Fitbit 10-K 1 Mar 17 2016 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2016	Feb. 17, 2017	Jul. 01, 2016
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, 2016
Document Fiscal Year Focus	2,016
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			$ 2
Common Class A [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		177,757,412
Common Class B [Member]
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		48,450,746

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 301,320	$ 535,846
Marketable securities	404,693	128,632
Accounts receivable, net	477,825	469,260
Inventories	230,387	178,146
Prepaid expenses and other current assets	66,346	43,530
Total current assets	1,480,571	1,355,414
Property and equipment, net	76,553	44,501
Goodwill	51,036	22,157
Intangible assets, net	27,521	12,216
Deferred tax assets	174,097	83,020
Other assets	10,448	1,758
Total assets	1,820,226	1,519,066
Current liabilities:
Accounts payable	313,773	260,842
Accrued liabilities	390,561	200,099
Deferred revenue	49,904	44,448
Income taxes payable	7,694	2,868
Total current liabilities	761,932	508,257
Other liabilities	59,762	29,358
Total liabilities	821,694	537,615
Commitments and contingencies (Note 7)
Stockholders’ equity:
Preferred stock, $0.0001 par value, 10,000,000 shares authorized	0	0
Additional paid-in capital	859,345	737,820
Accumulated other comprehensive income (loss)	(978)	691
Retained earnings	140,142	242,919
Total stockholders’ equity	998,532	981,451
Total liabilities and stockholders’ equity	1,820,226	1,519,066
Common Class A [Member]
Stockholders’ equity:
Common stock	18	10
Common Class B [Member]
Stockholders’ equity:
Common stock	$ 5	$ 11

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2016	Dec. 31, 2015
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, authorized (in shares)	10,000,000	10,000,000
Common stock issued (in shares)		40,875,583
Common stock outstanding (in shares)		40,875,583
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)	177,212,531	99,416,351
Common stock outstanding (in shares)	177,212,531	99,416,351
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)	48,450,746	115,365,222
Common stock outstanding (in shares)	48,450,746	115,365,222

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Revenue	$ 2,169,461	$ 1,857,998	$ 745,433
Cost of revenue	1,323,577	956,935	387,776
Gross profit	845,884	901,063	357,657
Operating expenses:
Research and development	320,191	150,035	54,167
Sales and marketing	491,255	332,741	112,005
General and administrative	146,903	77,793	33,556
Change in contingent consideration	0	(7,704)	0
Total operating expenses	958,349	552,865	199,728
Operating income (loss)	(112,465)	348,198	157,929
Interest income (expense), net	3,156	(1,019)	(2,222)
Other income (expense), net	14	(59,230)	(15,934)
Income (loss) before income taxes	(109,295)	287,949	139,773
Income tax expense (benefit)	(6,518)	112,272	7,996
Net income (loss)	(102,777)	175,677	131,777
Less: noncumulative dividends to preferred stockholders	0	(2,526)	(5,326)
Less: undistributed earnings to participating securities	0	(59,133)	(98,103)
Net income (loss) attributable to common stockholders—basic	(102,777)	114,018	28,348
Add: adjustments for undistributed earnings to participating securities	0	8,821	10,175
Net income (loss) attributable to common stockholders—diluted	$ (102,777)	$ 122,839	$ 38,523
Net income (loss) per share attributable to common stockholders:
Basic (in dollars per share)	$ (0.47)	$ 0.88	$ 0.70
Diluted (in dollars per share)	$ (0.47)	$ 0.75	$ 0.63
Shares used to compute net income (loss) per share attributable to common stockholders:
Basic (in shares)	220,405	129,886	40,351
Diluted (in shares)	220,405	164,213	61,179

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ (102,777)	$ 175,677	$ 131,777
Cash flow hedges:
Change in unrealized gain on cash flow hedges, net of tax expense (benefit) of ($1,251), $1,509, and $ —, respectively	9,422	1,276	0
Less reclassification for realized net gains included in net income (loss), net of tax expense of $509, $759, and $ —, respectively	(10,650)	(525)	0
Net change, net of tax	(1,228)	751	0
Available-for-sale investments:
Change in unrealized loss on investments	(126)	(63)	0
Less reclassification for realized net gains included in net income (loss)	(6)	8	0
Net change, net of tax	(132)	(55)	0
Change in foreign currency translation adjustment, net of tax	(309)	(42)	37
Comprehensive income (loss)	$ (104,446)	$ 176,331	$ 131,814

Consolidated Statements of Com6

Consolidated Statements of Comprehensive Income (Loss) (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Change in unrealized gain on cash flow hedges, tax	$ (1,251)	$ 1,509	$ 0
Reclassification for realized net gains included in net income, tax	$ 509	$ 759	$ 0

Consolidated Statements of Rede

Consolidated Statements of Redeemable Convertible Preferred Stock and 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, 2013	139,503,915
Beginning balance at Dec. 31, 2013	$ 66,236
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)	347,568
Issuance of redeemable convertible preferred stock upon exercise of redeemable convertible preferred stock warrants	$ 1,578
Ending balance (in shares) at Dec. 31, 2014	139,851,483
Ending balance at Dec. 31, 2014	$ 67,814
Beginning balance (in shares) at Dec. 31, 2013		40,140,159
Beginning balance at Dec. 31, 2013	(63,466)	$ 4	$ 1,065	$ 0	$ (64,535)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock upon exercise of stock options (in shares)		735,424
Issuance of common stock upon exercise of stock options	97		97
Stock-based compensation expense	6,804		6,804
Excess tax benefit from stock-based compensation	13		13
Net income (loss)	131,777				131,777
Other comprehensive Income (loss)	37			37
Ending balance (in shares) at Dec. 31, 2014		40,875,583
Ending balance at Dec. 31, 2014	$ 75,262	$ 4	7,979	37	67,242
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
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
Excess tax benefit from stock-based compensation	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)
Excess tax benefit from stock-based compensation	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

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash Flows from Operating Activities
Net income (loss)	$ (102,777)	$ 175,677	$ 131,777
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Provision for doubtful accounts	339	1,115	864
Provision for excess and obsolete inventory	4,993	5,060	2,964
Depreciation	36,046	19,405	6,131
Amortization of intangible assets	2,087	1,702	0
Accelerated depreciation of property and equipment	19,805	1,206	1,004
Amortization of issuance costs and discount on debt	466	961	795
Stock-based compensation	79,432	41,024	6,804
Deferred income taxes	(98,734)	(42,538)	(42,001)
Excess of tax benefit from stock-based compensation	(29,186)	(32,100)	(13)
Revaluation of redeemable convertible preferred stock warrant liability	0	56,655	13,272
Change in contingent consideration	0	(7,704)	0
Other	(423)	(263)	0
Changes in operating assets and liabilities, net of acquisitions:
Accounts receivable	(8,701)	(231,100)	(158,788)
Inventories	(61,975)	(68,108)	(61,595)
Prepaid expenses and other assets	(37,876)	(29,215)	(9,679)
Fitbit Force recall reserve	(3,869)	(17,354)	(60,462)
Accounts payable	45,654	56,759	123,761
Accrued liabilities and other liabilities	213,361	138,748	47,733
Deferred revenue	5,456	34,891	3,403
Income taxes payable	45,436	4,336	12,804
Net cash provided by operating activities	109,534	109,157	18,774
Cash Flows from Investing Activities
Purchase of property and equipment	(78,640)	(30,566)	(26,495)
Purchase of marketable securities	(638,055)	(230,935)	0
Sales of marketable securities	46,511	58,011	0
Maturities of marketable securities	315,774	44,500	0
Acquisitions, net of cash acquired	(38,256)	(11,037)	0
Change in restricted cash	0	0	2,310
Net cash used in investing activities	(392,666)	(170,027)	(24,185)
Cash Flows from Financing Activities
Payment of offering costs	(1,236)	(5,089)	0
Proceeds from issuance of common stock	25,969	4,018	97
Excess of tax benefit from stock-based compensation	29,186	32,100	13
Taxes paid related to net share settlement of restricted stock units	(4,939)	0	0
Proceeds from public offerings, net of underwriting discounts and commissions	0	505,275	0
Proceeds from issuance of debt and revolving credit facility	0	160,000	163,000
Repayment of debt	0	(294,503)	(41,346)
Payment of issuance costs	0	(748)	(2,575)
Proceeds from exercise of redeemable convertible preferred stock warrants	0	0	75
Net cash provided by financing activities	48,980	401,053	119,264
Net increase (decrease) in cash and cash equivalents	(234,152)	340,183	113,853
Effect of exchange rate on cash and cash equivalents	(374)	37	45
Cash and cash equivalents at beginning of period	535,846	195,626	81,728
Cash and cash equivalents at end of period	301,320	535,846	195,626
Supplemental Disclosure
Cash paid for interest	624	1,157	835
Cash paid for income taxes	34,014	150,923	34,616
Supplemental Disclosure of Non-Cash Investing and Financing Activity
Purchase of property and equipment included in accounts payable and accrued liabilities	19,778	10,534	2,492
Conversion of redeemable convertible preferred stock into Class B common stock	0	124,492	0
Reclassification of redeemable convertible preferred stock warrant liability to additional paid in capital	0	15,774	0
Issuance of redeemable convertible preferred stock upon net exercise of redeemable convertible preferred stock warrants	0	56,678	0
Issuance of redeemable convertible preferred stock warrants in connection with debt financing	0	0	0
Deferred offering costs included in accounts payable and accruals	0	1,080	0
Issuance of common stock in connection with acquisitions	0	13,317	0
Contingent consideration related to acquisitions	$ 0	$ (7,704)	$ 0

Business Overview and Basis of

Business Overview and Basis of Presentation	12 Months Ended
	Dec. 31, 2016
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 transforming the way millions of people around the world achieve their health and fitness goals. The Fitbit platform combines connected health and fitness devices with software and services, including an online dashboard and mobile apps, data analytics, motivational and social tools, personalized insights, and virtual coaching through customized fitness plans and interactive workouts. The Company sells devices through diversified sales channels that include distributors, retailers, a corporate wellness offering, and Fitbit.com. The Company was incorporated in Delaware in 2007. 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. The Company did not adjust operating results for quarters prior to 2016. Use of Estimates The preparation of consolidated financial statements in accordance with generally accepted accounting principles in the United States of America (“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 estimates and assumptions made by management 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 reporting segments, the recoverability of intangible assets and their useful lives, the valuations of deferred income tax assets and uncertain tax positions, and accruals for the Fitbit Force recall. 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 Adjustment During the first quarter of 2016, the Company identified an error, which resulted in an understatement of income tax expense by $3.0 million for the year ended December 31, 2015. The Company recorded an out-of-period adjustment to correct the error in the quarter ended April 2, 2016. The Company assessed the materiality of this error and concluded the error was not material to the 2015 and 2016 consolidated financial statements, and therefore, the Company recorded the correction in the first quarter of 2016. 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. The Company recorded $15.0 million of revenue during 2016 related to the transaction. 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 three years , and will be expensed as advertising services are received.

