Wayfair (W) 10-K 25 Feb 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2018	Feb. 18, 2019	Jun. 30, 2018
Entity Registrant Name	Wayfair Inc.
Entity Central Index Key	1,616,707
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Entity Public Float			$ 6.7
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Class A common stock
Entity Common Stock, Shares Outstanding		63,032,847
Class B common stock
Entity Common Stock, Shares Outstanding		28,070,781

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets
Cash and cash equivalents	$ 849,461	$ 558,960
Short-term investments	114,278	61,032
Accounts receivable, net of allowance of $9,312 and $7,000 at December 31, 2018 and December 31, 2017, respectively	50,603	37,948
Inventories	46,164	28,042
Prepaid expenses and other current assets	195,430	130,838
Total current assets	1,255,936	816,820
Property and equipment, net	606,977	361,141
Goodwill and intangible assets, net	2,585	3,105
Long-term investments	6,526	21,561
Other noncurrent assets	18,826	10,776
Total assets	1,890,850	1,213,403
Current liabilities
Accounts payable	650,174	440,366
Accrued expenses	212,997	120,247
Deferred revenue	148,057	94,116
Other current liabilities	127,995	85,026
Total current liabilities	1,139,223	739,755
Lease financing obligations, net of current portion	183,056	82,580
Long-term debt	738,904	332,905
Other liabilities	160,388	106,492
Total liabilities	2,221,571	1,261,732
Commitments and contingencies (Note 7)
Convertible preferred stock, $0.001 par value per share: 10,000,000 shares authorized and none issued at December 31, 2018 and December 31, 2017	0	0
Stockholders’ deficit:
Additional paid-in capital	753,657	537,212
Accumulated deficit	(1,082,689)	(583,266)
Accumulated other comprehensive loss	(1,780)	(2,363)
Total stockholders' deficit	(330,721)	(48,329)
Total liabilities and stockholders' deficit	1,890,850	1,213,403
Class A common stock
Stockholders’ deficit:
Common stock	63	57
Class B common stock
Stockholders’ deficit:
Common stock	$ 28	$ 31

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accounts receivable allowance	$ 9,312	$ 7,000
Convertible redeemable preferred units, par value (in dollars per share)	$ 0.001	$ 0.001
Convertible redeemable preferred units, shares authorized (in shares)	10,000,000	10,000,000
Convertible redeemable preferred units, shares issued (in shares)	0	0
Class A common stock
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	500,000,000	500,000,000
Common stock, shares issued (in shares)	62,329,701	57,398,983
Common stock, shares outstanding (in shares)	62,329,701	57,398,983
Class B common stock
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	164,000,000	164,000,000
Common stock, shares issued (in shares)	28,417,882	30,809,627
Common stock, shares outstanding (in shares)	28,417,882	30,809,627

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Net revenue	$ 6,779,174	$ 4,720,895	$ 3,380,360
Cost of goods sold	5,192,451	3,602,072	2,572,549
Gross profit	1,586,723	1,118,823	807,811
Operating expenses:
Customer service and merchant fees	260,046	169,516	127,883
Advertising	774,189	549,959	409,125
Selling, operations, technology, general and administrative	1,025,767	634,801	467,020
Total operating expenses	2,060,002	1,354,276	1,004,028
Loss from operations	(473,279)	(235,453)	(196,217)
Interest (expense) income, net	(28,560)	(9,433)	694
Other (expense) income, net	(204)	758	1,756
Loss before income taxes	(502,043)	(244,128)	(193,767)
Provision for income taxes	2,037	486	608
Net loss	$ (504,080)	$ (244,614)	$ (194,375)
Net loss per share, basic and diluted (in dollars per share)	$ (5.63)	$ (2.81)	$ (2.29)
Weighted average number of common stock outstanding used in computing per share amounts, basic and diluted (in shares)	89,472	86,983	84,977

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net loss	$ (504,080)	$ (244,614)	$ (194,375)
Other comprehensive loss:
Foreign currency translation adjustments	553	(2,196)	(102)
Net unrealized gain (loss) on available-for-sale investments	30	(180)	251
Comprehensive loss	$ (503,497)	$ (246,990)	$ (194,226)

