Balance Sheet Components (Tables) - USD ($)
$ in Thousands

3 Months Ended

Mar. 31, 2018

Apr. 01, 2017

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Schedule of deferred revenue

Changes in the total short-term and long-term deferred revenue balance were as follows (in thousands):

	March 31, 2018
Beginning balances	42,432
Deferral of revenue	7,380
Recognition of deferred revenue	(13,364		)
Ending balances	$	36,448

Schedule of accounts receivable reserves

Revenue returns reserve activities were as follows (in thousands):

	Three Months Ended
	March 31, 2018			April 1, 2017
Beginning balances	$	109,872		$	98,851
Increases	26,073			41,626
Write-offs/Returns taken	(53,333		)	(72,160		)
Ending balances	$	82,612		$	68,317

Schedule of inventories

Inventories consisted of the following (in thousands):

	March 31, 2018		December 31, 2017
Components	5,006		3,825
Finished goods	140,367		120,070
Total inventories	$	145,373	$	123,895

Schedule of prepaid expenses and other current asset

Prepaid expenses and other current assets consisted of the following (in thousands):

	March 31, 2018		December 31, 2017
POP displays, net	$	12,767	$	14,750
Prepaid marketing	9,103		6,074
Derivative assets	833		—
Prepaid expenses	14,070		24,204
Insurance receivable	900		37,300
Other	21,436		14,941
Total prepaid expenses and other current assets	$	59,109	$	97,269

Schedule of property and equipment

Property and equipment, net, consisted of the following (in thousands):

	March 31, 2018			December 31, 2017
Tooling and manufacturing equipment	$	65,202		$	66,854
Furniture and office equipment	22,327			20,942
Purchased and internally-developed software	18,957			18,112
Leasehold improvements	63,374			58,431
Total property and equipment	169,860			164,339
Less: Accumulated depreciation and amortization	(65,330		)	(59,431		)
Property and equipment, net	$	104,530		$	104,908

Schedule of goodwill

The increase in the carrying amount of goodwill during the three months ended March 31, 2018 was attributable to an acquisition in February 2018. See Note 11 for additional information.

	Goodwill
Balance at December 31, 2017	$	51,036
Goodwill acquired	9,893
Balance at March 31, 2018	$	60,929

Net change, net of tax

$ (326)

$ 67

Schedule of intangible assets (excluding goodwill)

The increase in the carrying amount of goodwill during the three months ended March 31, 2018 was attributable to an acquisition in February 2018. See Note 11 for additional information.

	March 31, 2018							December 31, 2017							Weighted Average Remaining Useful Life (years)
	Gross		Accumulated Amortization			Net		Gross		Accumulated Amortization			Net
Developed technology	$	35,988	$	(10,394	)	$	25,594	$	30,588	$	(8,738	)	$	21,850	3.8
Customer relationships	3,918		(164		)	3,754		—		—			—		6.9
Trademarks and other	1,150		(701		)	449		1,278		(772		)	506		0.8
Total intangible assets, net	$	41,056	$	(11,259	)	$	29,797	$	31,866	$	(9,510	)	$	22,356

Schedule of estimated future amortization expense

The estimated future amortization expense of acquired finite-lived intangible assets to be charged to cost of revenue and operating expenses after March 31, 2018 was as follows (in thousands):

	Cost of Revenue		Operating Expenses		Total
Remaining 2018	$	5,602	$	579	$	6,181
2019	6,690		771		7,461
2020	5,910		588		6,498
2021	5,910		541		6,451
2022	1,236		541		1,777
Thereafter	246		1,183		1,429
Total finite-lived intangible assets, net	$	25,594	$	4,203	$	29,797

Schedule of accrued liabilities

Accrued liabilities consisted of the following (in thousands):

	March 31, 2018		December 31, 2017
Product warranty	$	71,975	$	87,882
Accrued revenue reserve from returns	82,612		—
Accrued manufacturing expense and freight	31,962		41,901
Accrued sales incentives	71,175		111,592
Accrued sales and marketing	21,741		44,401
Accrued research and development	5,276		8,983
Accrued co-op advertising and marketing development funds	21,688		30,408
Employee-related liabilities	18,204		33,266
Sales taxes and VAT payable	22,447		21,340
Inventory received but not billed	8,935		10,526
Accrued legal settlements and fees	8,365		36,693
Derivative liabilities	430		2,138
Other	17,574		23,007
Accrued liabilities	$	382,384	$	452,137

Schedule of product warranty reserves

Product warranty reserve activities were as follows (in thousands)⁽¹⁾:

	Three Months Ended
	March 31, 2018			April 1, 2017
Beginning balances	$	87,882		$	99,923
Charged to cost of revenue	(2,481		)	18,937
Changes related to pre-existing warranties	(3,402		)	(2,440		)
Settlement of claims	(10,024		)	(25,961		)
Ending balances	$	71,975		$	90,459

⁽¹⁾  Does not include reserves established as a result of the recall of the Fitbit Force. See the section titled “—Fitbit Force Recall Reserve” in the Company’s Annual Report on Form 10-K for additional information regarding such reserves.

Schedule of accumulated other comprehensive income

The components and activity of accumulated other comprehensive income (“AOCI”), net of tax, were as follows (in thousands):

	Unrealized Gains (Losses) on Cash Flow Hedges		Currency Translation Adjustments		Unrealized Gains (Losses) on Available-for-Sale Investments			Total
Balance at December 31, 2017	$	66	$	—	$	(75	)	$	(9	)
Other comprehensive income (loss) before reclassifications	664		—		(326		)	338
Amounts reclassified from AOCI	—		—		—			—
Other comprehensive income (loss)	664		—		(326		)	338
Balance at March 31, 2018	$	730	$	—	$	(401	)	$	329