Significant Accounting Policies

Significant Accounting Policies	12 Months Ended
	Dec. 31, 2016
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 certain of the Company’s 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 $ 11.7 million , $(1.3) million , and $(2.7) million for 2016 , 2015 , and 2014 , respectively. The Company translates the assets and liabilities of its non-U.S. dollar functional currency subsidiaries into U.S. dollars using exchange rates in effect at the end of each period. Revenue and expenses for these subsidiaries are translated using rates that approximate those in effect during the related period. Gains and losses from translations are recognized in foreign currency translation included in accumulated other comprehensive income (loss). 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, accounts receivables, and derivative instruments. 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 are 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 market on a first-in, first-out basis. 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 $6.3 million as of December 31, 2016 and were immaterial as of December 31, 2015. 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 period between achieving technological feasibility and 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, 2016, and 2015, and the fair value of the reporting unit exceeded the carrying value. 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 connected health and fitness devices and accessories. 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 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 FitStar 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 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 FitStar subscription-based services is based on the price charged when sold separately. Amounts allocated to the delivered health and fitness 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. 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, 2016 , actual returns have primarily been open-box 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. 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 2016 , 2015 , and 2014 were $316.8 million , $237.0 million , and $71.9 million , respectively. Co-op advertising costs are recorded as a reduction to revenue, and for 2016 , 2015 , and 2014 were $52.9 million , $38.3 million , and $12.7 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 the product will operate under normal use for a period of one year from date of original purchase except in the European Union where the Company provides a two -year warranty. The Company has the obligation, at its option, to either repair or replace the defective product. At the time revenue is recognized, an estimate of future warranty costs is recorded as a component of cost of revenues. Factors that affect the warranty obligation include product failure rates, service delivery costs incurred in correcting the product failures, and warranty policies. The warranty obligation does not consider historical experience of the Fitbit Force product as a separate reserve has been established for the Fitbit Force recall. The Company’s products are manufactured by third-party contract manufacturers, and in certain cases, the Company may have recourse to such third-party contract manufacturers. 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, net of estimated forfeitures, 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 and shares issued under our 2015 Employee Stock Purchase Plan, or 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. The Company recognizes a benefit from stock-based compensation in equity to the extent that an incremental tax benefit is realized by following the ordering provisions of the tax laws. 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 its expense 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 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 the Company’s 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 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 EPS, net income attributable to common stockholders for basic EPS 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 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. In 2016, basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period without consideration of common stock equivalents or the participation rights of the preferred stock as they do not share in losses. As the Company was in a net loss position in 2016, basic net loss per share attributable to common stockholders is 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 In May 2014, the Financial Accounting Standards Board, or FASB, issued ASU 2014-09 (ASC 606), Revenue from Contracts with Customers , 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. This 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. ASU 2014-09 will become effective for the Company on January 1, 2018 and can be adopted either retrospectively to each prior reporting period presented or as a cumulative effect adjustment as of the date of adoption. In April 2016, the FASB issued ASU 2016-10, which clarifies guidance on identifying performance obligations and licensing implementation. The Company currently expects to adopt the revenue standards as of January 1, 2018, utilizing the modified retrospective transition method. Although the new standard may, in certain circumstances, impact the timing of when revenue is recognized for product shipped, and the timing and classification of certain sales incentives, the Company believes the new guidance is consistent with its current revenue policy. Therefore, the Company does not currently expect adoption to have a material impact on the consolidated financial statements. The Company is continuing to evaluate the impact of the adoption and preliminary assessments are subject to change. In February 2016, the FASB issued ASU 2016-02, Leases . This ASU requires lease assets and lease liabilities arising from leases, including operating leases, to be recognized 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; however, the Company does not expect th

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2016
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, 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 December 31, 2015 Level 1 Level 2 Level 3 Total Assets: Money market funds $ 248,128 $ — $ — $ 248,128 U.S. government agencies — 113,314 — 113,314 Corporate debt securities — 193,964 — 193,964 Derivative assets — 6,002 — 6,002 Total $ 248,128 $ 313,280 $ — $ 561,408 Liabilities: Derivative liabilities $ — $ 2,640 $ — $ 2,640 The fair value of the Company’s Level 1 financial instruments is based on quoted market prices in active markets for identical instruments. The fair value of the Company’s Level 2 financial instruments is based on observable inputs other than quoted prices in active markets for identical assets and liabilities, quoted prices for identical or similar assets or liabilities in inactive markets, or other inputs that are observable or can be corroborated by observable market data. In addition, Level 2 assets and liabilities include derivative financial instruments associated with hedging activity, which are further discussed in Note 3. Derivative financial instruments are initially measured at fair value on the contract date and are subsequently remeasured to fair value at each reporting date using inputs such as spot rates, forward rates, and discount rates. There is not an active market for each hedge contract, but the inputs used to calculate the value of the instruments are tied to active markets. The unexercised warrants to purchase redeemable convertible preferred stock were converted into warrants to purchase shares of Class B common stock upon the closing of the IPO in 2015. As a result, the Company revalued and reclassified the redeemable convertible preferred stock liability to additional paid-in capital upon the closing of the IPO. The fair value of the warrant liability was calculated using an option-pricing model. The following table sets forth a summary of the changes in the fair value of the redeemable convertible preferred stock warrant liability (in thousands): Balance at December 31, 2014 $ 15,797 Change in fair value 56,655 Settlement of warrant liability upon exercise (56,678 ) Reclassification of unexercised warrants to additional paid in capital upon the IPO (15,774 ) Balance at December 31, 2015 $ — The Company’s acquisition-related contingent consideration was determined using the Monte Carlo simulation method. The increases or decreases in the fair value of the contingent consideration payable could result from changes in the anticipated fair value of the Company’s common stock, stock price volatility, and probability of various market-based scenarios. As the fair value measure is based on significant inputs that are not observable in the market, they are categorized as Level 3. For additional information on the contingent consideration, see Note 12. The following table sets forth a summary of the changes in the fair value of the acquisition-related contingent consideration (in thousands): Balance at December 31, 2014 $ — Addition from acquisition 7,704 Change in fair value of contingent consideration (7,704 ) Balance at December 31, 2015 $ — There was no activity relating to Level 3 financial assets or liabilities during 2016, and there were no Level 3 financial assets or liabilities as of December 31, 2016. There have been no transfers between fair value measurement levels during 2016 , 2015 , and 2014 .

Financial Instruments

Financial Instruments	12 Months Ended
	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Financial Instruments	Financial Instruments Cash, Cash Equivalents, and Marketable Securities 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 sets forth the cash, cash equivalents, and marketable securities as of December 31, 2015 (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Cash and Cash Equivalents Marketable Securities Cash $ 109,072 $ — $ — $ 109,072 $ 109,072 $ — Money market funds 248,128 — — 248,128 248,128 — U.S. government agencies 113,315 3 (4 ) 113,314 63,464 49,850 Corporate debt securities 194,018 1 (55 ) 193,964 115,182 78,782 Total $ 664,533 $ 4 $ (59 ) $ 664,478 $ 535,846 $ 128,632 The gross unrealized gains or losses on marketable securities as of December 31, 2016 and December 31, 2015 were not material. There were no available-for-sale investments as of December 31, 2016 and December 31, 2015 that have been in a continuous unrealized loss position for greater than twelve months. The following table classifies marketable securities by contractual maturities (in thousands): December 31, 2016 December 31, 2015 Due in one year $ 355,152 $ 128,632 Due in one to two years 49,541 — Total $ 404,693 $ 128,632 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 Beginning in the third quarter of 2015, 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. All elements of the hedged transaction are included in the effectiveness assessment. 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 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 outstanding contracts with a total notional amount of $20.0 million and $20.9 million in cash flow hedges for forecasted revenue and expense transactions, respectively, as of December 31, 2016 , and $254.1 million in cash flow hedges for forecasted revenue transactions as of December 31, 2015 . 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 $177.0 million and $104.8 million as of December 31, 2016 and December 31, 2015 , 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, 2016 December 31, 2015 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 $ — $ 3,116 $ — Cash flow designated hedges Accrued liabilities — 1,428 — 1,327 Hedges not designated Prepaid expense and other current assets 9,812 — 2,886 — Hedges not designated Accrued liabilities — 2,352 — 1,313 Total fair value of derivative instruments $ 10,625 $ 3,780 $ 6,002 $ 2,640 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 2016 2015 2014 Foreign exchange cash flow hedges: Gain (loss) recognized in OCI—effective portion $ 8,171 $ 2,785 $ — Gain (loss) reclassified from OCI into income—effective portion Revenue 10,153 2,183 — Gain (loss) reclassified from OCI into income—effective portion Operating expenses 17 (899 ) — Gain (loss) recognized in income—ineffective portion Other expense, net (1,026 ) 202 — Foreign exchange balance sheet hedges: Gain (loss) recognized in income Other expense, net $ 10,916 $ 5,861 $ 211 As of December 31, 2016 , all net derivative gains related to the Company’s cash flow hedges will be reclassified from OCI into net income 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 under the master netting arrangements as of December 31, 2016 and December 31, 2015 (in thousands): December 31, 2016 Gross Amounts of Recognized Assets Gross Amounts Offset in the Condensed Consolidated Balance Sheets Net Amounts Presented in Condensed Consolidated Balance Sheets Gross Amounts Not Offset in Condensed Consolidated Balance Sheets Financial Instruments Cash Collateral Received Net Amount Foreign exchange contracts $ 10,625 $ — $ 10,625 $ 3,780 $ — $ 6,845 December 31, 2015 Gross Amounts of Recognized Assets Gross Amounts Offset in the Condensed Consolidated Balance Sheets Net Amounts Presented in Condensed Consolidated Balance Sheets Gross Amounts Not Offset in Condensed Consolidated Balance Sheets Financial Instruments Cash Collateral Received Net Amount Foreign exchange contracts $ 6,002 $ — $ 6,002 $ 2,100 $ — $ 3,902 The following table sets forth the available offsetting of net derivative liabilities under the master netting arrangements as of December 31, 2016 and December 31, 2015 (in thousands): December 31, 2016 Gross Amounts of Recognized Liabilities Gross Amounts Offset in the Condensed Consolidated Balance Sheets Net Amounts Presented in Condensed Consolidated Balance Sheets Gross Amounts Not Offset in Condensed Consolidated Balance Sheets Financial Instruments Cash Collateral Pledged Net Amount Foreign exchange contracts $ 3,780 $ — $ 3,780 $ 3,780 $ — $ — December 31, 2015 Gross Amounts of Recognized Liabilities Gross Amounts Offset in the Condensed Consolidated Balance Sheets Net Amounts Presented in Condensed Consolidated Balance Sheets Gross Amounts Not Offset in Condensed Consolidated Balance Sheets Financial Instruments Cash Collateral Pledged Net Amount Foreign exchange contracts $ 2,640 $ — $ 2,640 $ 2,100 $ — $ 540

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Balance Sheet Components	Balance Sheet Components Accounts Receivable Reserves Changes in accounts receivable reserves were as follows (in thousands): Allowance for Doubtful Accounts Revenue Reserve Balance at December 31, 2013 $ 743 $ 15,416 Increases 864 42,740 Write-offs/returns taken (769 ) (31,597 ) 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 in the revenue returns reserve include provisions for open box returns and stock rotations. Inventories Inventories consisted of the following (in thousands): December 31, 2016 2015 Components $ 1,035 $ 5,359 Finished goods 229,352 172,787 Total inventories $ 230,387 $ 178,146 Prepaid Expenses and Other Current Assets Prepaid expenses and other current assets consisted of the following (in thousands): December 31, 2016 2015 POP displays, net $ 22,804 $ 9,990 Derivative assets 10,625 6,002 Prepaid marketing 5,764 150 Other 27,153 27,388 Total prepaid expenses and other current assets $ 66,346 $ 43,530 Property and Equipment, Net Property and equipment, net, consisted of the following (in thousands): December 31, 2016 2015 Tooling and manufacturing equipment $ 60,944 $ 53,092 Furniture and office equipment 14,424 6,809 Purchased and internally-developed software 12,032 3,794 Leasehold improvements 28,489 8,388 Total property and equipment 115,889 72,083 Less: Accumulated depreciation and amortization (39,336 ) (27,582 ) Property and equipment, net $ 76,553 $ 44,501 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, 2014 $ — Goodwill acquired 22,562 Subsequent goodwill adjustments (405 ) Balance at December 31, 2015 22,157 Goodwill acquired 28,879 Balance at December 31, 2016 $ 51,036 The carrying amounts of the intangible assets as of December 31, 2016 and December 31, 2015 were as follows (in thousands, except useful life). See Note 12 for additional information. In-process research and development is not amortized until the completion or abandonment of the related development. December 31, 2016 December 31, 2015 Weighted Average Remaining Useful Life (years) Gross Accumulated Amortization Net Gross Accumulated Amortization Net Developed technology $ 26,092 $ (3,247 ) $ 22,845 $ 12,640 $ (1,442 ) $ 11,198 4.8 Trademarks and other 1,278 (542 ) 736 1,278 (260 ) 1,018 3.5 Total finite-lived intangible assets subject to amortization, net 27,370 (3,789 ) 23,581 13,918 (1,702 ) 12,216 In-process research and development 3,940 — 3,940 — — — Total intangible assets, net $ 31,310 $ (3,789 ) $ 27,521 $ 13,918 $ (1,702 ) $ 12,216 Total amortization expense related to intangible assets was $2.1 million and $1.7 million for 2016 and 2015 , respectively. There were no recognized intangible assets outstanding as of December 31, 2014. The estimated future amortization expense of acquired finite-lived intangible assets to be charged to cost of revenue and operating expenses after 2016 , is as follows (in thousands): Cost of Revenue Operating Expenses Total 2017 $ 5,436 $ 230 $ 5,666 2018 5,276 230 5,506 2019 4,496 230 4,726 2020 3,716 46 3,762 2021 3,557 — 3,557 Thereafter 364 — 364 Total intangible assets, net $ 22,845 $ 736 $ 23,581 Accrued Liabilities Accrued liabilities consisted of the following (in thousands): December 31, 2016 2015 Product warranty $ 99,923 $ 40,212 Accrued manufacturing expense and freight 75,579 10,723 Accrued sales incentives 74,181 24,324 Accrued sales and marketing 41,948 33,389 Accrued co-op advertising and marketing development funds 40,002 29,077 Employee related liabilities 13,934 27,394 Sales taxes and VAT payable 8,891 8,349 Inventory received but not billed 7,363 4,292 Accrued legal fees 3,963 3,138 Derivative liabilities 3,780 2,640 Customer deposits 1,923 2,062 Other 19,074 14,499 Accrued liabilities $ 390,561 $ 200,099 Product warranty reserve activities were as follows (in thousands): Reserve For Product Warranty (1) Balance at December 31, 2013 $ 8,480 Charged to cost of revenue 19,462 Settlement of claims (7,844 ) Balance at December 31, 2014 20,098 Charged to cost of revenue 55,642 Changes in estimate related to pre-existing warranties (8,968 ) Settlement of claims (26,560 ) Balance at December 31, 2015 $ 40,212 Charged to cost of revenue 134,771 Changes in estimate related to pre-existing warranties 4,073 Settlement of claims (79,133 ) Balance at December 31, 2016 $ 99,923 (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. During 2016, changes related to pre-existing warranties resulted primarily from a reduction 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 product 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, 2013 $ 82,938 Charged to revenue 8,112 Charged to cost of revenue 11,339 Charged to general and administrative 505 Settlement of claims (80,418 ) 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 During 2014, legal fees of $2.9 million were recognized as incurred, in addition to legal settlement costs of $0.5 million related to the Fitbit Force recall, which were included in general and administrative costs in the consolidated statement of operations. 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. 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, 2014 $ — $ 37 $ — $ 37 Other comprehensive income (loss) before reclassifications 1,276 (42 ) (63 ) 1,171 Amounts reclassified from AOCI (525 ) — 8 (517 ) Other comprehensive income (loss) 751 (42 ) (55 ) 654 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 consisted only of currency translation adjustments of an immaterial amount in 2014.

