due to a full valuation allowance on its U.S. deferred tax assets. The Company expects to complete its analysis within the measurement period in accordance with SAB 118. The calculation of the Company’s deferred tax asset balance involves the use of estimates, assumptions, and judgments while taking into account estimates of the amounts and type of future taxable income. Actual future operating results and the underlying amount and type of income could differ materially from its estimates, assumptions, and judgments, thereby impacting its financial position and operating results. The Company includes interest and penalties related to unrecognized tax benefits within income tax expense. Interest and penalties related to unrecognized tax benefits have been recognized in the appropriate periods presented.
Net Income (Loss) per Share Attributable to Common Stockholders	Net Income (Loss) per Share Attributable to Common Stockholders Basic and diluted net income (loss) per share attributable to common stockholders is presented in conformity with the two-class method required for participating securities. The Company considers its redeemable convertible preferred stock to be participating securities. The holders of the redeemable convertible preferred stock did not have a contractual obligation to share in losses. In accordance with the two-class method, earnings allocated to these participating securities and the related number of outstanding shares of the participating securities, which include contractual participation rights in undistributed earnings, have been excluded from the computation of basic and diluted net income per share attributable to common stockholders. For the calculation of diluted net income per share, net income attributable to common stockholders for basic net income per share is adjusted by the effect of dilutive securities. Diluted net income per share attributable to common stockholders is computed by dividing the net income attributable to common stockholders by the weighted-average number of common shares outstanding, including all potentially dilutive common shares, if the effect of such shares is dilutive. In connection with the Company’s Initial Public Offering (“IPO”) in 2015, the Company established two classes of authorized common stock: Class A common stock and Class B common stock. As a result, all then-outstanding shares of common stock were converted into shares of Class B common stock. The rights of the holders of Class A common stock and Class B common stock are identical, except with respect to voting and conversion. Each share of Class A common stock is entitled to one vote per share and each share of Class B common stock is entitled to ten votes per share. Each share of Class B common stock is convertible at any time at the option of the stockholder into one share of Class A common stock, generally automatically converts into Class A common stock upon a transfer, and has no expiration date. The Company applies the two-class method of calculating earnings per share, but as the dividend rights of both classes are identical, basic and diluted earnings per share are the same for both classes. As the Company was in a net loss position in 2017 and 2016, basic net loss per share attributable to common stockholders was the same as diluted net loss per share attributable to common stockholders as the inclusion of all potential shares of common stock outstanding would have been anti-dilutive.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Accounting Pronouncements Not Yet Adopted In May 2014, the Financial Accounting Standards Board (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. ASU 2014-09 will replace most existing revenue recognition guidance in U.S. GAAP when it becomes effective. 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. In doing so, companies will need to use more judgment and make more estimates than under the currently effective guidance. These judgments may include identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price and allocating the transaction price to each separate performance obligation. In April 2016, the FASB issued ASU 2016-10, which clarifies guidance on identifying performance obligations and licensing implementation. The Company will adopt ASU 2014-09 effective January 1, 2018, utilizing the modified retrospective transition method. Upon adoption, the Company will recognize the cumulative effect of adopting this guidance as an adjustment to its opening accumulated deficit balance. The Company expects this adjustment to be immaterial to its consolidated financial statements. Prior periods will not be retrospectively adjusted. The Company has assessed the impact of the guidance, which includes evaluating customer contracts across the organization, developing policies, processes and tools to report financial results, and implementing and evaluating the Company’s internal control over financial reporting that will be necessary under the new standard. 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. In addition, the Company has determined that the presentation of certain reserve balances currently shown net within accounts receivable will be presented as refund liabilities within current liabilities upon adoption. The Company has also determined that the impact of accounting for costs incurred to obtain a contract is immaterial. The Company believes the new guidance is materially consistent with its historical revenue recognition policy. Overall, the Company does not currently expect the adoption to have a material impact on its consolidated financial statements. In January 2016, the FASB issued ASU No. 2016-01, F inancial 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 will adopt ASU 2016-01 in its first quarter of 2018. The Company is currently evaluating the impact of this guidance on its consolidated financial statements. In February 2016, 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 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 situation 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 will become effective for the Company on January 1, 2018 and early adoption is permitted. The Company does not currently expect the adoption of this guidance to have a material impact on its 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 will become effective for the Company on January 1, 2018. The Company does not expect the adoption of this guidance to have a material impact 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 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 will become effective for the Company on January 1, 2018 with early adoption permitted. The amendments to ASU 2017-09 should be applied prospectively to an award modified on or after the adoption date. The Company does not currently expect the adoption of this guidance to have a material impact on its 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 guidance and better portray the economic results of risk management activities in the financial statements. The guidance 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 will become effective for the Company on January 1, 2019 with early adoption permitted. The Company plans to early adopt this new guidance in the first quarter of 2018 and does not expect the adoption will have a material impact on its consolidated financial statements. Accounting Pronouncements Recently Adopted In March 2016, the FASB issued ASU 2016-09, Compensation-Stock Compensation (Topic 718). ASU 2016-09 simplifies several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. Upon adoption, ASU 2016-09 requires that excess tax benefits for share-based payments be recorded as a reduction of income tax expense and reflected within operating cash flows, rather than being recorded within equity and reflected within financing cash flows. ASU 2016-09 also permits the repurchase of more of an employee’s shares for tax withholding purposes without triggering liability accounting, clarifies that all cash payments made on an employee’s behalf for withheld shares should be presented as a financing activity on the Company’s cash flows statement, and provides an accounting policy election to account for forfeitures as they occur. ASU 2016-09 became effective for the Company on January 1, 2017. The adoption of ASU 2016-09 resulted in a cumulative effect adjustment of $4.9 million to increase retained earnings as of January 1, 2017, related to the recognition of previously unrecognized excess tax benefits using the modified retrospective method. The Company elected to apply the change in presentation of excess tax benefits in the consolidated statement of cash flows retrospectively, which resulted in an increase in net cash provided by operations and a decrease in net cash provided by financing activities of $29.2 million for 2016 and $32.1 million for 2015. The Company also elected to make an accounting policy change to recognize forfeitures starting on January 1, 2017 on a prospective basis.

Significant Accounting Polici24

Significant Accounting Policies Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Schedule of useful lives of property and equipment	The useful lives of the property and equipment are as follows: Tooling and manufacturing equipment One to three years Furniture and office equipment Three years Purchased software Three years Capitalized internally-developed software Two to eight years Leasehold improvements Shorter of remaining lease term or ten years Property and equipment, net, consisted of the following (in thousands): December 31, 2017 2016 Tooling and manufacturing equipment $ 66,854 $ 60,944 Furniture and office equipment 20,942 14,424 Purchased and internally-developed software 18,112 12,032 Leasehold improvements 58,431 28,489 Total property and equipment 164,339 115,889 Less: Accumulated depreciation and amortization (59,431 ) (39,336 ) Property and equipment, net $ 104,908 $ 76,553

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2017
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): 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 $ — $ 2,138 $ 208 $ 2,346 December 31, 2016 Level 1 Level 2 Level 3 Total Assets: Money market funds $ 50,125 $ — $ — $ 50,125 U.S. government agencies — 86,526 — 86,526 Corporate debt securities — 390,286 — 390,286 Derivative assets — 10,625 — 10,625 Total $ 50,125 $ 487,437 $ — $ 537,562 Liabilities: Derivative liabilities $ — $ 3,780 $ — $ 3,780

Financial Instruments (Tables)

Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of available for sale securities including fair value and gross unrealized losses	The following table sets forth the 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 sets forth the cash, cash equivalents, and marketable securities as of December 31, 2016 (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Cash and Cash Equivalents Marketable Securities Cash $ 179,076 $ — $ — $ 179,076 $ 179,076 $ — Money market funds 50,125 — — 50,125 50,125 — U.S. government agencies 86,533 8 (15 ) 86,526 — 86,526 Corporate debt securities 390,466 24 (204 ) 390,286 72,119 318,167 Total $ 706,200 $ 32 $ (219 ) $ 706,013 $ 301,320 $ 404,693 The following table classifies marketable securities by contractual maturities (in thousands): December 31, 2017 December 31, 2016 Due in one year $ 319,112 $ 355,152 Due in one to two years 18,222 49,541 Total $ 337,334 $ 404,693
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 dates presented (in thousands): December 31, 2017 December 31, 2016 Balance Sheet Location Fair Value Derivative Assets Fair Value Derivative Liabilities Fair Value Derivative Assets Fair Value Derivative Liabilities Cash flow designated hedges Prepaid expense and other current assets $ — $ — $ 813 $ — Cash flow designated hedges Accrued liabilities — — — 1,428 Hedges not designated Prepaid expense and other current assets — — 9,812 — Hedges not designated Accrued liabilities — 2,138 — 2,352 Total fair value of derivative instruments $ — $ 2,138 $ 10,625 $ 3,780
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 consolidated statement of operations for the periods presented (in thousands): Year Ended December 31, Income Statement Location 2017 2016 2015 Foreign exchange cash flow hedges: Gain (loss) recognized in OCI—effective portion $ (19,436 ) $ 8,171 $ 2,785 Gain (loss) reclassified from OCI into income—effective portion Revenue (18,532 ) 10,153 2,183 Gain (loss) reclassified from OCI into income—effective portion Operating expenses (1,405 ) 17 (899 ) Gain (loss) recognized in income—ineffective portion Other income (expense), net 21 (1,026 ) 202 Gain (loss) recognized in income—excluded time value portion Other income (expense), net 1,771 — — Foreign exchange balance sheet hedges: Gain (loss) recognized in income Other income (expense), net $ (10,516 ) $ 10,916 $ 5,861
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 consolidated statement of operations for the periods presented (in thousands): Year Ended December 31, Income Statement Location 2017 2016 2015 Foreign exchange cash flow hedges: Gain (loss) recognized in OCI—effective portion $ (19,436 ) $ 8,171 $ 2,785 Gain (loss) reclassified from OCI into income—effective portion Revenue (18,532 ) 10,153 2,183 Gain (loss) reclassified from OCI into income—effective portion Operating expenses (1,405 ) 17 (899 ) Gain (loss) recognized in income—ineffective portion Other income (expense), net 21 (1,026 ) 202 Gain (loss) recognized in income—excluded time value portion Other income (expense), net 1,771 — — Foreign exchange balance sheet hedges: Gain (loss) recognized in income Other income (expense), net $ (10,516 ) $ 10,916 $ 5,861
Schedule of offsetting derivative assets	The following table sets forth the available offsetting of net derivative assets and net derivative liabilities under the master netting arrangements as of December 31, 2017 and December 31, 2016 (in thousands): December 31, 2017 Gross Amounts Offset in the Consolidated Balance Sheets Gross Amounts Not Offset in Consolidated Balance Sheets Gross Amount Recognized Gross Amount Offset Net Amount Presented Financial Instruments Cash Collateral Received Net Amount Foreign exchange contracts assets $ — $ — $ — $ — $ — $ — Foreign exchange contracts liabilities 2,138 — 2,138 — — 2,138 December 31, 2016 Gross Amounts Offset in the Consolidated Balance Sheets Gross Amounts Not Offset in Consolidated Balance Sheets Gross Amounts Recognized Gross Amounts Offset Net Amount Presented Financial Instruments Cash Collateral Received Net Amount Foreign exchange contracts assets $ 10,625 $ — $ 10,625 $ 3,780 $ — $ 6,845 Foreign exchange contracts liabilities 3,780 — 3,780 3,780 — —
Schedule of offsetting derivative liabilities	The following table sets forth the available offsetting of net derivative assets and net derivative liabilities under the master netting arrangements as of December 31, 2017 and December 31, 2016 (in thousands): December 31, 2017 Gross Amounts Offset in the Consolidated Balance Sheets Gross Amounts Not Offset in Consolidated Balance Sheets Gross Amount Recognized Gross Amount Offset Net Amount Presented Financial Instruments Cash Collateral Received Net Amount Foreign exchange contracts assets $ — $ — $ — $ — $ — $ — Foreign exchange contracts liabilities 2,138 — 2,138 — — 2,138 December 31, 2016 Gross Amounts Offset in the Consolidated Balance Sheets Gross Amounts Not Offset in Consolidated Balance Sheets Gross Amounts Recognized Gross Amounts Offset Net Amount Presented Financial Instruments Cash Collateral Received Net Amount Foreign exchange contracts assets $ 10,625 $ — $ 10,625 $ 3,780 $ — $ 6,845 Foreign exchange contracts liabilities 3,780 — 3,780 3,780 — —

Balance Sheet Components (Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of accounts receivable and revenue reserves	Changes in the allowance for doubtful accounts and revenue returns reserve were as follows (in thousands): Allowance for Doubtful Accounts Revenue Returns Reserve (1) Balance at December 31, 2014 838 26,559 Increases 1,115 169,677 Write-offs/returns taken (128 ) (122,191 ) Balance at December 31, 2015 1,825 74,045 Increases 339 275,815 Write-offs/returns taken (1,882 ) (251,009 ) Balance at December 31, 2016 282 98,851 Increases (2) 30,551 229,610 Write-offs/returns taken (2) (21,604 ) (218,589 ) Balance at December 31, 2017 $ 9,229 $ 109,872
Schedule of inventories	Inventories consisted of the following (in thousands): December 31, 2017 2016 Components $ 3,825 $ 1,035 Finished goods 120,070 229,352 Total inventories $ 123,895 $ 230,387
Schedule of prepaid expenses and other current asset	Prepaid expenses and other current assets consisted of the following (in thousands): December 31, 2017 2016 POP displays, net $ 14,750 $ 22,804 Prepaid expenses 24,204 17,161 Derivative assets — 10,625 Prepaid marketing 6,074 5,764 Insurance receivable 37,300 — Other 14,941 9,511 Total prepaid expenses and other current assets $ 97,269 $ 65,865
Schedule of property and equipment	The useful lives of the property and equipment are as follows: Tooling and manufacturing equipment One to three years Furniture and office equipment Three years Purchased software Three years Capitalized internally-developed software Two to eight years Leasehold improvements Shorter of remaining lease term or ten years Property and equipment, net, consisted of the following (in thousands): December 31, 2017 2016 Tooling and manufacturing equipment $ 66,854 $ 60,944 Furniture and office equipment 20,942 14,424 Purchased and internally-developed software 18,112 12,032 Leasehold improvements 58,431 28,489 Total property and equipment 164,339 115,889 Less: Accumulated depreciation and amortization (59,431 ) (39,336 ) Property and equipment, net $ 104,908 $ 76,553
Schedule of goodwill	The changes in the carrying amount of goodwill were as follows (in thousands). See Note 12 for additional information. Goodwill Balance at December 31, 2015 $ 22,157 Goodwill acquired 28,879 Balance at December 31, 2016 $ 51,036 Goodwill acquired — Balance at December 31, 2017 $ 51,036
Schedule of intangible assets (excluding goodwill)	The carrying amounts of the intangible assets as of December 31, 2017 and December 31, 2016 were as follows (in thousands, except useful life). See Note 12 for additional information. During 2017, the development was completed for certain technology that was in-process at the time of its acquisition. Accordingly, $3.9 million of in-process research and development recorded in the initial purchase price allocation has been reclassified to developed technology as of December 31, 2017. December 31, 2017 December 31, 2016 Weighted Average Remaining Useful Life (years) Gross Accumulated Amortization Net Gross Accumulated Amortization Net Developed technology $ 30,588 $ (8,738 ) $ 21,850 $ 26,092 $ (3,247 ) $ 22,845 4.0 Trademarks and other 1,278 (772 ) 506 1,278 (542 ) 736 0.9 Total finite-lived intangible assets subject to amortization, net 31,866 (9,510 ) 22,356 27,370 (3,789 ) 23,581 In-process research and development — — — 3,940 — 3,940 Total intangible assets, net $ 31,866 $ (9,510 ) $ 22,356 $ 31,310 $ (3,789 ) $ 27,521
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 2017 , is as follows (in thousands): Cost of Revenue Operating Expenses Total 2018 $ 6,120 $ 230 $ 6,350 2019 5,340 230 5,570 2020 4,560 46 4,606 2021 4,560 — 4,560 2022 1,270 — 1,270 Total intangible assets, net $ 21,850 $ 506 $ 22,356
Schedule of accrued liabilities	Accrued liabilities consisted of the following (in thousands): December 31, 2017 2016 Product warranty $ 87,882 $ 99,923 Accrued manufacturing expense and freight 41,901 75,579 Accrued sales incentives 111,592 74,181 Accrued sales and marketing 44,401 41,948 Accrued research and development 8,983 5,989 Accrued co-op advertising and marketing development funds 30,408 40,002 Employee-related liabilities 33,266 13,934 Sales taxes and VAT payable 21,340 8,891 Inventory received but not billed 10,526 7,363 Accrued legal settlements and fees 36,693 3,963 Derivative liabilities 2,138 3,780 Other 23,007 15,008 Accrued liabilities $ 452,137 $ 390,561
Schedule of product warranty reserves	Product warranty reserve activities were as follows (in thousands): Reserve For Product Warranty (1) Balance at December 31, 2014 $ 20,098 Charged to cost of revenue 84,184 Changes in estimate related to pre-existing warranties (8,968 ) Settlement of claims (55,102 ) Balance at December 31, 2015 $ 40,212 Charged to cost of revenue 185,434 Changes in estimate related to pre-existing warranties 4,072 Settlement of claims (129,795 ) Balance at December 31, 2016 $ 99,923 Charged to cost of revenue 53,840 Changes in estimate related to pre-existing warranties 11,788 Settlement of claims (77,669 ) Balance at December 31, 2017 $ 87,882 (1) Does not include reserves established as a result of the recall of the Fitbit Force. See the section titled “—Fitbit Force Recall Reserve” for additional information regarding such reserves.
Schedule of product recall reserves	Fitbit Force recall reserve activities were as follows (in thousands): Reserve For Fitbit Force Recall Balance at December 31, 2014 $ 22,476 Charged to cost of revenue (5,755 ) Charged to general and administrative (1,174 ) Settlement of claims (10,425 ) Balance at December 31, 2015 5,122 Settlement of claims (3,869 ) Balance at December 31, 2016 1,253 Settlement of claims (789 ) Balance at December 31, 2017 $ 464
Schedule of restructuring reserve activities	The restructuring reserve activities were as follows (in thousands): Restructuring Reserve Balance at December 31, 2016 $ — Restructuring charges 6,375 Cash paid (4,983 ) Other - noncash (1,392 ) Balance at December 31, 2017 $ —
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 on Cash Flow Hedges Currency Translation Adjustments Unrealized Gains (Losses) on Available-for-Sale Investments Total Balance at December 31, 2015 $ 751 $ (5 ) $ (55 ) $ 691 Other comprehensive income (loss) before reclassifications 9,422 (309 ) (126 ) 8,987 Amounts reclassified from AOCI (10,650 ) — (6 ) (10,656 ) Other comprehensive income (loss) (1,228 ) (309 ) (132 ) (1,669 ) Balance at December 31, 2016 (477 ) (314 ) (187 ) (978 ) Other comprehensive income (loss) before reclassifications (19,422 ) 314 125 (18,983 ) Amounts reclassified from AOCI 19,965 — (13 ) 19,952 Other comprehensive income (loss) 543 314 112 969 Balance at December 31, 2017 $ 66 $ — $ (75 ) $ (9 )

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Schedule of future minimum rental payments for operating leases	Future minimum payments under the leases as of December 31, 2017 were as follows (in thousands): Year ending December 31, Amounts 2018 $ 40,856 2019 46,713 2020 42,942 2021 41,331 2022 41,477 Thereafter 58,879 Total $ 272,198

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of stock option activity	Activity under the 2007 Plan and 2015 Plan is as follows (in thousands except per share amounts): Stock Options Outstanding Number of Shares Subject to Stock Options Weighted– Average Exercise Price Weighted– Average Remaining Contractual Term (in years) Aggregate Intrinsic Value Balance—December 31, 2016 34,454 $ 3.85 Granted 1,150 $ 5.63 Exercised (7,164 ) $ 1.22 Canceled (7,054 ) $ 9.35 Balance—December 31, 2017 21,386 $ 3.01 5.7 $ 64,582 Stock options exercisable—December 31, 2017 16,827 $ 2.52 5.3 $ 57,472 Stock options vested and expected to vest—December 31, 2017 21,386 $ 3.01 5.7 $ 64,582
Schedule of restricted stock unit activity	RSU activity under the equity incentive plans is as follows: RSUs Outstanding Weighted- Average Grant Date Fair Value (in thousands) Unvested balance—December 31, 2016 11,578 $ 16.85 Granted 18,912 6.47 Vested (6,191 ) 12.57 Forfeited or canceled (5,111 ) 12.61 Unvested balance—December 31, 2017 19,188 9.13
Schedule of stock-based compensation expense	Total stock-based compensation recognized was as follows (in thousands): Year Ended December 31, 2017 2016 2015 Cost of revenue $ 5,312 $ 4,797 $ 4,739 Research and development 54,123 47,207 18,251 Sales and marketing 14,959 11,575 7,419 General and administrative 17,187 15,853 10,615 Total stock-based compensation expense $ 91,581 $ 79,432 $ 41,024
Schedule of options valuation assumptions	The fair value of the stock option awards, warrants and awards issued under the 2015 ESPP granted to employees was estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions: Year Ended December 31, 2017 2016 2015 Employee stock options Expected term (in years) 6.25 6.25 6.25 Volatility 32.2% 40.7% 52.1% - 56.9% Risk-free interest rate 2.1% 1.6% 1.5% - 1.9% Dividend yield —% —% —% Warrants Expected term (in years) 9.5 — — Volatility 32.0% —% —% Risk-free interest rate 2.1% —% —% Dividend yield —% —% —% Employee stock purchase plan Expected term (in years) 0.5 0.5 0.5 – 0.9 Volatility 27.7% - 31.3% 30.1% - 39.0% 27.7% - 35.0% Risk-free interest rate 1.0% - 1.4% 0.4% - 0.6% 0.3% Dividend yield —% —% —%

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Schedule of components of income (loss) before income taxes	The following table presents domestic and foreign components of income (loss) before income taxes for the periods presented (in thousands): Year Ended December 31, 2017 2016 2015 United States $ (158,187 ) $ 5,577 $ 286,380 Foreign (36,457 ) (114,872 ) 1,569 Total $ (194,644 ) $ (109,295 ) $ 287,949
Schedule of components for income tax expense	The income tax expense (benefit) is composed of the following (in thousands): Year Ended December 31, 2017 2016 2015 Current: Federal $ (87,961 ) $ 78,782 $ 140,396 State (8,429 ) 9,878 13,307 Foreign 5,032 5,256 1,107 Total current (91,358 ) 93,916 154,810 Deferred: Federal 154,817 (87,584 ) (33,421 ) State 18,902 (11,622 ) (8,941 ) Foreign 187 (1,228 ) (176 ) Total deferred 173,906 (100,434 ) (42,538 ) Total income tax expense (benefit) $ 82,548 $ (6,518 ) $ 112,272
Schedule of reconciliation of effective tax to statutory federal rate	The reconciliation of the Company’s effective tax rate to the statutory federal rate is as follows: Year Ended December 31, 2017 2016 2015 Tax at federal statutory rate 35.0 % 35.0 % 35.0 % State taxes, net of federal effect (5.4 ) 4.3 1.5 Foreign rate differential (9.3 ) (38.9 ) (0.8 ) Tax credits 4.1 9.0 (2.0 ) Domestic production activities deduction (3.5 ) 5.0 (3.3 ) Stock-based compensation (1) (5.3 ) (4.6 ) 1.7 Change in prior year reserves (2.0 ) 1.9 — Out-of-period adjustment — (2.8 ) — Warrant fair value adjustment — — 6.9 Change in valuation allowance (35.2 ) — — Effect of change in tax rate due to Tax Act (23.4 ) — — Other 2.6 (2.9 ) — Effective tax rate (42.4 )% 6.0 % 39.0 %
Schedule of tax effects of temporary differences for deferred tax assets	The tax effects of temporary differences that give rise to significant portions of the deferred tax assets were as follows (in thousands): December 31, 2017 2016 Deferred tax assets: Net operating losses and credits $ 23,338 $ 9,446 Fixed assets and intangible assets 10,625 16,272 Accruals and reserves 49,886 112,915 Stock-based compensation 12,154 17,864 Inventory 4,345 8,513 Other 3,325 11,134 Total deferred tax assets 103,673 176,144 Less: valuation allowance (99,570 ) — Deferred tax assets, net of valuation allowance 4,103 176,144 Deferred tax liabilities: Other (369 ) (353 ) Total deferred tax liabilities (369 ) (353 ) Net deferred tax assets $ 3,734 $ 175,791
Schedule of reconciliation of unrecognized tax benefits	A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands): December 31, 2017 2016 2015 Balance at beginning of year $ 35,584 $ 23,518 $ 10,594 Reductions based on tax positions related to prior year (6,335 ) (2,100 ) (18 ) Additions based on tax positions related to prior year 108 2,809 — Additions based on tax positions related to current year 9,289 11,357 12,942 Reductions due to tax authorities’ settlements (8,603 ) — — Reductions due to expiration of statutes of limitation (105 ) — — Balance at end of year $ 29,938 $ 35,584 $ 23,518

Net Income (Loss) per Share A31

Net Income (Loss) per Share Attributable to Common Stockholders (Tables)	12 Months Ended
	Dec. 31, 2017
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 income (loss) per share attributable to common stockholders (in thousands, except per share amounts): Year Ended December 31, 2017 2016 2015 Numerator: Net income (loss) $ (277,192 ) $ (102,777 ) $ 175,677 Less: noncumulative dividends to preferred stockholders — — (2,526 ) Less: undistributed earnings to participating securities — — (59,133 ) Net income (loss) attributable to common stockholders—basic (277,192 ) (102,777 ) 114,018 Add: adjustments to undistributed earnings to participating securities — — 8,821 Net income (loss) attributable to common stockholders—diluted $ (277,192 ) $ (102,777 ) $ 122,839 Denominator: Weighted-average shares of common stock—basic for Class A and Class B 232,032 220,405 129,886 Effect of dilutive securities — — 34,327 Weighted-average shares of common stock—diluted for Class A and Class B 232,032 220,405 164,213 Net income (loss) per share attributable to common stockholders: Basic $ (1.19 ) $ (0.47 ) $ 0.88 Diluted $ (1.19 ) $ (0.47 ) $ 0.75
Schedule of antidilutive securities excluded from earnings per share	The following potentially dilutive common shares on a weighted average basis were excluded from the computation of diluted net income (loss) per share for the periods presented because including them would have been antidilutive (in thousands): December 31, 2017 2016 2015 Stock options to purchase common stock 17,469 34,454 445 RSUs 10,030 11,578 692 Warrants 216 — — Diluted common stock subject to vesting 84 — — Diluted impact of ESPP 162 — — Redeemable stock unites — — 65,903 Redeemable convertible preferred stock warrants — — 921 Total 27,961 46,032 67,961

Significant Customer Informat32

Significant Customer Information and Other Information (Tables)	12 Months Ended
	Dec. 31, 2017
Risks and Uncertainties [Abstract]
Schedules of concentration of risk	Retailers and distributors with revenue equal to or greater than 10% of total revenue were as follows: December 31, 2017 2016 2015 C 13 % 14 % 14 % A * 14 15 B * 10 14 * Revenue was less than 10%. Retailers and distributors that accounted for equal to or greater than 10% of accounts receivable at December 31, 2017 and 2016 were as follows: December 31, 2017 2016 C 17 % 19 % B 13 * E 11 * A * 16 D * 12 * Accounts receivable were less than 10%. Revenue by geographic region, based on ship-to destinations, was as follows (in thousands): December 31, 2017 2016 2015 United States $ 944,052 $ 1,539,600 $ 1,381,152 Americas excluding United States 116,330 110,111 92,252 Europe, Middle East, and Africa 440,135 389,154 208,767 APAC 115,002 130,596 175,827 Total $ 1,615,519 $ 2,169,461 $ 1,857,998

Acquisitions (Tables)

Acquisitions (Tables)	12 Months Ended
	Dec. 31, 2017
Business Combinations [Abstract]
Schedule of fair value of assets acquired and liabilities assumed	The following table summarizes the fair value of assets acquired and liabilities assumed (in thousands): Goodwill $ 22,157 Developed and core technology 12,640 Customer relationships 128 Trademarks 1,150 Assumed liabilities, net of assets (3,552 ) Total $ 32,523