Debt

Debt	12 Months Ended
	Dec. 31, 2016
Debt Disclosure [Abstract]
Debt	Debt 2015 Credit Agreement In December 2015, the Company entered into a second amended and restated credit agreement, or Senior Facility, with Silicon Valley Bank, or 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. This Senior Facility allows the Company to borrow up to $250.0 million , including up to a $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. Borrowings under the Senior Facility may be drawn as Alternate Base Rate, or ABR, loans or Eurodollar loans, and matures in December 2020. ABR loans bear interest at a variable rate equal to the applicable margin plus the highest of (i) the prime rate, (ii) the federal funds effective rate plus 0.5% , and (iii) the Eurodollar rate plus 1.0% , but in any case at a minimum rate of 3.25% per annum. Eurodollar loans bear interest at a variable rate based on the LIBOR rate and Eurodollar reserve requirements, but in any case at a minimum rate of 1.0% per annum. The Company has the option to repay our borrowings under the Senior Facility without penalty prior to maturity. The Senior Facility requires the Company to comply with certain financial covenants, including maintaining a consolidated fixed charge coverage ratio of at least 1.15:1 , and a consolidated leverage ratio of less than 3:1 . The Senior Facility also requires the Company to comply with certain 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. Pursuant to the terms of the Senior Facility, the Company excluded certain expenses from the consolidated fixed charge coverage ratio calculation with the approval of its lenders, and as a result, the Company was in compliance with all covenants as of December 31, 2016 . Obligations under the credit facility are collateralized by substantially all of our assets, excluding our intellectual property. As of December 31, 2016 , there were no outstanding borrowings under the Senior Facility. Based on the Company’s forecasts, the Company may fail to meet certain of its financial covenants under the Senior Facility during 2017. The Company currently does not have any borrowings outstanding under the Senior Facility and has $38.0 million in letters of credit issued under the line. The Company believes that should it fail to meet any of the financial covenants under the line of credit, the Company would be able to negotiate revised terms or fund outstanding letters of credit with available cash balances. 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 2016 , 2015 , and 2014 was $0.5 million , $1.0 million , and $0.8 million , respectively. As of December 31, 2016, capitalized issuance costs were $ 1.3 million . Letters of Credit As of December 31, 2016 and 2015 , the Company had outstanding letters of credit of $38.0 million and $17.1 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, 2016
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 2017 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 $29.9 million , $8.4 million , and $4.1 million for 2016 , 2015 , and 2014 , respectively. Future minimum payments under the leases as of December 31, 2016 were as follows (in thousands): Year ending December 31, Amounts 2017 $ 33,867 2018 41,222 2019 44,943 2020 41,176 2021 40,148 Thereafter 99,004 Total $ 300,360 Purchase Commitments The aggregate amount of purchase orders open as of December 31, 2016 was approximately $158.4 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. 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 seeks 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. No trial date has been set. 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 seeks 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 has been stayed pending 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, 2016 through May 17, 2016. On August 23, 2016, the ALJ concluded that the Company did not misappropriate any Jawbone trade secrets. On October 20, 2016, the ITC terminated the investigation in the ITC. On December 19, 2016, Jawbone filed a notice of appeal with the Federal Circuit. 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 has been transferred to the U.S. District Court for the Northern District of California. 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. On October 29, 2015, the Company filed a complaint for patent infringement against Jawbone in the United States District Court for the District of Delaware, asserting that Jawbone’s activity trackers infringe certain U.S. patents. This case has been stayed pending the ITC investigation. 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 asserts that Jawbone’s products infringe certain U.S. patents. The complaint seeks 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 based on changed circumstances. On August 12, 2016, the Company was notified by Jawbone that Jawbone had received a confidential subpoena from the U.S. Attorney’s Office in the Northern District of California requesting certain Fitbit 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 from the same office. Fitbit is cooperating with the U.S. Attorney’s Office. The Company intends to vigorously defend and prosecute each of the Jawbone litigation matters and, based on its review, the Company believes it has valid defenses and claims with respect to each of these matters. However, litigation is inherently uncertain, and any judgment or injunctive relief entered against the Company or any adverse settlement could materially and adversely impact its business, financial condition, operating results, and prospects. Regarding the five matters still in the early stages of litigation, the Company is unable to estimate a reasonably possible loss or range of loss, if any, that may result from these matters. In addition, these litigation matters are complex, likely to involve significant management time and attention, and the cost of defending and prosecuting these matters is likely to be expensive, regardless of outcome. 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. Plaintiffs have amended their complaint four times, and on January 15, 2016, the Company moved to dismiss the Fourth Amended Complaint. On July 15, 2016, the Court denied the motion to dismiss. Trial is currently scheduled for July 10, 2017. 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. The Company has filed a motion to compel arbitration, and a hearing has been scheduled for March 16, 2017. 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 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, or the Securities Act, and the Securities Exchange Act of 1934, as amended, or the Exchange Act, 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 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, its board members, underwriters of our IPO, and a number of its investors. Plaintiff alleges that the IPO registration statement contained material misstatements about the Company’s products. Plaintiff seeks to represent a class of persons who purchased the Company’s common stock in and/or traceable to the IPO. Plaintiff seeks 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 November 11, 2016, a derivative lawsuit was filed in the U.S. District Court for the Northern District of California naming as defendants the Company and certain of its officers and board members. Plaintiffs allege breach of fiduciary duty based on the same alleged set of facts in the federal and state securities class action litigation. The court has ordered a stay in the litigation. The Company believes that the plaintiffs’ allegations in these actions 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. 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, 2016
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 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, 2016 , 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, 2016 , 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, 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. As of December 31, 2015 , the Company 99.4 million shares of Class A common stock were issued and outstanding and 115.4 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. 2007 Equity Incentive Plan The Amended and Restated 2007 Stock Plan (the “2007 Plan”) provides 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 Equity Incentive Plan (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, 2016 , 6.7 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 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 Employee Stock Purchase Plan (“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. 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, 2015 44,362 $ 3.20 Granted 955 $ 14.06 Exercised (8,619 ) $ 1.21 $ 121,268 Canceled (2,244 ) $ 5.54 Balance—December 31, 2016 34,454 $ 3.85 6.8 $ 145,568 Stock options exercisable—December 31, 2016 20,654 $ 2.51 6.2 $ 107,573 Stock options vested and expected to vest—December 31, 2016 34,223 $ 3.82 6.8 $ 145,097 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 $7.32 as of December 31, 2016 . 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, 2015 3,292 $ 34 Granted 10,356 13.48 Vested (1,117 ) 31.5 Forfeited or canceled (953 ) 27.87 Unvested balance—December 31, 2016 11,578 16.85 Stock-Based Compensation Expense Total stock-based compensation recognized was as follows (in thousands): Year Ended December 31, 2016 2015 2014 Cost of revenue $ 4,797 $ 4,739 $ 890 Research and development 47,207 18,251 2,350 Sales and marketing 11,575 7,419 1,295 General and administrative 15,853 10,615 2,269 Total stock-based compensation expense $ 79,432 $ 41,024 $ 6,804 The weighted-average grant date fair value of stock options granted during 2016 , 2015 , and 2014 was $14.06 , $10.67 , and $2.40 per share, respectively. The total grant date fair value of stock options that vested during 2016 , 2015 , and 2014 was $32.9 million , $18.6 million , and $1.7 million , respectively. As of December 31, 2016 , the total unrecognized compensation expense related to unvested stock options, net of estimated forfeitures, was $49.8 million , which the Company expects to recognize over an estimated weighted average period of 2.1 years . As of December 31, 2016 , the total unrecognized compensation expense related to unvested RSUs, net of estimated forfeitures, was $163.7 million , which the Company expects to recognize over an estimated weighted average period of 3.2 years . Stock-based compensation is measured at the grant date based on the fair value of the award and is recognized as expense, net of estimated forfeitures, 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 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. In addition, the Company is required to estimate the amount of stock-based compensation that it expects to be forfeited based on historical experience. 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 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, 2016 2015 2014 Employee stock options Expected term (in years) 6.25 6.25 6.25 Volatility 40.7% 52.1% - 56.9% 54.83% – 60.94% Risk-free interest rate 1.6% 1.5% - 1.9% 1.73% – 2.04% Dividend yield —% —% —% Employee stock purchase plan Expected term (in years) 0.5 0.5 – 0.9 — Volatility 30.1% - 39.0% 27.7% - 35.0% —% Risk-free interest rate 0.4% - 0.6% 0.3% —% Dividend yield —% —% —%