Selected Unaudited Quarterly 34

Selected Unaudited Quarterly Financial Data (Tables)	12 Months Ended
	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Schedule of quarterly financial information	The following tables show a summary of the Company’s unaudited quarterly financial information for each of the four quarters of 2017 and 2016 (in thousands, except per share amounts): Three Months Ended December 31, 2017 (3) September 30, 2017 (2) July 1, 2017 April 1, 2017 (1) Revenue $ 570,756 $ 392,522 $ 353,299 $ 298,942 Gross profit $ 248,597 $ 174,760 $ 149,245 $ 118,299 Net loss $ (45,470 ) $ (113,403 ) $ (58,240 ) $ (60,079 ) Net loss per share attributable to common stockholders—basic $ (0.19 ) $ (0.48 ) $ (0.25 ) $ (0.27 ) Net loss per share attributable to common stockholders—diluted $ (0.19 ) $ (0.48 ) $ (0.25 ) $ (0.27 ) Three Months Ended December 31, 2016 October 1, 2016 July 2, 2016 April 2, 2016 Revenue $ 573,775 $ 503,802 $ 586,528 $ 505,356 Gross profit $ 126,502 $ 240,658 $ 244,969 $ 233,755 Net income (loss) $ (146,273 ) $ 26,120 $ 6,341 $ 11,035 Net income (loss) per share attributable to common stockholders—basic $ (0.65 ) $ 0.12 $ 0.03 $ 0.05 Net income (loss) per share attributable to common stockholders—diluted $ (0.65 ) $ 0.11 $ 0.03 $ 0.05

Business Overview and Basis o35

Business Overview and Basis of Presentation (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Jul. 01, 2017	Jan. 31, 2018	Sep. 30, 2017	Dec. 31, 2017	Sep. 30, 2017	Jul. 01, 2017	Apr. 01, 2017	Dec. 31, 2016	Oct. 01, 2016	Jul. 02, 2016	Apr. 02, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Organization, Consolidation And Presentation Of Financial Statements [Line Items]
Revenue					$ 570,756	$ 392,522	$ 353,299	$ 298,942	$ 573,775	$ 503,802	$ 586,528	$ 505,356	$ 1,615,519	$ 2,169,461	$ 1,857,998
Cost of revenue													924,618	1,323,577	956,935
Bad debt expense													7,893	339	$ 1,115
Insurance receivable					37,300				0				$ 37,300	0
Advertising credit, expected period of use													4 years
Advertising Barter Transactions [Member]
Organization, Consolidation And Presentation Of Financial Statements [Line Items]
Fair value of revenue in nonmonetary transaction														15,000
Fair value of cost of goods sold in nonmonetary transaction														7,000
Remaining advertising credit					12,200				$ 13,000				$ 12,200	13,000
Cash received in nonmonetary transaction														$ 2,000
Advertising credit utilized													$ 800
Wynit Distribution [Member]
Organization, Consolidation And Presentation Of Financial Statements [Line Items]
Revenue						8,100
Cost of revenue	$ 9,000					35,800
Cost of revenue associated with shipments	1,400					5,500
Insurance proceeds received					21,400
Bad debt expense	7,600					30,300
Customer Concentration Risk [Member] | Revenue [Member]
Organization, Consolidation And Presentation Of Financial Statements [Line Items]
Concentration risk, percentage				11.00%
Customer Concentration Risk [Member] | Accounts Receivable [Member]
Organization, Consolidation And Presentation Of Financial Statements [Line Items]
Concentration risk, percentage		19.00%
Understated Long-term Deferred Revenue [Member]
Organization, Consolidation And Presentation Of Financial Statements [Line Items]
Out-of-period adjustment					7,300
Understated Deferred Tax Assets and Income Tax Payable [Member]
Organization, Consolidation And Presentation Of Financial Statements [Line Items]
Out-of-period adjustment					$ 1,700
Prepaid expenses and other current assets [Member] | Wynit Distribution [Member]
Organization, Consolidation And Presentation Of Financial Statements [Line Items]
Insurance receivable	26,800			$ 26,800		26,800
Allowance for Doubtful Accounts [Member] | Wynit Distribution [Member]
Organization, Consolidation And Presentation Of Financial Statements [Line Items]
Insurance receivable	22,700			22,700		22,700
Cost of revenue [Member] | Wynit Distribution [Member]
Organization, Consolidation And Presentation Of Financial Statements [Line Items]
Insurance receivable	$ 4,100			$ 4,100		$ 4,100
Subsequent Event [Member] | Wynit Distribution [Member]
Organization, Consolidation And Presentation Of Financial Statements [Line Items]
Insurance proceeds received			$ 5,400

Significant Accounting Polici36

Significant Accounting Policies - Property, Plant and Equipment (Details)	12 Months Ended
	Dec. 31, 2017
Tooling and manufacturing equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	1 year
Tooling and manufacturing equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Furniture and office equipment [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Purchased software [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	3 years
Capitalized internally-developed software [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	2 years
Capitalized internally-developed software [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	8 years
Leasehold improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Property and equipment, useful life	10 years

Significant Accounting Polici37

Significant Accounting Policies - Narrative (Details)	6 Months Ended	12 Months Ended
	Jul. 01, 2017USD ($)	Dec. 31, 2017USD ($)segmentvotereporting_unit	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)classvote	Jan. 01, 2017USD ($)	Jun. 30, 2015class
Significant Accounting Policies [Line Items]
Impairment loss recorded		$ 0
Foreign currency transaction gains (losses)	$ 2,600,000		$ 11,700,000	$ (1,300,000)
Capitalized software costs		$ 11,200,000	6,300,000
Number of operating segments | segment		1
Number of reporting units | reporting_unit		1
Best estimated selling price upgrade rights		$ 0.99
Advertising and promotion expenses		226,300,000	316,800,000	237,000,000
Cooperative advertising and promotion expense		$ 45,000,000	52,900,000	$ 38,300,000
Product warranty term		1 year
Number of classes of stock | class				2		2
Net cash provided by operations		$ 64,241,000	138,720,000	$ 141,257,000
Net cash provided by financing activities		$ (4,635,000)	(19,794,000)	(368,953,000)
Accounting Standards Update 2016-09 [Member]
Significant Accounting Policies [Line Items]
Net cash provided by operations			29,200,000	32,100,000
Net cash provided by financing activities			$ 29,200,000	$ 32,100,000
Accounting Standards Update 2016-09 [Member] | Retained Earnings [Member]
Significant Accounting Policies [Line Items]
Cumulative effect of new accounting pronouncement					$ 4,900,000
Common Class A [Member]
Significant Accounting Policies [Line Items]
Common stock, votes per share | vote		1		1
Common Class B [Member]
Significant Accounting Policies [Line Items]
Common stock, votes per share | vote		10		10
European Union [Member]
Significant Accounting Policies [Line Items]
Product warranty term		2 years
Customer Concentration Risk [Member] | Subscription-based Premium Services [Member]
Significant Accounting Policies [Line Items]
Percent of revenue (less than)		1.00%
Customer Concentration Risk [Member] | Corporate Dashboard Tool [Member]
Significant Accounting Policies [Line Items]
Percent of revenue (less than)		1.00%
Minimum [Member]
Significant Accounting Policies [Line Items]
Best estimate of the selling price		$ 1
Maximum [Member]
Significant Accounting Policies [Line Items]
Best estimate of the selling price		$ 3

Fair Value Measurements (Detail

Fair Value Measurements (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Assets:
Available for sale securities	$ 337,334,000	$ 404,693,000
U.S. government agencies [Member]
Assets:
Available for sale securities	73,029,000	86,526,000
Corporate debt securities [Member]
Assets:
Available for sale securities	264,305,000	318,167,000
Fair Value, Measurements, Recurring [Member]
Assets:
Money market funds	193,066,000	50,125,000
Derivative assets		10,625,000
Total	564,272,000	537,562,000
Liabilities:
Derivative liabilities	2,138,000	3,780,000
Stock warrant liability	208,000
Total liabilities	2,346,000
Fair Value, Measurements, Recurring [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	79,624,000	86,526,000
Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	291,582,000	390,286,000
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Money market funds	193,066,000	50,125,000
Derivative assets		0
Total	193,066,000	50,125,000
Liabilities:
Derivative liabilities	0	0
Stock warrant liability	0
Total liabilities	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets:
Money market funds	0	0
Derivative assets		10,625,000
Total	371,206,000	487,437,000
Liabilities:
Derivative liabilities	2,138,000	3,780,000
Stock warrant liability	0
Total liabilities	2,138,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	79,624,000	86,526,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	291,582,000	390,286,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Assets:
Money market funds	0	0
Derivative assets		0
Total	0	0
Liabilities:
Derivative liabilities	0	0
Stock warrant liability	208,000
Total liabilities	208,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	$ 0	$ 0

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjustments to the fair value of long lived assets	$ 0	$ 0	$ 0
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	564,272,000	537,562,000
Derivative liabilities	2,138,000	3,780,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities	0	$ 0
Other Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash paid for acquisition	$ 6,000,000

Financial Instruments - Narrati

Financial Instruments - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Gross unrealized gains (losses)	$ 0	$ 0
Available-for-sale securities in continuous loss position for one year or more, gross unrealized losses	0	0
Derivative, notional amount based on forecasted revenue	0
Derivative, notional amount based on forecasted expenses	0
Foreign currency exchange contract [Member] | Not designated as hedging instrument [Member]
Derivative [Line Items]
Derivative, notional amount	$ 141,200,000	177,000,000
Cash flow hedges [Member] | Foreign currency exchange contract [Member] | Designated as hedging instrument [Member]
Derivative [Line Items]
Derivative, notional amount based on forecasted revenue		20,000,000
Derivative, notional amount based on forecasted expenses		$ 20,900,000

Financial Instruments - Amortiz

Financial Instruments - Amortized to fair value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	$ 341,966	$ 301,320	$ 535,846	$ 195,626
Gross Unrealized Gains	16	32
Gross Unrealized Losses	(270)	(219)
Available for sale securities	337,334	404,693
Cash, cash equivalents, and available-for-sale securities, amortized cost	679,554	706,200
Fair Value	679,300	706,013
U.S. government agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	6,595	0
Available-for-sale debt securities, amortized cost	79,722	86,533
Gross Unrealized Gains	1	8
Gross Unrealized Losses	(99)	(15)
Available for sale securities	73,029	86,526
Fair Value	79,624	86,526
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	27,277	72,119
Available-for-sale debt securities, amortized cost	291,738	390,466
Gross Unrealized Gains	15	24
Gross Unrealized Losses	(171)	(204)
Available for sale securities	264,305	318,167
Fair Value	291,582	390,286
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	115,028	179,076
Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	$ 193,066	$ 50,125

Financial Instruments - Contrac

Financial Instruments - Contractual Maturity Dates (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Due in one year	$ 319,112	$ 355,152
Due in one to two years	18,222	49,541
Available for sale securities	$ 337,334	$ 404,693

Financial Instruments - Financi

Financial Instruments - Financial Position, Fair Value (Details) - Foreign currency exchange contract [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Fair Value Derivative Assets	$ 0	$ 10,625
Fair Value Derivative Liabilities	2,138	3,780
Designated as hedging instrument [Member] | Prepaid expenses and other current assets [Member]
Derivatives, Fair Value [Line Items]
Fair Value Derivative Assets	0	813
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	1,428
Not designated as hedging instrument [Member] | Prepaid expenses and other current assets [Member]
Derivatives, Fair Value [Line Items]
Fair Value Derivative Assets	0	9,812
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	$ 2,138	$ 2,352

Financial Instruments - Stateme

Financial Instruments - Statement of Operations and Other Comprehensive Income (Details) - Foreign currency exchange contract [Member] - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	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	$ (19,436,000)	$ 8,171,000	$ 2,785,000
Designated as hedging instrument [Member] | Cash flow hedges [Member] | Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from OCI into income—effective portion	(18,532,000)	10,153,000	2,183,000
Designated as hedging instrument [Member] | Cash flow hedges [Member] | Operating expenses [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from OCI into income—effective portion	(1,405,000)	17,000	(899,000)
Designated as hedging instrument [Member] | Cash flow hedges [Member] | Other income (expense), net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in income—ineffective portion	21,000	(1,026,000)	202,000
Gain (loss) recognized in income—excluded time value portion	1,771,000	0	0
Not designated as hedging instrument [Member] | Other income (expense), net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in income	$ (10,516,000)	$ 10,916,000	$ 5,861,000

Financial Instruments - Offsett

Financial Instruments - Offsetting of Foreign Currency Derivative Contracts (Details) - Foreign currency exchange contract [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Foreign exchange contracts liabilities [Line Items]
Gross Amounts Offset in the Consolidated Balance Sheets, Gross Amount Recognized	$ 2,138	$ 3,780
Gross Amounts Offset in the Consolidated Balance Sheets, Gross Amount Offset	0	0
Gross Amounts Offset in the Consolidated Balance Sheets, Net Amount Presented	2,138	3,780
Gross Amounts Not Offset in the Consolidated Balance Sheets, Financial Instruments	0	3,780
Gross Amounts Not Offset in the Consolidated Balance Sheets, Cash Collateral Received	0	0
Gross Amounts Not Offset in the Consolidated Balance Sheets, Net Amount	2,138	0
Foreign exchange contracts assets [Line Items]
Gross Amounts Offset in the Consolidated Balance Sheets, Gross Amount Recognized	0	10,625
Gross Amounts Offset in the Consolidated Balance Sheets, Gross Amount Offset	0	0
Gross Amounts Offset in the Consolidated Balance Sheets, Net Amount Presented	0	10,625
Gross Amounts Not Offset in the Consolidated Balance Sheets, Financial Instruments	0	3,780
Gross Amounts Not Offset in the Consolidated Balance Sheets, Cash Collateral Received	0	0
Gross Amounts Not Offset in the Consolidated Balance Sheets, Net Amount	$ 0	$ 6,845

Balance Sheet Components - Acco

Balance Sheet Components - Account Receivable and Revenue Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balances	$ 282	$ 1,825	$ 838
Increases	30,551	339	1,115
Write-offs/returns taken	(21,604)	(1,882)	(128)
Ending balances	9,229	282	1,825
Revenue Reserve [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balances	98,851	74,045	26,559
Increases	229,610	275,815	169,677
Write-offs/returns taken	(218,589)	(251,009)	(122,191)
Ending balances	$ 109,872	$ 98,851	$ 74,045

Balance Sheet Components - Inve

Balance Sheet Components - Inventories (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Components	$ 3,825	$ 1,035
Finished goods	120,070	229,352
Total inventories	$ 123,895	$ 230,387

Balance Sheet Components - Prep

Balance Sheet Components - Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
POP displays, net	$ 14,750	$ 22,804
Prepaid expenses	24,204	17,161
Derivative assets	0	10,625
Prepaid marketing	6,074	5,764
Insurance receivable	37,300	0
Other	14,941	9,511
Total prepaid expenses and other current assets	$ 97,269	$ 65,865

Balance Sheet Components - Prop

Balance Sheet Components - Property and Equipment, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 164,339	$ 115,889
Less: Accumulated depreciation and amortization	(59,431)	(39,336)
Property and equipment, net	104,908	76,553
Depreciation expense, net	40,000	36,000
Tooling and manufacturing equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	66,854	60,944
Furniture and office equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	20,942	14,424
Purchased and internally-developed software [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	18,112	12,032
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 58,431	$ 28,489

Balance Sheet Components - Good

Balance Sheet Components - Goodwill Roll forward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Beginning balance	$ 51,036	$ 22,157
Goodwill acquired	0	28,879
Ending balance	$ 51,036	$ 51,036

Balance Sheet Components - Inta

Balance Sheet Components - Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
Gross	$ 31,866	$ 27,370
Accumulated Amortization	(9,510)	(3,789)
Net	22,356	23,581
Intangible Assets, Gross	31,866	31,310
Intangible assets, net	22,356	27,521
Amortization of intangible assets	5,722	2,087	$ 1,702
In process research and development [Member]
Indefinite-lived Intangible Assets [Line Items]
Indefinite-Lived Intangible Assets (Excluding Goodwill)	0	3,940
Developed technology [Member]
Finite-Lived Intangible Assets [Line Items]
Gross	30,588	26,092
Accumulated Amortization	(8,738)	(3,247)
Net	$ 21,850	22,845
Weighted Average Remaining Useful Life (years)	4 years
Trademarks and other [Member]
Finite-Lived Intangible Assets [Line Items]
Gross	$ 1,278	1,278
Accumulated Amortization	(772)	(542)
Net	$ 506	$ 736
Weighted Average Remaining Useful Life (years)	10 months 24 days

Balance Sheet Components - Esti

Balance Sheet Components - Estimated Future Amortization Expense (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
2,018	$ 6,350
2,019	5,570
2,020	4,606
2,021	4,560
2,022	1,270
Net	22,356	$ 23,581
Cost of revenue [Member]
Finite-Lived Intangible Assets [Line Items]
2,018	6,120
2,019	5,340
2,020	4,560
2,021	4,560
2,022	1,270
Net	21,850
Operating expenses [Member]
Finite-Lived Intangible Assets [Line Items]
2,018	230
2,019	230
2,020	46
2,021	0
2,022	0
Net	$ 506

Balance Sheet Components - Accr

Balance Sheet Components - Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Product warranty	$ 87,882	$ 99,923
Accrued manufacturing expense and freight	41,901	75,579
Accrued sales incentives	111,592	74,181
Accrued sales and marketing	44,401	41,948
Accrued research and development	8,983	5,989
Accrued co-op advertising and marketing development funds	30,408	40,002
Employee-related liabilities	33,266	13,934
Sales taxes and VAT payable	21,340	8,891
Inventory received but not billed	10,526	7,363
Accrued legal settlements and fees	36,693	3,963
Derivative liabilities	2,138	3,780
Other	23,007	15,008
Accrued liabilities	$ 452,137	$ 390,561

Balance Sheet Components - Prod

Balance Sheet Components - Product Warranty (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Standard and Extended Product Warranty, Increase (Decrease) [Roll Forward]
Beginning balances	$ 99,923	$ 40,212	$ 20,098
Charged to cost of revenue	53,840	185,434	84,184
Changes in estimate related to pre-existing warranties	11,788	4,072	(8,968)
Settlement of claims	77,669	129,795	55,102
Ending balances	$ 87,882	99,923	$ 40,212
Restatement Adjustment [Member] | Immaterial Reclassification of Cost of Revenue and Settlement of Claims [Member]
Movement in Standard and Extended Product Warranty, Increase (Decrease) [Roll Forward]
Charged to cost of revenue		50,700
Settlement of claims		$ 50,700

Balance Sheet Components - Rese

Balance Sheet Components - Reserve Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
General and administrative [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Legal settlement costs (benefit)	$ 33,300
Product Recall Reserves [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balances	1,253	$ 5,122	$ 22,476
Charged to cost of revenue			(5,755)
Charged to general and administrative			(1,174)
Settlement of claims	(789)	(3,869)	(10,425)
Ending balances	$ 464	$ 1,253	5,122
Charged to revenues			(5,800)
Charges to general and administrative expenses			(1,200)
Product Recall Reserves [Member] | General and administrative [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Legal settlement costs (benefit)			$ (4,400)