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2016
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, 2016 2015 2014 United States $ 5,577 $ 286,380 $ 150,137 Foreign (114,872 ) 1,569 (10,364 ) Total $ (109,295 ) $ 287,949 $ 139,773 The income tax expense (benefit) is composed of the following (in thousands): Year Ended December 31, 2016 2015 2014 Current: Federal $ 78,025 $ 140,396 $ 47,565 State 9,878 13,307 2,319 Foreign 4,313 1,107 113 Total current 92,216 154,810 49,997 Deferred: Federal (86,827 ) (33,421 ) (39,339 ) State (11,622 ) (8,941 ) (2,651 ) Foreign (285 ) (176 ) (11 ) Total deferred (98,734 ) (42,538 ) (42,001 ) Total income tax expense (benefit) $ (6,518 ) $ 112,272 $ 7,996 The reconciliation of the Company’s effective tax rate to the statutory federal rate is as follows: Year Ended December 31, 2016 2015 2014 Tax at federal statutory rate 35.0 % 35.0 % 35.0 % State taxes, net of federal effect 4.3 1.5 (0.2 ) Foreign rate differential (38.9 ) (0.8 ) 2.7 Tax credits 9.0 (2.0 ) (1.1 ) Domestic production activities deduction 5.0 (3.3 ) (2.6 ) Stock-based compensation (4.6 ) 1.7 — Change in prior year reserves 1.9 — — Out-of-period adjustment (2.8 ) — — Warrant fair value adjustment — 6.9 3.3 Change in valuation allowance — — (32.0 ) Other (2.9 ) — 0.6 Effective tax rate 6.0 % 39.0 % 5.7 % For 2016, the Company recorded a benefit for income taxes of $(6.5) million , for an effective tax rate of 6.0% . The effective tax rate is different than the statutory federal tax rate primarily due to losses incurred in certain foreign jurisdictions for which we will not realize a tax benefit. The decrease in our provision was also a result of additional federal research and development credits and a reduction in state taxes. For 2015, the Company recorded an expense for income taxes of $112.3 million , for an effective tax rate of 39% . The effective tax rate is higher than the statutory federal tax rate primarily due to state taxes, net of federal benefit and certain permanent differences related to the non-deductible change in fair value of the redeemable convertible preferred stock warrant liability and non-deductible stock-based compensation expense, partially offset by non-taxable income associated with the change in contingent consideration from the FitStar acquisition and a permanent domestic production activities deduction. The tax effects of temporary differences that give rise to significant portions of the deferred tax assets were as follows (in thousands): December 31, 2016 2015 Net operating losses and credits $ 9,446 $ 5,225 Fixed assets and intangible assets 16,272 1,227 Accruals and reserves 111,216 54,891 Fitbit Force recall reserve 469 1,910 Stock-based compensation 17,864 9,642 Inventory 8,513 3,477 Other 10,665 7,089 Gross deferred tax assets 174,445 83,461 Debt issuance costs (353 ) (441 ) Net deferred tax assets $ 174,092 $ 83,020 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 our net deferred tax assets is dependent upon our 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. As of December 31, 2016 , the Company continued to believe that it was more-likely-than-not that it would have future taxable income sufficient to realize the benefit of the Company’s net deferred tax assets. As of December 31, 2016, the Company has federal net operating loss carryforwards of $2.2 million which expire beginning after 2033 and California net operating loss carryforwards of $2.2 million which expire beginning after 2033. As of December 31, 2016, the Company has federal research tax credit carryforwards of approximately $0.2 million , which if not utilized, begin to expire after 2030, California research tax credit carryforwards of approximately $16.2 million , which do not expire, Massachusetts research tax credit carryforwards of approximately $0.4 million , which if not utilized, begin to expire after 2031, and California hiring tax credit carryforwards of approximately $0.1 million , which if not utilized, begin to expire after 2026. As of December 31, 2016, the Company has United Kingdom net operating loss carryforwards of $16.5 million , which do not expire. These United Kingdom net operating losses are related to excess stock-based compensation, and accordingly no deferred tax asset is recognized for such amounts prior to the adoption of ASU 2016-09. Any deferred tax asset recognized on adoption of ASU 2016-09 related to excess stock-based compensation, net of any applicable valuation allowance, will be recorded to retained earnings as of the date of adoption. 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. It is the intention of the Company to indefinitely reinvest the earnings of the Company’s foreign subsidiaries. The Company does not provide for U.S. income taxes on the earnings of its foreign subsidiaries as such earnings are to be reinvested indefinitely. If these earnings were distributed to the United States in the form of dividends or if the shares of the relevant foreign subsidiaries were sold or otherwise transferred the Company would be subject to additional U.S. income taxes, subject to adjustment for foreign tax credits, and foreign withholding taxes. As of December 31, 2016 , there was $0.6 million of cumulative foreign earnings upon which U.S. income taxes have not been provided. Determining the unrecognized deferred tax liability related to investments in these non-U.S. subsidiaries that are indefinitely reinvested is not practical. We are subject to income taxes in the U.S. (federal and state) and numerous foreign jurisdictions. Significant judgment is required in evaluating our tax positions and determining our provision for income taxes. During the ordinary course of business, there are many transactions and calculations for which the ultimate tax determination is uncertain. We establish 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 we believe that certain positions might be challenged despite our belief that our tax return positions are fully supportable. We adjust 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, 2016 and 2015, the Company has $35.6 million and $23.5 million of unrecognized tax benefits. A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands): December 31, 2016 2015 2014 Balance at beginning of year $ 23,518 $ 10,594 $ 7,991 Reductions based on tax positions related to prior year (2,100 ) (18 ) (418 ) Additions based on tax positions related to prior year 2,809 — — Additions based on tax positions related to current year 11,357 12,942 3,021 Balance at end of year $ 35,584 $ 23,518 $ 10,594 At December 31, 2016 , the total amount of gross unrecognized tax benefits was $35.6 million , all of which would affect the effective tax rate if recognized. The Company does not have any tax positions as of December 31, 2016 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, 2016 and 2015, the Company has accrued $2.2 million and $0.6 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. The Company is under examination by the Internal Revenue Service for the tax years 2013 and 2014 and under state examinations for the tax years 2013 through 2016. For federal, state and non-U.S. tax returns, the Company is generally no longer subject to tax examinations for years prior to 2008.

Net Income (Loss) per Share Att

Net Income (Loss) per Share Attributable to Common Stockholders	12 Months Ended
	Dec. 31, 2016
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, 2016 2015 2014 Numerator: Net income (loss) $ (102,777 ) $ 175,677 $ 131,777 Less: noncumulative dividends to preferred stockholders — (2,526 ) (5,326 ) Less: undistributed earnings to participating securities — (59,133 ) (98,103 ) Net income (loss) attributable to common stockholders—basic (102,777 ) 114,018 28,348 Add: adjustments to undistributed earnings to participating securities — 8,821 10,175 Net income (loss) attributable to common stockholders—diluted $ (102,777 ) $ 122,839 $ 38,523 Denominator: Weighted-average shares of common stock—basic for Class A and Class B 220,405 129,886 40,351 Effect of dilutive securities — 34,327 20,828 Weighted-average shares of common stock—diluted for Class A and Class B 220,405 164,213 61,179 Net income (loss) per share attributable to common stockholders: Basic $ (0.47 ) $ 0.88 $ 0.70 Diluted $ (0.47 ) $ 0.75 $ 0.63 The following potentially dilutive common shares (in thousands) were excluded from the computation of diluted net income (loss) per share for the periods presented because including them would have been antidilutive: December 31, 2016 2015 2014 Stock options to purchase common stock 34,454 445 4,420 Restricted stock units 11,578 692 — Redeemable convertible preferred stock — 65,903 139,851 Redeemable convertible preferred stock warrants — 921 1,955 Total 46,032 67,961 146,226

Significant Customer Informatio

Significant Customer Information and Other Information	12 Months Ended
	Dec. 31, 2016
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, 2016 2015 2014 A 14 % 15 % 13 % C 14 14 11 B 10 14 12 * Revenue was less than 10%. Retailers and distributors that accounted for equal to or greater than 10% of accounts receivable at December 31, 2016 and 2015 were as follows: December 31, 2016 2015 C 19 % 23 % A 16 15 D 12 * B * 19 * 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, 2016 2015 2014 United States $ 1,539,600 $ 1,381,152 $ 562,553 Americas excluding United States 110,111 92,252 38,576 Europe, Middle East, and Africa 389,154 208,767 60,699 APAC 130,596 175,827 83,605 Total $ 2,169,461 $ 1,857,998 $ 745,433 As of December 31, 2016 and 2015 , long-lived assets, which represent property and equipment, located outside the United States were $30.1 million and $28.9 million , respectively.

Acquisitions

Acquisitions	12 Months Ended
	Dec. 31, 2016
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, 2016
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 2016 and 2015 (in thousands, except per share amounts): 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 Three Months Ended December 31, 2015 September 30, 2015 June 30, 2015 March 31, 2015 Revenue $ 711,570 $ 409,262 $ 400,412 $ 336,754 Gross profit $ 348,299 $ 196,013 $ 187,542 $ 169,209 Net income $ 64,165 $ 45,834 $ 17,681 $ 47,997 Net income per share attributable to common stockholders—basic $ 0.30 $ 0.22 $ 0.09 $ 0.26 Net income per share attributable to common stockholders—diluted $ 0.26 $ 0.19 $ 0.07 $ 0.22

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2016
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events Restructuring In January 2017, the Company announced a reorganization, including a reduction in workforce. This reorganization impacted 107 employees or approximately 6% of the Company’s global workforce. The Company anticipates that it will complete the reorganization by the end of the second quarter of 2017, and estimates it will incur approximately $ 3.7 million in restructuring expenses to be recorded in the first quarter of 2017. Stock Option Exchange The Company intends to seek stockholder approval for a program under which certain employees may relinquish out-of-the-money stock options at the time of the exchange program in return for a fewer number of RSUs. An estimate of the financial impact of the exchange program is impracticable to be made until the terms of the exchange program are finalized.

Significant Accounting Polici23

Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2016
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. The Company did not adjust operating results for quarters prior to 2016.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in accordance with generally accepted accounting principles in the United States of America (“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 estimates and assumptions made by management 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 reporting segments, the recoverability of intangible assets and their useful lives, the valuations of deferred income tax assets and uncertain tax positions, and accruals for the Fitbit Force recall. 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 certain of the Company’s 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 $ 11.7 million , $(1.3) million , and $(2.7) million for 2016 , 2015 , and 2014 , respectively. The Company translates the assets and liabilities of its non-U.S. dollar functional currency subsidiaries into U.S. dollars using exchange rates in effect at the end of each period. Revenue and expenses for these subsidiaries are translated using rates that approximate those in effect during the related period. Gains and losses from translations are recognized in foreign currency translation included in accumulated other comprehensive income (loss).
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, accounts receivables, and derivative instruments. 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 are 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 market on a first-in, first-out basis. 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 period between achieving technological feasibility and 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.
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 connected health and fitness devices and accessories. 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 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 FitStar 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 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 FitStar subscription-based services is based on the price charged when sold separately. Amounts allocated to the delivered health and fitness 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. 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, 2016 , actual returns have primarily been open-box 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. 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.
Product Warranty	Product Warranty The Company offers a standard product warranty that the product will operate under normal use for a period of one year from date of original purchase except in the European Union where the Company provides a two -year warranty. The Company has the obligation, at its option, to either repair or replace the defective product. At the time revenue is recognized, an estimate of future warranty costs is recorded as a component of cost of revenues. Factors that affect the warranty obligation include product failure rates, service delivery costs incurred in correcting the product failures, and warranty policies. The warranty obligation does not consider historical experience of the Fitbit Force product as a separate reserve has been established for the Fitbit Force recall. The Company’s products are manufactured by third-party contract manufacturers, and in certain cases, the Company may have recourse to such third-party contract manufacturers.
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, net of estimated forfeitures, 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 and shares issued under our 2015 Employee Stock Purchase Plan, or 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. The Company recognizes a benefit from stock-based compensation in equity to the extent that an incremental tax benefit is realized by following the ordering provisions of the tax laws.
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 its expense 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 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 the Company’s 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 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 EPS, net income attributable to common stockholders for basic EPS 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 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. In 2016, basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period without consideration of common stock equivalents or the participation rights of the preferred stock as they do not share in losses. As the Company was in a net loss position in 2016, basic net loss per share attributable to common stockholders is 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 In May 2014, the Financial Accounting Standards Board, or FASB, issued ASU 2014-09 (ASC 606), Revenue from Contracts with Customers , 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. This 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. ASU 2014-09 will become effective for the Company on January 1, 2018 and can be adopted either retrospectively to each prior reporting period presented or as a cumulative effect adjustment as of the date of adoption. In April 2016, the FASB issued ASU 2016-10, which clarifies guidance on identifying performance obligations and licensing implementation. The Company currently expects to adopt the revenue standards as of January 1, 2018, utilizing the modified retrospective transition method. Although the new standard may, in certain circumstances, impact the timing of when revenue is recognized for product shipped, and the timing and classification of certain sales incentives, the Company believes the new guidance is consistent with its current revenue policy. Therefore, the Company does not currently expect adoption to have a material impact on the consolidated financial statements. The Company is continuing to evaluate the impact of the adoption and preliminary assessments are subject to change. In February 2016, the FASB issued ASU 2016-02, Leases . This ASU requires lease assets and lease liabilities arising from leases, including operating leases, to be recognized 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; however, the Company does not expect the adoption to have a material impact on its statements of operations. In March 2016, the FASB issued ASU 2016-09, Compensation — Stock Compensation (Topic 718). This ASU 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. The Company adopted ASU 2016-09 as of January 1, 2017. Any deferred tax asset recognized on adoption of ASU 2016-09 related to excess stock-based compensation, net of any applicable valuation allowance, was recorded to retained earnings as of the date of adoption. The Company does not currently expect the adoption to have a material impact on the consolidated financial statements. In June 2016, the FASB issued ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. This ASU 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 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) . This ASU provides guidance intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. ASU 2016-15 will become effective for the Company on January 1, 2018 and early adoption is permitted. The Company is currently evaluating the impact of this guidance on its consolidated financial statements.