Balance Sheet Components - Rest

Balance Sheet Components - Restructuring Reserve Activities (Details) $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	Jan. 31, 2017USD ($)employee	Apr. 01, 2017USD ($)	Dec. 31, 2017USD ($)
Restructuring Reserve [Roll Forward]
Balance at December 31, 2016	$ 0	$ 0	$ 0
Restructuring charges	$ 6,400	$ 6,300	6,375
Cash paid			(4,983)
Other - noncash			(1,392)
Balance at December 31, 2017			$ 0
Employee Severance [Member]
Restructuring Cost and Reserve [Line Items]
Number of positions eliminated in reorganization | employee	110
Percentage of workforce reduction in reorganization	6.00%

Balance Sheet Components - Accu

Balance Sheet Components - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 998,532	$ 981,451	$ 75,262
Other comprehensive income (loss)	969	(1,669)	654
Ending balance	823,963	998,532	981,451
Unrealized Gains on Cash Flow Hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(477)	751
Other comprehensive income (loss) before reclassifications	(19,422)	9,422
Amounts reclassified from AOCI	19,965	(10,650)
Other comprehensive income (loss)	543	(1,228)
Ending balance	66	(477)	751
Currency Translation Adjustments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(314)	(5)
Other comprehensive income (loss) before reclassifications	314	(309)
Amounts reclassified from AOCI	0	0
Other comprehensive income (loss)	314	(309)
Ending balance	0	(314)	(5)
Unrealized Gains (Losses) on Available-for-Sale Investments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(187)	(55)
Other comprehensive income (loss) before reclassifications	125	(126)
Amounts reclassified from AOCI	(13)	(6)
Other comprehensive income (loss)	112	(132)
Ending balance	(75)	(187)	(55)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(978)	691	37
Other comprehensive income (loss) before reclassifications	(18,983)	8,987
Amounts reclassified from AOCI	19,952	(10,656)
Other comprehensive income (loss)	969	(1,669)	654
Ending balance	$ (9)	$ (978)	$ 691

Long-Term Debt (Details)

Long-Term Debt (Details) - USD ($)	May 03, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	May 02, 2017
Line of Credit Facility [Line Items]
Interest expense		$ 1,100,000	$ 500,000	$ 1,000,000
Capitalized issuance costs		600,000
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Credit facility, amount outstanding		36,900,000	$ 38,000,000
2015 Credit Agreement [Member] | Federal Funds Effective Rate [Member]
Line of Credit Facility [Line Items]
Credit facility, basis spread on variable rate	0.50%
2015 Credit Agreement [Member] | Eurodollar [Member]
Line of Credit Facility [Line Items]
Credit facility, basis spread on variable rate	1.00%
2015 Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Debt Instrument, interest rate, stated percentage	1.50%
2015 Credit Agreement [Member] | London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Debt Instrument, interest rate, stated percentage	2.00%
2015 Credit Agreement [Member] | Line of Credit [Member]
Line of Credit Facility [Line Items]
Credit facility, borrowing capacity	$ 100,000,000			250,000,000	$ 250,000,000
Minimum amount of unrestricted cash	200,000,000
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

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) $ in Millions	Jan. 19, 2018USD ($)	May 10, 2016investor	Feb. 16, 2016lawsuit	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Rent expense				$ 40	$ 29.9	$ 8.4
Number of plaintiffs | investor		5
Purchase Commitment [Member]
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Purchase commitment				157.7
Accrued Liabilities				$ 21.8
Pending Litigation [Member] | PurePulse Class Action Lawsuit [Member]
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Purported class action lawsuits filed against Company | lawsuit			2
Subsequent Event [Member]
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Litigation settlement, amount	$ 33.3

Commitments and Contingencies60

Commitments and Contingencies - Future Minimum Payments (Details) $ in Thousands	Dec. 31, 2017USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2,018	$ 40,856
2,019	46,713
2,020	42,942
2,021	41,331
2,022	41,477
Thereafter	58,879
Total	$ 272,198

Stockholders' Equity - Narrativ

Stockholders' Equity - Narrative (Details) $ / shares in Units, $ in Thousands	Jul. 20, 2017USD ($)shares	Nov. 30, 2015USD ($)$ / sharesshares	Jun. 30, 2015USD ($)class$ / sharesshares	May 31, 2015	Dec. 31, 2017USD ($)vote$ / sharesshares	Dec. 31, 2016USD ($)$ / sharesshares	Dec. 31, 2015USD ($)classvote$ / sharesshares	Jul. 19, 2017shares	Jul. 10, 2017$ / sharesshares
Class of Stock [Line Items]
Proceeds from public offerings, net of underwriting discounts and commissions | $		$ 84,400	$ 420,900		$ 0	$ 0	$ 505,275
Preferred stock, authorized (in shares)					10,000,000	10,000,000
Preferred stock, par value (in dollars per share) | $ / shares					$ 0.0001	$ 0.0001
Preferred stock, shares issued (in shares)					0
Preferred stock, shares outstanding (in shares)					0
Number of classes of stock | class			2				2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value (in dollars per share) | $ / shares					$ 5.63	$ 14.06	$ 10.67
Total grant date fair value of options that vested | $					$ 20,200	$ 32,900	$ 18,600
Unrecognized compensation expense related to unvested options, net of estimated forfeitures | $					$ 11,300
Unrecognized compensation expense related to unvested options, estimated weighted average period					1 year 4 months 24 days
Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock converted and reclassified into Class B Common Stock (in shares)			400,000
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to unvested options, net of estimated forfeitures | $					$ 159,000
Unrecognized compensation expense related to unvested options, estimated weighted average period					2 years 2 months 12 days
Amended And Restated 2007 Stock Plan [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years
Vesting percentage					25.00%
Requisite period					1 year
Expiration period					10 years
2015 Plan [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years
Expiration period					10 years
Maximum percentage of purchase price of fair value for employee purchase					100.00%
2015 Plan [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years
Expiration period					10 years
Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock converted and reclassified into Class B Common Stock (in shares)			141,300,000
Common Class A [Member]
Class of Stock [Line Items]
Common stock authorized (in shares)					600,000,000	600,000,000
Common stock, par value (in dollars per share) | $ / shares					$ 0.0001	$ 0.0001
Common stock issued (in shares)					207,453,624	177,212,531
Common stock outstanding (in shares)					207,453,624	177,212,531
Common stock, votes per share | vote					1		1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period					10 years
Number of securities called by each warrant									500,000
Exercise price of warrant (in usd per share) | $ / shares									$ 5.23
Vested shares					100,000
Common Class A [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of exercise price (in dollars per share) | $ / shares					$ 5.71
Common Class A [Member] | 2015 Plan [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual increase in shares reserved for issuance				5.00%
Common stock reserved for future issuance (in shares)					12,500,000
Common Class A [Member] | 2015 Employee Stock Purchase Plan [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual increase in shares reserved for issuance				1.00%
Common stock reserved for future issuance (in shares)					4,500,000
Common stock discount available to employees				15.00%
Common stock, fair value of market value of stock, available to employees, percent				85.00%
Common Class B [Member]
Class of Stock [Line Items]
Common stock authorized (in shares)					350,000,000	350,000,000
Common stock, par value (in dollars per share) | $ / shares					$ 0.0001	$ 0.0001
Common stock issued (in shares)					31,302,898	48,450,746
Common stock outstanding (in shares)					31,302,898	48,450,746
Common stock, votes per share | vote					10		10
Minimum [Member] | Amended And Restated 2007 Stock Plan [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					3 years
Maximum [Member] | Amended And Restated 2007 Stock Plan [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					4 years
Eligible Employees [Member] | 2015 Plan [Member] | Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Underwater options vested								3,700,000
Underwater options tendered, percent								85.00%
Stock issued in exchange (in shares)	1,800,000
Stock issued in exchange | $	$ 8,500
Common Stock [Member]
Class of Stock [Line Items]
Issuance of common stock upon public offering, net of offering costs (in shares)		3,000,000					25,387,500,000
Common Stock [Member] | IPO [Member]
Class of Stock [Line Items]
Issuance of common stock upon public offering, net of offering costs (in shares)			22,400,000
Share price (in dollars per share) | $ / shares			$ 20
Underwriting discounts and commissions | $			$ 26,900
Offering costs incurred | $			$ 5,000
Common Stock [Member] | IPO [Member] | Select Stockholders [Member]
Class of Stock [Line Items]
Shares sold by certain shareholders (in shares)			19,700,000
Common Stock [Member] | Over-Allotment Option [Member]
Class of Stock [Line Items]
Issuance of common stock upon public offering, net of offering costs (in shares)		2,600,000	5,500,000
Share price (in dollars per share) | $ / shares		$ 29
Underwriting discounts and commissions | $		$ 2,600
Offering costs incurred | $		$ 1,200
Common Stock [Member] | Over-Allotment Option [Member] | Select Stockholders [Member]
Class of Stock [Line Items]
Shares sold by certain shareholders (in shares)		16,600,000

Stockholders' Equity - Stock Op

Stockholders' Equity - Stock Option Activity (Details) - 2007 And 2015 Equity Incentive Plan [Member] $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Balance, beginning of period (in shares) | shares	34,454
Granted (in shares) | shares	1,150
Exercised (in shares) | shares	(7,164)
Canceled (in shares) | shares	(7,054)
Balance, end of period (in shares) | shares	21,386
Options exercisable (in shares) | shares	16,827
Options vested and expected to vest (in shares) | shares	21,386
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) | $ / shares	$ 3.85
Granted, weighted average exercise price (in dollars per share) | $ / shares	5.63
Exercised, weighted average exercise price (in dollars per share) | $ / shares	1.22
Canceled, weighted average exercise price (in dollars per share) | $ / shares	9.35
Balance, weighted average exercise price, end of period (in dollars per share) | $ / shares	3.01
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	2.52
Options vested and expected to vest, weighted average exercise price (in dollars per share) | $ / shares	$ 3.01
Options outstanding, weighted average remaining contractual term	5 years 8 months 22 days
Options exercisable, weighted average remaining contractual term	5 years 3 months 22 days
Options vested and expected to vest, weighted average remaining contractual term	5 years 8 months 22 days
Options exercised, aggregate intrinsic value | $
Options outstanding, aggregate intrinsic value | $	64,582
Options exercisable, aggregate intrinsic value | $	57,472
Options vested and expected to vest, aggregate intrinsic value | $	$ 64,582

Stockholders' Equity - Restrict

Stockholders' Equity - Restricted Stock Unit Activity (Details) - Restricted Stock Units (RSUs) [Member] shares in Thousands	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Unvested balance—December 31 (in shares) | shares	11,578
Granted (in shares) | shares	18,912
Vested (in shares) | shares	(6,191)
Forfeited or canceled (in shares) | shares	(5,111)
Unvested balance—December 31 (in shares) | shares	19,188
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Unvested balance—December 31 (in dollars per share) | $ / shares	$ 16.85
Granted (in dollars per share) | $ / shares	6.47
Vested (in dollars per share) | $ / shares	12.57
Forfeited or canceled (in dollars per share) | $ / shares	12.61
Unvested balance—December 31 (in dollars per share) | $ / shares	$ 9.13

Stockholders' Equity - Stock Co

Stockholders' Equity - Stock Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 91,581	$ 79,432	$ 41,024
Cost of revenue [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	5,312	4,797	4,739
Research and development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	54,123	47,207	18,251
Sales and marketing [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	14,959	11,575	7,419
General and administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 17,187	$ 15,853	$ 10,615

Stockholders' Equity - Valuatio

Stockholders' Equity - Valuation Assumptions (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	6 years 3 months	6 years 3 months	6 years 3 months
Volatility rate	32.20%	40.70%
Minimum volatility rate			52.10%
Maximum volatility rate			56.90%
Risk-free interest rate	2.10%	1.60%
Minimum risk-free interest rate			1.50%
Maximum risk-free interest rate			1.90%
Dividend yield rate	0.00%	0.00%	0.00%
Warrant [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	6 years 3 months
Volatility rate	32.00%	0.00%	0.00%
Risk-free interest rate	2.07%	0.00%	0.00%
Dividend yield rate	0.00%	0.00%	0.00%
Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	6 months	6 months
Minimum volatility rate	27.70%	30.10%	27.70%
Maximum volatility rate	31.30%	39.00%	35.00%
Risk-free interest rate			0.30%
Minimum risk-free interest rate	1.00%	0.40%
Maximum risk-free interest rate	1.40%	0.60%
Dividend yield rate	0.00%	0.00%	0.00%
Employee Stock Purchase Plan [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term			6 months
Employee Stock Purchase Plan [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term			10 months 24 days

Income Taxes - Components of In

Income Taxes - Components of Income (Loss) Before Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
United States	$ (158,187)	$ 5,577	$ 286,380
Foreign	(36,457)	(114,872)	1,569
Income (loss) before income taxes	$ (194,644)	$ (109,295)	$ 287,949

Income Taxes - Components of 67

Income Taxes - Components of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal	$ (87,961)	$ 78,782	$ 140,396
State	(8,429)	9,878	13,307
Foreign	5,032	5,256	1,107
Total current	(91,358)	93,916	154,810
Deferred:
Federal	154,817	(87,584)	(33,421)
State	18,902	(11,622)	(8,941)
Foreign	187	(1,228)	(176)
Total deferred	173,906	(100,434)	(42,538)
Total income tax expense (benefit)	$ 82,548	$ (6,518)	$ 112,272

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Effective Tax to Statutory Federal Rate (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Tax at federal statutory rate	35.00%	35.00%	35.00%
State taxes, net of federal effect	(5.40%)	4.30%	1.50%
Foreign rate differential	(9.30%)	(38.90%)	(0.80%)
Tax credits	4.10%	9.00%	(2.00%)
Domestic production activities deduction	(3.50%)	5.00%	(3.30%)
Stock-based compensation(1)	(5.30%)	(4.60%)	1.70%
Change in prior year reserves	(2.00%)	1.90%	0.00%
Out-of-period adjustment	0.00%	(2.80%)	0.00%
Warrant fair value adjustment	0.00%	0.00%	6.90%
Change in valuation allowance	(35.20%)	0.00%	0.00%
Effect of change in tax rate due to Tax Act	(23.40%)	0.00%	0.00%
Other	2.60%	(2.90%)	0.00%
Effective tax rate	(42.40%)	6.00%	39.00%

Income Taxes - Tax Effects of T

Income Taxes - Tax Effects of Temporary Differences for Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Net operating losses and credits	$ 23,338	$ 9,446
Fixed assets and intangible assets	10,625	16,272
Accruals and reserves	49,886	112,915
Stock-based compensation	12,154	17,864
Inventory	4,345	8,513
Other	3,325	11,134
Total deferred tax assets	103,673	176,144
Less: valuation allowance	(99,570)	0
Deferred tax assets, net of valuation allowance	4,103	176,144
Other	(369)	(353)
Total deferred tax liabilities	(369)	(353)
Net deferred tax assets	$ 3,734	$ 175,791

Income Taxes - Reconciliation70

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	$ 35,584	$ 23,518	$ 10,594
Reductions based on tax positions related to prior year	(6,335)	(2,100)	(18)
Additions based on tax positions related to prior year	108	2,809	0
Additions based on tax positions related to current year	9,289	11,357	12,942
Reductions due to tax authorities’ settlements	(8,603)	0	0
Reductions due to expiration of statutes of limitation	(105)	0	0
Balance at end of year	$ 29,938	$ 35,584	$ 23,518

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Thousands	Dec. 22, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Examination [Line Items]
Income tax expense (benefit)		$ 82,548	$ (6,518)	$ 112,272
Effective income tax rate		(42.40%)	6.00%	39.00%
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Deferred Tax Liability, Provisional Income Tax (Expense) Benefit	$ 45,500
Valuation allowance		$ 99,570	$ 0
Federal operating loss carryforwards			2,200
Unrecognized tax benefits		29,938	35,584	$ 23,518	$ 10,594
Unrecognized that would affect the effective tax rate if recognized		24,800
Unrecognized tax benefits, interest on income taxes expense		$ 2,400	2,200
Domestic Tax Authority [Member]
Income Tax Examination [Line Items]
Deferred tax assets, research credit carryforwards			1,600
California [Member]
Income Tax Examination [Line Items]
State net operating loss carryforwards			25,800
California [Member] | State and Local Jurisdiction [Member]
Income Tax Examination [Line Items]
Deferred tax assets, research credit carryforwards			25,600
Tax credit carryforwards			300
Other States [Member]
Income Tax Examination [Line Items]
State net operating loss carryforwards			27,200
Massachusetts [Member] | State and Local Jurisdiction [Member]
Income Tax Examination [Line Items]
Deferred tax assets, research credit carryforwards			1,200
United Kingdom [Member]
Income Tax Examination [Line Items]
Foreign net operating loss carryforwards			$ 15,600

Net Income (Loss) per Share A72

Net Income (Loss) per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jul. 01, 2017	Apr. 01, 2017	Dec. 31, 2016	Oct. 01, 2016	Jul. 02, 2016	Apr. 02, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income (loss)	$ (45,470)	$ (113,403)	$ (58,240)	$ (60,079)	$ (146,273)	$ 26,120	$ 6,341	$ 11,035	$ (277,192)	$ (102,777)	$ 175,677
Less: noncumulative dividends to preferred stockholders									0	0	(2,526)
Less: undistributed earnings to participating securities									0	0	(59,133)
Net income (loss) attributable to common stockholders—basic									(277,192)	(102,777)	114,018
Add: adjustments to undistributed earnings to participating securities									0	0	8,821
Net Income (Loss) Available to Common Stockholders, Diluted									$ (277,192)	$ (102,777)	$ 122,839
Denominator:
Weighted-average shares of common stock— basic for Class A and Class B (in shares)									232,032	220,405	129,886
Effect of dilutive securities (in shares)									0	0	34,327
Weighted-average shares of common stock—diluted for Class A and Class B (in shares)									232,032	220,405	164,213
Net income (loss) per share attributable to common stockholders:
Basic (in dollars per share)	$ (0.19)	$ (0.48)	$ (0.25)	$ (0.27)	$ (0.65)	$ 0.12	$ 0.03	$ 0.05	$ (1.19)	$ (0.47)	$ 0.88
Diluted (in dollars per share)	$ (0.19)	$ (0.48)	$ (0.25)	$ (0.27)	$ (0.65)	$ 0.11	$ 0.03	$ 0.05	$ (1.19)	$ (0.47)	$ 0.75

Net Income (Loss) per Share A73

Net Income (Loss) per Share Attributable to Common Stockholders - Antidilutive Securities (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	27,961	46,032	67,961
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)	162	0	0
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	17,469	34,454	445
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)	10,030	11,578	692
Warrant [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)	216	0	0
Diluted Common Stock Subject to Vesting [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)	84	0	0
Redeemable convertible preferred stock and stock options to purchase common stock [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)	0	0	65,903
Redeemable Convertible Preferred Stock [Member] | Preferred 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)	0	0	921