Significant Accounting Polici24

Significant Accounting Policies Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016 2015 Tooling and manufacturing equipment $ 60,944 $ 53,092 Furniture and office equipment 14,424 6,809 Purchased and internally-developed software 12,032 3,794 Leasehold improvements 28,489 8,388 Total property and equipment 115,889 72,083 Less: Accumulated depreciation and amortization (39,336 ) (27,582 ) Property and equipment, net $ 76,553 $ 44,501

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2016
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, 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 December 31, 2015 Level 1 Level 2 Level 3 Total Assets: Money market funds $ 248,128 $ — $ — $ 248,128 U.S. government agencies — 113,314 — 113,314 Corporate debt securities — 193,964 — 193,964 Derivative assets — 6,002 — 6,002 Total $ 248,128 $ 313,280 $ — $ 561,408 Liabilities: Derivative liabilities $ — $ 2,640 $ — $ 2,640
Schedule of changes in fair value of redeemable convertible preferred stock warrant liability	The following table sets forth a summary of the changes in the fair value of the acquisition-related contingent consideration (in thousands): Balance at December 31, 2014 $ — Addition from acquisition 7,704 Change in fair value of contingent consideration (7,704 ) Balance at December 31, 2015 $ — Balance at December 31, 2014 $ 15,797 Change in fair value 56,655 Settlement of warrant liability upon exercise (56,678 ) Reclassification of unexercised warrants to additional paid in capital upon the IPO (15,774 ) Balance at December 31, 2015 $ —

Financial Instruments (Tables)

Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of available for sale securities including fair value and gross unrealized losses	The following table classifies marketable securities by contractual maturities (in thousands): December 31, 2016 December 31, 2015 Due in one year $ 355,152 $ 128,632 Due in one to two years 49,541 — Total $ 404,693 $ 128,632 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 sets forth the cash, cash equivalents, and marketable securities as of December 31, 2015 (in thousands): Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Fair Value Cash and Cash Equivalents Marketable Securities Cash $ 109,072 $ — $ — $ 109,072 $ 109,072 $ — Money market funds 248,128 — — 248,128 248,128 — U.S. government agencies 113,315 3 (4 ) 113,314 63,464 49,850 Corporate debt securities 194,018 1 (55 ) 193,964 115,182 78,782 Total $ 664,533 $ 4 $ (59 ) $ 664,478 $ 535,846 $ 128,632
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, 2016 December 31, 2015 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 $ — $ 3,116 $ — Cash flow designated hedges Accrued liabilities — 1,428 — 1,327 Hedges not designated Prepaid expense and other current assets 9,812 — 2,886 — Hedges not designated Accrued liabilities — 2,352 — 1,313 Total fair value of derivative instruments $ 10,625 $ 3,780 $ 6,002 $ 2,640
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 2016 2015 2014 Foreign exchange cash flow hedges: Gain (loss) recognized in OCI—effective portion $ 8,171 $ 2,785 $ — Gain (loss) reclassified from OCI into income—effective portion Revenue 10,153 2,183 — Gain (loss) reclassified from OCI into income—effective portion Operating expenses 17 (899 ) — Gain (loss) recognized in income—ineffective portion Other expense, net (1,026 ) 202 — Foreign exchange balance sheet hedges: Gain (loss) recognized in income Other expense, net $ 10,916 $ 5,861 $ 211
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 2016 2015 2014 Foreign exchange cash flow hedges: Gain (loss) recognized in OCI—effective portion $ 8,171 $ 2,785 $ — Gain (loss) reclassified from OCI into income—effective portion Revenue 10,153 2,183 — Gain (loss) reclassified from OCI into income—effective portion Operating expenses 17 (899 ) — Gain (loss) recognized in income—ineffective portion Other expense, net (1,026 ) 202 — Foreign exchange balance sheet hedges: Gain (loss) recognized in income Other expense, net $ 10,916 $ 5,861 $ 211
Schedule of offsetting derivative assets	The following table sets forth the available offsetting of net derivative assets under the master netting arrangements as of December 31, 2016 and December 31, 2015 (in thousands): December 31, 2016 Gross Amounts of Recognized Assets Gross Amounts Offset in the Condensed Consolidated Balance Sheets Net Amounts Presented in Condensed Consolidated Balance Sheets Gross Amounts Not Offset in Condensed Consolidated Balance Sheets Financial Instruments Cash Collateral Received Net Amount Foreign exchange contracts $ 10,625 $ — $ 10,625 $ 3,780 $ — $ 6,845 December 31, 2015 Gross Amounts of Recognized Assets Gross Amounts Offset in the Condensed Consolidated Balance Sheets Net Amounts Presented in Condensed Consolidated Balance Sheets Gross Amounts Not Offset in Condensed Consolidated Balance Sheets Financial Instruments Cash Collateral Received Net Amount Foreign exchange contracts $ 6,002 $ — $ 6,002 $ 2,100 $ — $ 3,902
Schedule of offsetting derivative liabilities	The following table sets forth the available offsetting of net derivative liabilities under the master netting arrangements as of December 31, 2016 and December 31, 2015 (in thousands): December 31, 2016 Gross Amounts of Recognized Liabilities Gross Amounts Offset in the Condensed Consolidated Balance Sheets Net Amounts Presented in Condensed Consolidated Balance Sheets Gross Amounts Not Offset in Condensed Consolidated Balance Sheets Financial Instruments Cash Collateral Pledged Net Amount Foreign exchange contracts $ 3,780 $ — $ 3,780 $ 3,780 $ — $ — December 31, 2015 Gross Amounts of Recognized Liabilities Gross Amounts Offset in the Condensed Consolidated Balance Sheets Net Amounts Presented in Condensed Consolidated Balance Sheets Gross Amounts Not Offset in Condensed Consolidated Balance Sheets Financial Instruments Cash Collateral Pledged Net Amount Foreign exchange contracts $ 2,640 $ — $ 2,640 $ 2,100 $ — $ 540

Balance Sheet Components (Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of accounts receivable and revenue reserves	Changes in accounts receivable reserves were as follows (in thousands): Allowance for Doubtful Accounts Revenue Reserve Balance at December 31, 2013 $ 743 $ 15,416 Increases 864 42,740 Write-offs/returns taken (769 ) (31,597 ) 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
Schedule of inventories	Inventories consisted of the following (in thousands): December 31, 2016 2015 Components $ 1,035 $ 5,359 Finished goods 229,352 172,787 Total inventories $ 230,387 $ 178,146
Schedule of prepaid expenses and other current asset	Prepaid expenses and other current assets consisted of the following (in thousands): December 31, 2016 2015 POP displays, net $ 22,804 $ 9,990 Derivative assets 10,625 6,002 Prepaid marketing 5,764 150 Other 27,153 27,388 Total prepaid expenses and other current assets $ 66,346 $ 43,530
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, 2016 2015 Tooling and manufacturing equipment $ 60,944 $ 53,092 Furniture and office equipment 14,424 6,809 Purchased and internally-developed software 12,032 3,794 Leasehold improvements 28,489 8,388 Total property and equipment 115,889 72,083 Less: Accumulated depreciation and amortization (39,336 ) (27,582 ) Property and equipment, net $ 76,553 $ 44,501
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, 2014 $ — Goodwill acquired 22,562 Subsequent goodwill adjustments (405 ) Balance at December 31, 2015 22,157 Goodwill acquired 28,879 Balance at December 31, 2016 $ 51,036
Schedule of intangible assets (excluding goodwill)	The carrying amounts of the intangible assets as of December 31, 2016 and December 31, 2015 were as follows (in thousands, except useful life). See Note 12 for additional information. In-process research and development is not amortized until the completion or abandonment of the related development. December 31, 2016 December 31, 2015 Weighted Average Remaining Useful Life (years) Gross Accumulated Amortization Net Gross Accumulated Amortization Net Developed technology $ 26,092 $ (3,247 ) $ 22,845 $ 12,640 $ (1,442 ) $ 11,198 4.8 Trademarks and other 1,278 (542 ) 736 1,278 (260 ) 1,018 3.5 Total finite-lived intangible assets subject to amortization, net 27,370 (3,789 ) 23,581 13,918 (1,702 ) 12,216 In-process research and development 3,940 — 3,940 — — — Total intangible assets, net $ 31,310 $ (3,789 ) $ 27,521 $ 13,918 $ (1,702 ) $ 12,216
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 2016 , is as follows (in thousands): Cost of Revenue Operating Expenses Total 2017 $ 5,436 $ 230 $ 5,666 2018 5,276 230 5,506 2019 4,496 230 4,726 2020 3,716 46 3,762 2021 3,557 — 3,557 Thereafter 364 — 364 Total intangible assets, net $ 22,845 $ 736 $ 23,581
Schedule of accrued liabilities	Accrued liabilities consisted of the following (in thousands): December 31, 2016 2015 Product warranty $ 99,923 $ 40,212 Accrued manufacturing expense and freight 75,579 10,723 Accrued sales incentives 74,181 24,324 Accrued sales and marketing 41,948 33,389 Accrued co-op advertising and marketing development funds 40,002 29,077 Employee related liabilities 13,934 27,394 Sales taxes and VAT payable 8,891 8,349 Inventory received but not billed 7,363 4,292 Accrued legal fees 3,963 3,138 Derivative liabilities 3,780 2,640 Customer deposits 1,923 2,062 Other 19,074 14,499 Accrued liabilities $ 390,561 $ 200,099
Schedule of product warranty reserves	Product warranty reserve activities were as follows (in thousands): Reserve For Product Warranty (1) Balance at December 31, 2013 $ 8,480 Charged to cost of revenue 19,462 Settlement of claims (7,844 ) Balance at December 31, 2014 20,098 Charged to cost of revenue 55,642 Changes in estimate related to pre-existing warranties (8,968 ) Settlement of claims (26,560 ) Balance at December 31, 2015 $ 40,212 Charged to cost of revenue 134,771 Changes in estimate related to pre-existing warranties 4,073 Settlement of claims (79,133 ) Balance at December 31, 2016 $ 99,923 (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, 2013 $ 82,938 Charged to revenue 8,112 Charged to cost of revenue 11,339 Charged to general and administrative 505 Settlement of claims (80,418 ) 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
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, 2014 $ — $ 37 $ — $ 37 Other comprehensive income (loss) before reclassifications 1,276 (42 ) (63 ) 1,171 Amounts reclassified from AOCI (525 ) — 8 (517 ) Other comprehensive income (loss) 751 (42 ) (55 ) 654 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 )

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Schedule of future minimum rental payments for operating leases	Future minimum payments under the leases as of December 31, 2016 were as follows (in thousands): Year ending December 31, Amounts 2017 $ 33,867 2018 41,222 2019 44,943 2020 41,176 2021 40,148 Thereafter 99,004 Total $ 300,360

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2015 44,362 $ 3.20 Granted 955 $ 14.06 Exercised (8,619 ) $ 1.21 $ 121,268 Canceled (2,244 ) $ 5.54 Balance—December 31, 2016 34,454 $ 3.85 6.8 $ 145,568 Stock options exercisable—December 31, 2016 20,654 $ 2.51 6.2 $ 107,573 Stock options vested and expected to vest—December 31, 2016 34,223 $ 3.82 6.8 $ 145,097
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, 2015 3,292 $ 34 Granted 10,356 13.48 Vested (1,117 ) 31.5 Forfeited or canceled (953 ) 27.87 Unvested balance—December 31, 2016 11,578 16.85
Schedule of stock-based compensation expense	Total stock-based compensation recognized was as follows (in thousands): Year Ended December 31, 2016 2015 2014 Cost of revenue $ 4,797 $ 4,739 $ 890 Research and development 47,207 18,251 2,350 Sales and marketing 11,575 7,419 1,295 General and administrative 15,853 10,615 2,269 Total stock-based compensation expense $ 79,432 $ 41,024 $ 6,804
Schedule of options valuation assumptions	The fair value of the stock option awards 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, 2016 2015 2014 Employee stock options Expected term (in years) 6.25 6.25 6.25 Volatility 40.7% 52.1% - 56.9% 54.83% – 60.94% Risk-free interest rate 1.6% 1.5% - 1.9% 1.73% – 2.04% Dividend yield —% —% —% Employee stock purchase plan Expected term (in years) 0.5 0.5 – 0.9 — Volatility 30.1% - 39.0% 27.7% - 35.0% —% Risk-free interest rate 0.4% - 0.6% 0.3% —% Dividend yield —% —% —%