Significant Customer Informat74

Significant Customer Information and Other Information (Details) - USD ($) $ in Thousands	Jul. 01, 2017	Sep. 30, 2017	Dec. 31, 2017	Sep. 30, 2017	Jul. 01, 2017	Apr. 01, 2017	Dec. 31, 2016	Oct. 01, 2016	Jul. 02, 2016	Apr. 02, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Concentration Risk [Line Items]
Revenue			$ 570,756	$ 392,522	$ 353,299	$ 298,942	$ 573,775	$ 503,802	$ 586,528	$ 505,356	$ 1,615,519	$ 2,169,461	$ 1,857,998
Geographic Concentration Risk [Member] | Non-US [Member]
Concentration Risk [Line Items]
Long-lived assets including property and equipment			$ 30,000				$ 30,100				$ 30,000	$ 30,100
Revenue [Member] | Customer Concentration Risk [Member]
Concentration Risk [Line Items]
Concentration risk, percentage		11.00%
Revenue [Member] | Customer Concentration Risk [Member] | Customer C [Member]
Concentration Risk [Line Items]
Concentration risk, percentage											13.00%	14.00%	14.00%
Revenue [Member] | Customer Concentration Risk [Member] | Customer B [Member]
Concentration Risk [Line Items]
Concentration risk, percentage												10.00%	14.00%
Revenue [Member] | Customer Concentration Risk [Member] | Customer A [Member]
Concentration Risk [Line Items]
Concentration risk, percentage												14.00%	15.00%
Revenue [Member] | Geographic Concentration Risk [Member]
Concentration Risk [Line Items]
Revenue											$ 1,615,519	$ 2,169,461	$ 1,857,998
Revenue [Member] | Geographic Concentration Risk [Member] | United States [Member]
Concentration Risk [Line Items]
Revenue											944,052	1,539,600	1,381,152
Revenue [Member] | Geographic Concentration Risk [Member] | Americas excluding United States [Member]
Concentration Risk [Line Items]
Revenue											116,330	110,111	92,252
Revenue [Member] | Geographic Concentration Risk [Member] | Europe, Middle East, and Africa [Member]
Concentration Risk [Line Items]
Revenue											440,135	389,154	208,767
Revenue [Member] | Geographic Concentration Risk [Member] | APAC [Member]
Concentration Risk [Line Items]
Revenue											$ 115,002	$ 130,596	$ 175,827
Accounts Receivable [Member] | Customer Concentration Risk [Member]
Concentration Risk [Line Items]
Concentration risk, percentage	19.00%
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Customer C [Member]
Concentration Risk [Line Items]
Concentration risk, percentage											17.00%	19.00%
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Customer B [Member]
Concentration Risk [Line Items]
Concentration risk, percentage											13.00%
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Customer E [Member]
Concentration Risk [Line Items]
Concentration risk, percentage											11.00%
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Customer A [Member]
Concentration Risk [Line Items]
Concentration risk, percentage												16.00%
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Customer D [Member]
Concentration Risk [Line Items]
Concentration risk, percentage												12.00%

Acquisitions - Narrative (Detai

Acquisitions - Narrative (Details) - USD ($)	1 Months Ended			12 Months Ended
	Dec. 31, 2016	May 31, 2016	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 13, 2015
Business Acquisition [Line Items]
Goodwill	$ 51,036,000			$ 51,036,000	$ 51,036,000	$ 22,157,000
Change in contingent consideration				$ 0	0	$ (7,704,000)
Shares issued for contingent considerations (in shares)						0
Pebble Industries, Inc [Member]
Business Acquisition [Line Items]
Cash paid for acquisition	23,400,000
Goodwill	14,400,000				14,400,000
Assumed liabilities	600,000				600,000
Consideration held as security for indemnifications obligations	3,500,000
Pebble Industries, Inc [Member] | In process research and development [Member]
Business Acquisition [Line Items]
Intangibles acquired	$ 9,600,000				9,600,000
Amortization period	5 years
Vector Watch S.R.L. [Member]
Business Acquisition [Line Items]
Cash paid for acquisition	$ 15,000,000
Goodwill	11,400,000				11,400,000
Assumed liabilities	300,000				300,000
Consideration held as security for indemnifications obligations	2,300,000
Vector Watch S.R.L. [Member] | In process research and development [Member]
Business Acquisition [Line Items]
Intangibles acquired	$ 3,900,000				3,900,000
Amortization period	2 years 6 months
Coin, Inc [Member]
Business Acquisition [Line Items]
Cash paid for acquisition		$ 7,000,000
Goodwill		3,100,000
Coin, Inc [Member] | In process research and development [Member]
Business Acquisition [Line Items]
Intangibles acquired		$ 3,900,000
FitStar, Inc [Member]
Business Acquisition [Line Items]
Cash paid for acquisition			$ 11,500,000
Goodwill							$ 22,157,000
Assumed liabilities							3,552,000
Total consideration			$ 32,500,000
Shares repurchased (in shares)			24,949
Contingent consideration assumed in acquisition			$ 7,700,000
Contingent consideration liability			7,700,000			$ 0
Payment of contingent consideration						0
FitStar, Inc [Member] | Post Acquisition Compensation Expense [Member]
Business Acquisition [Line Items]
Business combination, contingent cash payments			1,200,000
FitStar, Inc [Member] | Other expense, net [Member]
Business Acquisition [Line Items]
Change in contingent consideration						7,700,000
FitStar, Inc [Member] | General and administrative [Member] | Post Acquisition Compensation Expense [Member]
Business Acquisition [Line Items]
Transaction costs					$ 1,300,000	$ 300,000
FitStar, Inc [Member] | Common Stock [Member]
Business Acquisition [Line Items]
Shares issued, amount			$ 13,300,000
Shares issued (in shares)			1,059,688
FitStar, Inc [Member] | Common Stock [Member] | Post Acquisition Compensation Expense [Member]
Business Acquisition [Line Items]
Shares issued (in shares)			308,216
Shares repurchased (in shares)			24,948
Shares repurchased, amount			$ 4,200,000
FitStar, Inc [Member] | Developed technology [Member]
Business Acquisition [Line Items]
Intangibles acquired							12,640,000
Amortization period			7 years
FitStar, Inc [Member] | Customer relationships [Member]
Business Acquisition [Line Items]
Intangibles acquired							128,000
Amortization period			1 year 3 months
FitStar, Inc [Member] | Trademarks [Member]
Business Acquisition [Line Items]
Intangibles acquired							$ 1,150,000
Amortization period			5 years

Acquisitions - Assets Acquired

Acquisitions - Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 13, 2015
Business Acquisition [Line Items]
Goodwill	$ 51,036	$ 51,036	$ 22,157
FitStar, Inc [Member]
Business Acquisition [Line Items]
Goodwill				$ 22,157
Assumed liabilities, net of assets				(3,552)
Total				32,523
FitStar, Inc [Member] | Developed and core technology [Member]
Business Acquisition [Line Items]
Intangibles acquired				12,640
FitStar, Inc [Member] | Customer relationships [Member]
Business Acquisition [Line Items]
Intangibles acquired				128
FitStar, Inc [Member] | Trademarks [Member]
Business Acquisition [Line Items]
Intangibles acquired				$ 1,150

Selected Unaudited Quarterly 77

Selected Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 22, 2017	Sep. 30, 2017	Jan. 31, 2017	Dec. 31, 2017	Sep. 30, 2017	Jul. 01, 2017	Apr. 01, 2017	Dec. 31, 2016	Oct. 01, 2016	Jul. 02, 2016	Apr. 02, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue				$ 570,756	$ 392,522	$ 353,299	$ 298,942	$ 573,775	$ 503,802	$ 586,528	$ 505,356	$ 1,615,519	$ 2,169,461	$ 1,857,998
Gross profit				248,597	174,760	149,245	118,299	126,502	240,658	244,969	233,755	690,901	845,884	901,063
Net loss				$ (45,470)	$ (113,403)	$ (58,240)	$ (60,079)	$ (146,273)	$ 26,120	$ 6,341	$ 11,035	$ (277,192)	$ (102,777)	$ 175,677
Net income (loss) per share attributable to common stockholders—basic (in dollars per share)				$ (0.19)	$ (0.48)	$ (0.25)	$ (0.27)	$ (0.65)	$ 0.12	$ 0.03	$ 0.05	$ (1.19)	$ (0.47)	$ 0.88
Net income (loss) per share attributable to common stockholders—diluted (in dollars per share)				$ (0.19)	$ (0.48)	$ (0.25)	$ (0.27)	$ (0.65)	$ 0.11	$ 0.03	$ 0.05	$ (1.19)	$ (0.47)	$ 0.75
Restructuring charges			$ 6,400				$ 6,300					$ 6,375
Accrued Income Taxes												(82,548)	$ 6,518	$ (112,272)
Cost of revenue												924,618	1,323,577	956,935
Bad debt expense												$ 7,893	$ 339	1,115
Valuation allowance recorded in the period					$ (111,400)
Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Change in Tax Rate, Deferred Tax Liability, Provisional Income Tax (Expense) Benefit	$ 45,500
Wynit Distribution [Member]
Revenue					8,100
Cost of revenue		$ 9,000			35,800
Bad debt expense		7,600			30,300
Cost of revenue associated with shipments		$ 1,400			$ 5,500
Accounting Standards Update 2016-09 [Member]
Accrued Income Taxes														$ 2,800

Subsequent Events Subsequent Ev

Subsequent Events Subsequent Events (Details) - USD ($) $ in Thousands	Jan. 19, 2018	Feb. 28, 2018	Dec. 31, 2017	Dec. 31, 2016
Subsequent Event [Line Items]
Insurance receivable			$ 37,300	$ 0
Subsequent Event [Member]
Subsequent Event [Line Items]
Litigation settlement, amount	$ 33,300
Litigation settlement expense	$ 1,000
General and administrative [Member]
Subsequent Event [Line Items]
Litigation settlement expense			33,300
Prepaid expenses and other current assets [Member]
Subsequent Event [Line Items]
Insurance receivable			$ 32,300
Twine Health, Inc. [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Consideration transferred		$ 17,500