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016 2015 2014 United States $ 5,577 $ 286,380 $ 150,137 Foreign (114,872 ) 1,569 (10,364 ) Total $ (109,295 ) $ 287,949 $ 139,773
Schedule of components for income tax expense	The income tax expense (benefit) is composed of the following (in thousands): Year Ended December 31, 2016 2015 2014 Current: Federal $ 78,025 $ 140,396 $ 47,565 State 9,878 13,307 2,319 Foreign 4,313 1,107 113 Total current 92,216 154,810 49,997 Deferred: Federal (86,827 ) (33,421 ) (39,339 ) State (11,622 ) (8,941 ) (2,651 ) Foreign (285 ) (176 ) (11 ) Total deferred (98,734 ) (42,538 ) (42,001 ) Total income tax expense (benefit) $ (6,518 ) $ 112,272 $ 7,996
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, 2016 2015 2014 Tax at federal statutory rate 35.0 % 35.0 % 35.0 % State taxes, net of federal effect 4.3 1.5 (0.2 ) Foreign rate differential (38.9 ) (0.8 ) 2.7 Tax credits 9.0 (2.0 ) (1.1 ) Domestic production activities deduction 5.0 (3.3 ) (2.6 ) Stock-based compensation (4.6 ) 1.7 — Change in prior year reserves 1.9 — — Out-of-period adjustment (2.8 ) — — Warrant fair value adjustment — 6.9 3.3 Change in valuation allowance — — (32.0 ) Other (2.9 ) — 0.6 Effective tax rate 6.0 % 39.0 % 5.7 %
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, 2016 2015 Net operating losses and credits $ 9,446 $ 5,225 Fixed assets and intangible assets 16,272 1,227 Accruals and reserves 111,216 54,891 Fitbit Force recall reserve 469 1,910 Stock-based compensation 17,864 9,642 Inventory 8,513 3,477 Other 10,665 7,089 Gross deferred tax assets 174,445 83,461 Debt issuance costs (353 ) (441 ) Net deferred tax assets $ 174,092 $ 83,020
Schedule of reconciliation of unrecognized tax benefits	As of December 31, 2016 and 2015, the Company has $35.6 million and $23.5 million of unrecognized tax benefits. A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands): December 31, 2016 2015 2014 Balance at beginning of year $ 23,518 $ 10,594 $ 7,991 Reductions based on tax positions related to prior year (2,100 ) (18 ) (418 ) Additions based on tax positions related to prior year 2,809 — — Additions based on tax positions related to current year 11,357 12,942 3,021 Balance at end of year $ 35,584 $ 23,518 $ 10,594

Net Income (Loss) per Share A31

Net Income (Loss) per Share Attributable to Common Stockholders (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016 2015 2014 Numerator: Net income (loss) $ (102,777 ) $ 175,677 $ 131,777 Less: noncumulative dividends to preferred stockholders — (2,526 ) (5,326 ) Less: undistributed earnings to participating securities — (59,133 ) (98,103 ) Net income (loss) attributable to common stockholders—basic (102,777 ) 114,018 28,348 Add: adjustments to undistributed earnings to participating securities — 8,821 10,175 Net income (loss) attributable to common stockholders—diluted $ (102,777 ) $ 122,839 $ 38,523 Denominator: Weighted-average shares of common stock—basic for Class A and Class B 220,405 129,886 40,351 Effect of dilutive securities — 34,327 20,828 Weighted-average shares of common stock—diluted for Class A and Class B 220,405 164,213 61,179 Net income (loss) per share attributable to common stockholders: Basic $ (0.47 ) $ 0.88 $ 0.70 Diluted $ (0.47 ) $ 0.75 $ 0.63
Schedule of antidilutive securities excluded from earnings per share	The following potentially dilutive common shares (in thousands) were excluded from the computation of diluted net income (loss) per share for the periods presented because including them would have been antidilutive: December 31, 2016 2015 2014 Stock options to purchase common stock 34,454 445 4,420 Restricted stock units 11,578 692 — Redeemable convertible preferred stock — 65,903 139,851 Redeemable convertible preferred stock warrants — 921 1,955 Total 46,032 67,961 146,226

Significant Customer Informat32

Significant Customer Information and Other Information (Tables)	12 Months Ended
	Dec. 31, 2016
Risks and Uncertainties [Abstract]
Schedules of concentration of risk	Revenue by geographic region, based on ship-to destinations, was as follows (in thousands): December 31, 2016 2015 2014 United States $ 1,539,600 $ 1,381,152 $ 562,553 Americas excluding United States 110,111 92,252 38,576 Europe, Middle East, and Africa 389,154 208,767 60,699 APAC 130,596 175,827 83,605 Total $ 2,169,461 $ 1,857,998 $ 745,433 Retailers and distributors with revenue equal to or greater than 10% of total revenue were as follows: December 31, 2016 2015 2014 A 14 % 15 % 13 % C 14 14 11 B 10 14 12 * Revenue was less than 10%. Retailers and distributors that accounted for equal to or greater than 10% of accounts receivable at December 31, 2016 and 2015 were as follows: December 31, 2016 2015 C 19 % 23 % A 16 15 D 12 * B * 19 * Accounts receivable were less than 10%.

Acquisitions - (Tables)

Acquisitions - (Tables)	12 Months Ended
	Dec. 31, 2016
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, 2016
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 2016 and 2015 (in thousands, except per share amounts): 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 Three Months Ended December 31, 2015 September 30, 2015 June 30, 2015 March 31, 2015 Revenue $ 711,570 $ 409,262 $ 400,412 $ 336,754 Gross profit $ 348,299 $ 196,013 $ 187,542 $ 169,209 Net income $ 64,165 $ 45,834 $ 17,681 $ 47,997 Net income per share attributable to common stockholders—basic $ 0.30 $ 0.22 $ 0.09 $ 0.26 Net income per share attributable to common stockholders—diluted $ 0.26 $ 0.19 $ 0.07 $ 0.22

Business Overview and Basis o35

Business Overview and Basis of Presentation (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Apr. 02, 2016	Dec. 31, 2016
Advertising Barter Transactions [Member]
Organization, Consolidation And Presentation Of Financial Statements [Line Items]
Remaining advertising credit		$ 15
Remaining advertising credit		$ 13
Advertising credit, expected period of use (over)		3 years
Understated Income Tax Expense [Member]
Organization, Consolidation And Presentation Of Financial Statements [Line Items]
Out-of-period adjustment	$ 3

Significant Accounting Polici36

Significant Accounting Policies - Property, Plant and Equipment (Details)	12 Months Ended
	Dec. 31, 2016
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)	12 Months Ended
	Dec. 31, 2016USD ($)segmentvotereporting_unit	Dec. 31, 2015USD ($)classvote	Dec. 31, 2014USD ($)
Significant Accounting Policies [Line Items]
Impairment loss recorded	$ 0
Foreign currency transaction gains (losses)	11,700,000	$ (1,300,000)	$ (2,700,000)
Capitalized software costs	$ 6,300,000	0
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	316,800,000	237,000,000	71,900,000
Cooperative advertising and promotion expense	$ 52,900,000	$ 38,300,000	$ 12,700,000
Product warranty term	1 year
Number of classes of stock | class		2
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, 2016	Dec. 31, 2015
Assets:
Available for sale securities	$ 404,693,000	$ 128,632,000
Liabilities:
Derivative liabilities	3,780,000	2,640,000
U.S. government agencies [Member]
Assets:
Available for sale securities	86,526,000	49,850,000
Corporate debt securities [Member]
Assets:
Available for sale securities	318,167,000	78,782,000
Fair Value, Measurements, Recurring [Member]
Assets:
Money market funds	50,125,000	248,128,000
Derivative assets	10,625,000	6,002,000
Total	537,562,000	561,408,000
Liabilities:
Derivative liabilities	3,780,000	2,640,000
Fair Value, Measurements, Recurring [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	86,526,000	113,314,000
Fair Value, Measurements, Recurring [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	390,286,000	193,964,000
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets:
Money market funds	50,125,000	248,128,000
Derivative assets	0	0
Total	50,125,000	248,128,000
Liabilities:
Derivative liabilities	0	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	6,002,000
Total	487,437,000	313,280,000
Liabilities:
Derivative liabilities	3,780,000	2,640,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. government agencies [Member]
Assets:
Available for sale securities	86,526,000	113,314,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate debt securities [Member]
Assets:
Available for sale securities	390,286,000	193,964,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Assets:
Money market funds	0	0
Derivative assets	0	0
Total	0	0
Liabilities:
Derivative liabilities	0	0
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 - Chang

Fair Value Measurements - Change in fair value (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015USD ($)
Acquisition Contingent Consideration [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at December 31, 2014	$ 0
Addition from acquisition	7,704
Change in fair value	7,704
Balance at December 31, 2015	0
Redeemable Convertible Preferred Stock Warrant [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at December 31, 2014	15,797
Change in fair value	56,655
Settlement of warrant liability upon exercise	(56,678)
Reclassification of unexercised warrants to additional paid in capital upon the IPO	(15,774)
Balance at December 31, 2015	$ 0

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	$ 2,640,000		$ 3,780,000
Adjustments to the fair value of long lived assets	0	$ 0
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	561,408,000		537,562,000
Derivative liabilities	2,640,000		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

Financial Instruments - Narrati

Financial Instruments - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
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
Foreign currency exchange contract [Member] | Not designated as hedging instrument [Member]
Derivative [Line Items]
Derivative, notional amount	177,000,000	104,800,000
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	254,100,000
Derivative, notional amount based on forecasted expenses	$ 20,900,000	$ 254,100,000

Financial Instruments - Amortiz

Financial Instruments - Amortized to fair value (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	$ 301,320	$ 535,846	$ 195,626	$ 81,728
Gross Unrealized Gains	32	4
Gross Unrealized Losses	(219)	(59)
Available for sale securities	404,693	128,632
Cash, cash equivalents, and available-for-sale securities, amortized cost	706,200	664,533
Fair Value	706,013	664,478
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	179,076	109,072
Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	50,125	248,128
U.S. government agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents		63,464
Available-for-sale debt securities, amortized cost	86,533	113,315
Gross Unrealized Gains	8	3
Gross Unrealized Losses	(15)	(4)
Available for sale securities	86,526	49,850
Fair Value	86,526	113,314
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	72,119	115,182
Available-for-sale debt securities, amortized cost	390,466	194,018
Gross Unrealized Gains	24	1
Gross Unrealized Losses	(204)	(55)
Available for sale securities	318,167	78,782
Fair Value	$ 390,286	$ 193,964

Financial Instruments - Contrac

Financial Instruments - Contractual Maturity Dates (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Due in one year	$ 355,152	$ 128,632
Due in one to two years	49,541	0
Available for sale securities	$ 404,693	$ 128,632

Financial Instruments - Financi

Financial Instruments - Financial Position, Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Fair Value Derivative Liabilities	$ 3,780	$ 2,640
Foreign currency exchange contract [Member]
Derivatives, Fair Value [Line Items]
Fair Value Derivative Assets	10,625	6,002
Fair Value Derivative Liabilities	3,780	2,640
Foreign currency exchange contract [Member] | Designated as hedging instrument [Member] | Prepaid expenses and other current assets [Member]
Derivatives, Fair Value [Line Items]
Fair Value Derivative Assets	813	3,116
Fair Value Derivative Liabilities	0	0
Foreign currency exchange contract [Member] | Designated as hedging instrument [Member] | Accrued liabilities [Member]
Derivatives, Fair Value [Line Items]
Fair Value Derivative Assets	0	0
Fair Value Derivative Liabilities	1,428	1,327
Foreign currency exchange contract [Member] | Not designated as hedging instrument [Member] | Prepaid expenses and other current assets [Member]
Derivatives, Fair Value [Line Items]
Fair Value Derivative Assets	9,812	2,886
Fair Value Derivative Liabilities	0	0
Foreign currency exchange contract [Member] | Not designated as hedging instrument [Member] | Accrued liabilities [Member]
Derivatives, Fair Value [Line Items]
Fair Value Derivative Assets	0	0
Fair Value Derivative Liabilities	$ 2,352	$ 1,313

Financial Instruments - Stateme

Financial Instruments - Statement of Operations and Other Comprehensive Income (Details) - Foreign currency exchange contract [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Designated as hedging instrument [Member] | Cash flow hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in OCI—effective portion	$ 8,171	$ 2,785	$ 0
Revenue [Member] | Designated as hedging instrument [Member] | Cash flow hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from OCI into income—effective portion	10,153	2,183	0
Operating expenses [Member] | Designated as hedging instrument [Member] | Cash flow hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) reclassified from OCI into income—effective portion	17	(899)	0
Other expense, net [Member] | Designated as hedging instrument [Member] | Cash flow hedges [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in income—ineffective portion	(1,026)	202	0
Other expense, net [Member] | Not designated as hedging instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) recognized in income	$ 10,916	$ 5,861	$ 211

Financial Instruments - Offsett

Financial Instruments - Offsetting of Foreign Currency Derivative Contracts (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Liabilities	$ 3,780	$ 2,640
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Net Amounts Presented in Condensed Consolidated Balance Sheets	3,780	2,640
Gross Amounts Not Offset in Condensed Consolidated Balance Sheets, Financial Instruments	3,780	2,100
Gross Amounts Not Offset in Condensed Consolidated Balance Sheets, Cash Collateral Pledged	0	0
Net Amount	0	540
Foreign currency exchange contract [Member]
Offsetting Liabilities [Line Items]
Gross Amounts of Recognized Liabilities	3,780	2,640
Offsetting Assets [Line Items]
Gross Amounts of Recognized Assets	10,625	6,002
Gross Amounts Offset in the Condensed Consolidated Balance Sheets	0	0
Net Amounts Presented in Condensed Consolidated Balance Sheets	10,625	6,002
Gross Amounts Not Offset in Condensed Consolidated Balance Sheets, Financial Instruments	3,780	2,100
Gross Amounts Not Offset in Condensed Consolidated Balance Sheets, Cash Collateral Received	0	0
Net Amount	$ 6,845	$ 3,902

Balance Sheet Components - Acco

Balance Sheet Components - Account Receivable and Revenue Reserves (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Allowance for Doubtful Accounts [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balances	$ 1,825	$ 838	$ 743
Increases	339	1,115	864
Write-offs/returns taken	(1,882)	(128)	(769)
Ending balances	282	1,825	838
Revenue Reserve [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balances	74,045	26,559	15,416
Increases	275,815	169,677	42,740
Write-offs/returns taken	(251,009)	(122,191)	(31,597)
Ending balances	$ 98,851	$ 74,045	$ 26,559

Balance Sheet Components - Inve

Balance Sheet Components - Inventories (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Components	$ 1,035	$ 5,359
Finished goods	229,352	172,787
Total inventories	$ 230,387	$ 178,146

Balance Sheet Components - Prep

Balance Sheet Components - Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
POP displays, net	$ 22,804	$ 9,990
Derivative assets	10,625	6,002
Prepaid marketing	5,764	150
Other	27,153	27,388
Total prepaid expenses and other current assets	$ 66,346	$ 43,530

Balance Sheet Components - Prop

Balance Sheet Components - Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 115,889	$ 72,083
Less: Accumulated depreciation and amortization	(39,336)	(27,582)
Property and equipment, net	76,553	44,501
Tooling and manufacturing equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	60,944	53,092
Furniture and office equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	14,424	6,809
Purchased and internally-developed software [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	12,032	3,794
Leasehold improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 28,489	$ 8,388

Balance Sheet Components - Good

Balance Sheet Components - Goodwill Roll forward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Roll Forward]
Beginning balance	$ 22,157	$ 0
Goodwill acquired	28,879	22,562
Subsequent goodwill adjustments		(405)
Ending balance	$ 51,036	$ 22,157

Balance Sheet Components - Inta

Balance Sheet Components - Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Gross	$ 27,370	$ 13,918
Accumulated Amortization	(3,789)	(1,702)
Net	23,581	12,216
Intangible Assets, Gross	31,310	13,918
Intangible assets, net	27,521	12,216
Amortization of intangible assets	2,087	1,702	$ 0
In process research and development [Member]
Indefinite-lived Intangible Assets [Line Items]
Indefinite-Lived Intangible Assets (Excluding Goodwill)	3,940	0
Developed technology [Member]
Finite-Lived Intangible Assets [Line Items]
Gross	26,092	12,640
Accumulated Amortization	(3,247)	(1,442)
Net	$ 22,845	11,198
Weighted Average Remaining Useful Life (years)	4 years 9 months
Trademarks and other [Member]
Finite-Lived Intangible Assets [Line Items]
Gross	$ 1,278	1,278
Accumulated Amortization	(542)	(260)
Net	$ 736	$ 1,018
Weighted Average Remaining Useful Life (years)	3 years 6 months

Balance Sheet Components - Esti

Balance Sheet Components - Estimated Future Amortization Expense (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]
2,017	$ 5,666
2,018	5,506
2,019	4,726
2,020	3,762
2,021	3,557
Thereafter	364
Net	23,581	$ 12,216
Cost of revenue [Member]
Finite-Lived Intangible Assets [Line Items]
2,017	5,436
2,018	5,276
2,019	4,496
2,020	3,716
2,021	3,557
Thereafter	364
Net	22,845
Operating expenses [Member]
Finite-Lived Intangible Assets [Line Items]
2,017	230
2,018	230
2,019	230
2,020	46
2,021	0
Thereafter	0
Net	$ 736

Balance Sheet Components - Accr

Balance Sheet Components - Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Product warranty	$ 99,923	$ 40,212
Accrued manufacturing expense and freight	75,579	10,723
Accrued sales incentives	74,181	24,324
Accrued sales and marketing	41,948	33,389
Accrued co-op advertising and marketing development funds	40,002	29,077
Employee related liabilities	13,934	27,394
Sales taxes and VAT payable	8,891	8,349
Inventory received but not billed	7,363	4,292
Accrued legal fees	3,963	3,138
Derivative liabilities	3,780	2,640
Customer deposits	1,923	2,062
Other	19,074	14,499
Accrued liabilities	$ 390,561	$ 200,099

Balance Sheet Components - Prod

Balance Sheet Components - Product Warranty (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Movement in Standard and Extended Product Warranty, Increase (Decrease) [Roll Forward]
Beginning balances	$ 40,212	$ 20,098	$ 8,480
Charged to cost of revenue	134,771	55,642	19,462
Changes in estimate related to pre-existing warranties	4,073	(8,968)
Settlement of claims	(79,133)	(26,560)	(7,844)
Ending balances	$ 99,923	$ 40,212	$ 20,098

Balance Sheet Components - Rese

Balance Sheet Components - Reserve Accounts (Details) - Product Recall Reserves [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Movement in Valuation Allowances and Reserves [Roll Forward]
Beginning balances	$ 5,122	$ 22,476	$ 82,938
Charged to revenue			8,112
Charged to cost of revenue		(5,755)	11,339
Charged to general and administrative		(1,174)	505
Settlement of claims	(3,869)	(10,425)	(80,418)
Ending balances	$ 1,253	5,122	22,476
Charged to revenues		(5,800)
Charges to general and administrative expenses		(1,200)
General and administrative [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Legal fees			2,900
Legal settlement costs (benefit)		$ (4,400)	$ 500

Balance Sheet Components - Accu

Balance Sheet Components - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 981,451	$ 75,262	$ (63,466)
Other comprehensive income (loss)	(1,669)	654	37
Ending balance	998,532	981,451	75,262
Unrealized Gains (Losses) on Available-for-Sale Investments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	751	0
Other comprehensive income (loss) before reclassifications	9,422	1,276
Amounts reclassified from AOCI	(10,650)	(525)
Other comprehensive income (loss)	(1,228)	751
Ending balance	(477)	751	0
Currency Translation Adjustments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(5)	37
Other comprehensive income (loss) before reclassifications	(309)	(42)
Amounts reclassified from AOCI	0	0
Other comprehensive income (loss)	(309)	(42)
Ending balance	(314)	(5)	37
Unrealized Gains on Cash Flow Hedges [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(55)	0
Other comprehensive income (loss) before reclassifications	(126)	(63)
Amounts reclassified from AOCI	(6)	8
Other comprehensive income (loss)	(132)	(55)
Ending balance	(187)	(55)	0
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	691	37	0
Other comprehensive income (loss) before reclassifications	8,987	1,171
Amounts reclassified from AOCI	(10,656)	(517)
Other comprehensive income (loss)	(1,669)	654	37
Ending balance	$ (978)	$ 691	$ 37

Debt (Details)

Debt (Details)	12 Months Ended
	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Line of Credit Facility [Line Items]
Interest expense	$ 500,000	$ 1,000,000	$ 800,000
Capitalized issuance costs	1,300,000
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]
Line of Credit Facility [Line Items]
Credit facility, basis spread on variable rate		1.00%
Line of Credit [Member] | 2015 Credit Agreement [Member]
Line of Credit Facility [Line Items]
Credit facility, borrowing capacity		$ 250,000,000
Credit facility, amount outstanding	$ 0
Credit facility, minimum fixed charge coverage ratio	1.15
Credit facility, minimum leverage ratio	3
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Credit facility, amount outstanding	$ 38,000,000	17,100,000
Letter of Credit [Member] | 2015 Credit Agreement [Member]
Line of Credit Facility [Line Items]
Credit facility, borrowing capacity		50,000,000
Credit facility, amount outstanding	$ 38,000,000
Swing Line Loan [Member] | 2015 Credit Agreement [Member]
Line of Credit Facility [Line Items]
Credit facility, borrowing capacity		$ 25,000,000
Minimum [Member] | 2015 Credit Agreement [Member]
Line of Credit Facility [Line Items]
Debt Instrument, interest rate, stated percentage		3.25%

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) $ in Millions	May 10, 2016investor	Feb. 16, 2016lawsuit	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Rent expense			$ 29.9	$ 8.4	$ 4.1
Number of plaintiffs | investor	5
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
Purchase Commitment [Member]
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Purchase commitment			$ 158.4

Commitments and Contingencies60

Commitments and Contingencies - Future Minimum Payments (Details) $ in Thousands	Dec. 31, 2016USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2,017	$ 33,867
2,018	41,222
2,019	44,943
2,020	41,176
2,021	40,148
Thereafter	99,004
Total	$ 300,360

Stockholders' Equity - Narrativ

Stockholders' Equity - Narrative (Details) $ / shares in Units, $ in Thousands	1 Months Ended			12 Months Ended
	Nov. 30, 2015USD ($)$ / sharesshares	Jun. 30, 2015USD ($)$ / sharesshares	May 31, 2015	Dec. 31, 2016USD ($)vote$ / sharesshares	Dec. 31, 2015USD ($)classvote$ / sharesshares	Dec. 31, 2014USD ($)$ / shares
Class of Stock [Line Items]
Proceeds from public offerings, net of underwriting discounts and commissions | $	$ 84,400	$ 420,900		$ 0	$ 505,275	$ 0
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
Common stock issued (in shares)					40,875,583
Common stock outstanding (in shares)					40,875,583
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant date fair value (in dollars per share) | $ / shares				$ 14.06	$ 10.67	$ 2.40
Total grant date fair value of options that vested | $				$ 32,900	$ 18,600	$ 1,700
Unrecognized compensation expense related to unvested options | $				$ 49,800
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)				177,212,531	99,416,351
Common stock outstanding (in shares)				177,212,531	99,416,351
Common stock, votes per share | vote				1	1
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)				48,450,746	115,365,222
Common stock outstanding (in shares)				48,450,746	115,365,222
Common stock, votes per share | vote				10	10
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
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to unvested options, estimated weighted average period				2 years 1 month
Stock Options [Member] | Common Class A [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fair value of exercise price (in dollars per share) | $ / shares				$ 7.32
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to unvested options, estimated weighted average period				3 years 2 months
Unrecognized compensation expense related to unvested options, net of estimated forfeitures | $				$ 163,700
IPO [Member] | Common Stock [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
Over-Allotment Option [Member] | Common Stock [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
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] | Stock Options [Member] | Common Class A [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance (in shares)				6,700,000
Annual increase in shares reserved for issuance			5.00%
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
2015 Employee Stock Purchase Plan [Member] | Stock Options [Member] | Common Class A [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual increase in shares reserved for issuance			1.00%
Common stock discount available to employees			15.00%
Common stock, fair value of market value of stock, available to employees, percent			85.00%
Select Stockholders [Member] | IPO [Member] | Common Stock [Member]
Class of Stock [Line Items]
Shares sold by certain shareholders (in shares)		19,700,000
Select Stockholders [Member] | Over-Allotment Option [Member] | Common Stock [Member]
Class of Stock [Line Items]
Shares sold by certain shareholders (in shares)	16,600,000
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
Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock converted and reclassified into Class B Common Stock (in shares)		400,000

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, 2016USD ($)$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Balance, beginning of period (in shares) | shares	44,362
Granted (in shares) | shares	955
Exercised (in shares) | shares	(8,619)
Canceled (in shares) | shares	(2,244)
Balance, end of period (in shares) | shares	34,454
Options exercisable (in shares) | shares	20,654
Options vested and expected to vest (in shares) | shares	34,223
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	$ 3.20
Granted, weighted average exercise price (in dollars per share) | $ / shares	14.06
Exercised, weighted average exercise price (in dollars per share) | $ / shares	1.21
Canceled, weighted average exercise price (in dollars per share) | $ / shares	5.54
Balance, weighted average exercise price, end of period (in dollars per share) | $ / shares	3.85
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	2.51
Options vested and expected to vest, weighted average exercise price (in dollars per share) | $ / shares	$ 3.82
Options outstanding, weighted average remaining contractual term	6 years 10 months 2 days
Options exercisable, weighted average remaining contractual term	6 years 2 months 20 days
Options vested and expected to vest, weighted average remaining contractual term	6 years 10 months
Options exercised, aggregate intrinsic value | $	$ 121,268
Options outstanding, aggregate intrinsic value | $	145,568
Options exercisable, aggregate intrinsic value | $	107,573
Options vested and expected to vest, aggregate intrinsic value | $	$ 145,097

Stockholders' Equity - Restrict

Stockholders' Equity - Restricted Stock Unit Activity (Details) - Restricted Stock Units (RSUs) [Member] shares in Thousands	12 Months Ended
	Dec. 31, 2016$ / 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, 2015 (in shares) | shares	3,292
Granted (in shares) | shares	10,356
Vested (in shares) | shares	(1,117)
Forfeited or canceled (in shares) | shares	(953)
Unvested balance—December 31, 2016 (in shares) | shares	11,578
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, 2015 (in dollars per share) | $ / shares	$ 34.27
Granted (in dollars per share) | $ / shares	13.48
Vested (in dollars per share) | $ / shares	31.50
Forfeited or canceled (in dollars per share) | $ / shares	27.87
Unvested balance—December 31, 2016 (in dollars per share) | $ / shares	$ 16.85

Stockholders' Equity - Stock Co

Stockholders' Equity - Stock Compensation Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 79,432	$ 41,024	$ 6,804
Cost of revenue [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	4,797	4,739	890
Research and development [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	47,207	18,251	2,350
Sales and marketing [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	11,575	7,419	1,295
General and administrative [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 15,853	$ 10,615	$ 2,269

Stockholders' Equity - Valuatio

Stockholders' Equity - Valuation Assumptions (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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	40.70%
Minimum volatility rate		52.10%	54.83%
Maximum volatility rate		56.90%	60.94%
Risk-free interest rate	1.60%
Minimum risk-free interest rate		1.50%	1.73%
Maximum risk-free interest rate		1.90%	2.04%
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
Minimum volatility rate	30.10%	27.70%
Maximum volatility rate	39.00%	35.00%
Risk-free interest rate		0.30%
Minimum risk-free interest rate	0.40%
Maximum risk-free interest rate	0.60%
Dividend yield rate	0.00%	0.00%
Minimum [Member] | Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term		6 months
Maximum [Member] | Employee Stock Purchase Plan [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, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
United States	$ 5,577	$ 286,380	$ 150,137
Foreign	(114,872)	1,569	(10,364)
Income (loss) before income taxes	$ (109,295)	$ 287,949	$ 139,773

Income Taxes - Components of 67

Income Taxes - Components of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current:
Federal	$ 78,025	$ 140,396	$ 47,565
State	9,878	13,307	2,319
Foreign	4,313	1,107	113
Total current	92,216	154,810	49,997
Deferred:
Federal	(86,827)	(33,421)	(39,339)
State	(11,622)	(8,941)	(2,651)
Foreign	(285)	(176)	(11)
Total deferred	(98,734)	(42,538)	(42,001)
Total income tax expense (benefit)	$ (6,518)	$ 112,272	$ 7,996

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Effective Tax to Statutory Federal Rate (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Tax at federal statutory rate	35.00%	35.00%	35.00%
State taxes, net of federal effect	4.30%	1.50%	(0.20%)
Foreign rate differential	(38.90%)	(0.80%)	2.70%
Tax credits	9.00%	(2.00%)	(1.10%)
Domestic production activities deduction	5.00%	(3.30%)	(2.60%)
Stock-based compensation	(4.60%)	1.70%	0.00%
Change in prior year reserves	1.90%	0.00%	0.00%
Out-of-period adjustment	(2.80%)	0.00%	0.00%
Warrant fair value adjustment	0.00%	6.90%	3.30%
Change in valuation allowance	0.00%	0.00%	(32.00%)
Other	(2.90%)	0.00%	0.60%
Effective tax rate	6.00%	39.00%	5.70%

Income Taxes - Tax Effects of T

Income Taxes - Tax Effects of Temporary Differences for Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Net operating losses and credits	$ 9,446	$ 5,225
Fixed assets and intangible assets	16,272	1,227
Accruals and reserves	111,216	54,891
Fitbit Force recall reserve	469	1,910
Stock-based compensation	17,864	9,642
Inventory	8,513	3,477
Other	10,665	7,089
Gross deferred tax assets	174,445	83,461
Debt issuance costs	(353)	(441)
Net deferred tax assets	$ 174,092	$ 83,020

Income Taxes - Reconciliation70

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	$ 23,518	$ 10,594	$ 7,991
Reductions based on tax positions related to prior year	(2,100)	(18)	(418)
Additions based on tax positions related to prior year	2,809	0	0
Additions based on tax positions related to current year	11,357	12,942	3,021
Balance at end of year	$ 35,584	$ 23,518	$ 10,594

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Examination [Line Items]
Income tax expense (benefit)	$ (6,518)	$ 112,272	$ 7,996
Effective income tax rate	6.00%	39.00%	5.70%
Federal operating loss carryforwards	$ 2,200
Foreign net operating loss carryforwards	16,500
Undistributed earnings of foreign subsidiaries	600
Unrecognized tax benefits	35,584	$ 23,518	$ 10,594	$ 7,991
Unrecognized tax benefits, interest on income taxes expense	2,200	$ 600
Domestic Tax Authority [Member]
Income Tax Examination [Line Items]
Deferred tax assets, research credit carryforwards	200
California [Member]
Income Tax Examination [Line Items]
State net operating loss carryforwards	2,200
California [Member] | State and Local Jurisdiction [Member]
Income Tax Examination [Line Items]
Deferred tax assets, research credit carryforwards	16,200
Deferred Tax Assets, Tax Credit Carryforwards, General Business	100
Massachusetts [Member] | State and Local Jurisdiction [Member]
Income Tax Examination [Line Items]
Deferred tax assets, research credit carryforwards	$ 400

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, 2016	Oct. 01, 2016	Jul. 02, 2016	Apr. 02, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net income (loss)	$ (146,273)	$ 26,120	$ 6,341	$ 11,035	$ 64,165	$ 45,834	$ 17,681	$ 47,997	$ (102,777)	$ 175,677	$ 131,777
Less: noncumulative dividends to preferred stockholders									0	(2,526)	(5,326)
Less: undistributed earnings to participating securities									0	(59,133)	(98,103)
Net income (loss) attributable to common stockholders—basic									(102,777)	114,018	28,348
Add: adjustments to undistributed earnings to participating securities									0	8,821	10,175
Net income (loss) attributable to common stockholders—diluted									$ (102,777)	$ 122,839	$ 38,523
Denominator:
Weighted-average shares of common stock— basic for Class A and Class B (in shares)									220,405	129,886	40,351
Effect of dilutive securities (in shares)									0	34,327	20,828
Weighted-average shares of common stock—diluted for Class A and Class B (in shares)									220,405	164,213	61,179
Net income (loss) per share attributable to common stockholders:
Basic (in dollars per share)	$ (0.65)	$ 0.12	$ 0.03	$ 0.05	$ 0.30	$ 0.22	$ 0.09	$ 0.26	$ (0.47)	$ 0.88	$ 0.70
Diluted (in dollars per share)	$ (0.65)	$ 0.11	$ 0.03	$ 0.05	$ 0.26	$ 0.19	$ 0.07	$ 0.22	$ (0.47)	$ 0.75	$ 0.63

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, 2016	Dec. 31, 2015	Dec. 31, 2014
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	46,032	67,961	146,226
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)	34,454	445	4,420
Redeemable convertible preferred stock and stock options to purchase common 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	65,903	139,851
Restricted Stock Units (RSUs) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share, amount (in shares)	11,578	692	0
Redeemable Convertible Preferred Stock Warrant [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	921	1,955

Significant Customer Informat74

Significant Customer Information and Other Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Oct. 01, 2016	Jul. 02, 2016	Apr. 02, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Concentration Risk [Line Items]
Revenue	$ 573,775	$ 503,802	$ 586,528	$ 505,356	$ 711,570	$ 409,262	$ 400,412	$ 336,754	$ 2,169,461	$ 1,857,998	$ 745,433
Geographic Concentration Risk [Member] | Non-US [Member]
Concentration Risk [Line Items]
Long-lived assets including property and equipment	$ 30,100				$ 28,900				$ 30,100	$ 28,900
Revenue [Member] | Customer Concentration Risk [Member] | Customer A [Member]
Concentration Risk [Line Items]
Concentration risk, percentage									14.00%	15.00%	13.00%
Revenue [Member] | Customer Concentration Risk [Member] | Customer B [Member]
Concentration Risk [Line Items]
Concentration risk, percentage									10.00%	14.00%	12.00%
Revenue [Member] | Customer Concentration Risk [Member] | Customer C [Member]
Concentration Risk [Line Items]
Concentration risk, percentage									14.00%	14.00%	11.00%
Revenue [Member] | Geographic Concentration Risk [Member]
Concentration Risk [Line Items]
Revenue									$ 2,169,461	$ 1,857,998	$ 745,433
Revenue [Member] | Geographic Concentration Risk [Member] | United States [Member]
Concentration Risk [Line Items]
Revenue									1,539,600	1,381,152	562,553
Revenue [Member] | Geographic Concentration Risk [Member] | Americas excluding United States [Member]
Concentration Risk [Line Items]
Revenue									110,111	92,252	38,576
Revenue [Member] | Geographic Concentration Risk [Member] | Europe, Middle East, and Africa [Member]
Concentration Risk [Line Items]
Revenue									389,154	208,767	60,699
Revenue [Member] | Geographic Concentration Risk [Member] | APAC [Member]
Concentration Risk [Line Items]
Revenue									$ 130,596	$ 175,827	$ 83,605
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Customer A [Member]
Concentration Risk [Line Items]
Concentration risk, percentage									16.00%	15.00%
Accounts Receivable [Member] | Customer Concentration Risk [Member] | Customer B [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									19.00%	23.00%
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, 2016	Dec. 31, 2015	Dec. 31, 2014	Mar. 13, 2015
Business Acquisition [Line Items]
Goodwill	$ 51,036,000			$ 51,036,000	$ 22,157,000	$ 0
Change in contingent consideration				0	$ (7,704,000)	$ 0
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] | 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
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

Acquisitions - Assets Acquired

Acquisitions - Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Mar. 13, 2015	Dec. 31, 2014
Business Acquisition [Line Items]
Goodwill	$ 51,036	$ 22,157		$ 0
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	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Oct. 01, 2016	Jul. 02, 2016	Apr. 02, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 573,775	$ 503,802	$ 586,528	$ 505,356	$ 711,570	$ 409,262	$ 400,412	$ 336,754	$ 2,169,461	$ 1,857,998	$ 745,433
Gross profit	126,502	240,658	244,969	233,755	348,299	196,013	187,542	169,209	845,884	901,063	357,657
Net income (loss)	$ (146,273)	$ 26,120	$ 6,341	$ 11,035	$ 64,165	$ 45,834	$ 17,681	$ 47,997	$ (102,777)	$ 175,677	$ 131,777
Net income (loss) per share attributable to common stockholders—basic (in dollars per share)	$ (0.65)	$ 0.12	$ 0.03	$ 0.05	$ 0.30	$ 0.22	$ 0.09	$ 0.26	$ (0.47)	$ 0.88	$ 0.70
Net income (loss) per share attributable to common stockholders—diluted (in dollars per share)	$ (0.65)	$ 0.11	$ 0.03	$ 0.05	$ 0.26	$ 0.19	$ 0.07	$ 0.22	$ (0.47)	$ 0.75	$ 0.63

Subsequent Events (Details)

Subsequent Events (Details) - Employee Severance [Member] $ in Millions	6 Months Ended
	Jun. 30, 2017employee	Jan. 31, 2017USD ($)
Forecast [Member]
Subsequent Event [Line Items]
Number of positions eliminated in reorganization | employee	107
Percentage of workforce reduction in reorganization	6.00%
Subsequent Event [Member]
Subsequent Event [Line Items]
Reorganization costs expected costs expected in first quarter of 2017 | $		$ 3.7