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) shares in Thousands, $ in Thousands	Total	Class A and Class B Common Stock	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive (Loss) Income
Balance at Dec. 31, 2015	$ 242,545	$ 84	$ 378,162	$ (135,565)	$ (136)
Balance (in shares) at Dec. 31, 2015		84,311
Increase (Decrease) in Stockholders' Equity
Net loss	(194,375)			(194,375)
Other comprehensive income	149				149
Exercise of options to purchase common stock	209	$ 1	208
Exercise of options to purchase common stock (in shares)		70
Issuance of common stock upon vesting of RSUs	2	$ 2
Issuance of common stock upon vesting of RSUs (in shares)		1,963
Shares withheld related to net settlement of RSUs	(21,092)	$ (1)	(21,091)
Shares withheld related to net settlement of RSUs (in shares)		(525)
Equity compensation expense	51,494		51,494
Acquisition of a business	452		452
Acquisition of a business (in shares)		12
Balance at Dec. 31, 2016	79,384	$ 86	409,225	(329,940)	13
Balance (in shares) at Dec. 31, 2016		85,831
Increase (Decrease) in Stockholders' Equity
Net loss	(244,614)			(244,614)
Other comprehensive income	(2,376)				(2,376)
Exercise of options to purchase common stock	244	$ 0	244
Exercise of options to purchase common stock (in shares)		84
Issuance of common stock upon vesting of RSUs	2	$ 2
Issuance of common stock upon vesting of RSUs (in shares)		2,327
Shares withheld related to net settlement of RSUs	(1,562)	$ 0	(1,562)
Shares withheld related to net settlement of RSUs (in shares)		(33)
Equity compensation expense	71,380		71,380
Equity component of issuance of convertible notes, net (Note 14)	49,213		49,213
Balance at Dec. 31, 2017	(48,329)	$ 88	537,212	(583,266)	(2,363)
Balance (in shares) at Dec. 31, 2017		88,209
Increase (Decrease) in Stockholders' Equity
Adoption of ASU			8,712
Adoption of ASU | ASU 2016-09	0			(8,712)
Net loss	(504,080)			(504,080)
Other comprehensive income	583				583
Exercise of options to purchase common stock	138		138
Exercise of options to purchase common stock (in shares)		46
Issuance of common stock upon vesting of RSUs	3	$ 3
Issuance of common stock upon vesting of RSUs (in shares)		2,504
Shares withheld related to net settlement of RSUs	(1,284)		(1,284)
Shares withheld related to net settlement of RSUs (in shares)		11
Equity compensation expense	133,638		133,638
Equity component of issuance of convertible notes, net (Note 14)	83,953		83,953
Balance at Dec. 31, 2018	(330,721)	$ 91	$ 753,657	(1,082,689)	$ (1,780)
Balance (in shares) at Dec. 31, 2018		90,748
Increase (Decrease) in Stockholders' Equity
Adoption of ASU | ASU 2014-09	$ 4,657			$ 4,657

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net loss	$ (504,080)	$ (244,614)	$ (194,375)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	123,542	87,020	55,572
Equity based compensation	127,564	67,840	49,402
Amortization of discount and issuance costs on convertible notes	22,585	5,830	0
Other non-cash adjustments	(56)	1,198	331
Changes in operating assets and liabilities:
Accounts receivable	(12,792)	(18,172)	(9,217)
Inventories	(18,319)	(9,454)	1,351
Prepaid expenses and other current assets	(65,195)	(39,124)	(16,179)
Accounts payable and accrued expenses	285,064	104,184	126,013
Deferred revenue and other liabilities	134,705	81,354	51,914
Other assets	(8,157)	(2,428)	(1,998)
Net cash provided by operating activities	84,861	33,634	62,814
Cash flows from investing activities
Purchase of short-term and long-term investments	(99,002)	(54,551)	(88,112)
Sale and maturities of short-term investments	61,068	71,095	119,810
Purchase of property and equipment	(159,205)	(100,451)	(96,707)
Site and software development costs	(62,750)	(46,428)	(31,379)
Cash received from the sale of a business, net of cash sold	0	0	1,508
Other investing activities, net	(398)	0	(1,000)
Net cash used in investing activities	(260,287)	(130,335)	(95,880)
Cash flows from financing activities
Proceeds from issuance of convertible notes, net of issuance costs	562,047	420,449	0
Premiums paid for capped call confirmations	(93,438)	(44,160)	0
Taxes paid related to net share settlement of equity awards	(1,284)	(1,562)	(21,092)
Net proceeds from exercise of stock options	138	244	209
Net cash provided by (used in) financing activities	467,463	374,971	(20,883)
Effect of exchange rate changes on cash and cash equivalents	(1,536)	850	(387)
Net increase (decrease) in cash and cash equivalents	290,501	279,120	(54,336)
Cash and cash equivalents
Beginning of year	558,960	279,840	334,176
End of year	849,461	558,960	279,840
Supplemental Cash Flow Information
Cash paid for interest on long-term debt	1,554	0	0
Cash paid for interest on finance lease obligations	9,058	0	0
Purchase of property and equipment included in accounts payable and accrued expenses and in other liabilities	15,383	8,533	1,336
Construction costs capitalized under finance lease obligations and other leases	$ 125,796	$ 47,276	$ 53,894

Basis of Presentation

Basis of Presentation	12 Months Ended
	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation Wayfair Inc. (the "Company") is one of the world's largest online destinations for the home. Through its e-commerce business model, the Company offers visually inspired browsing, compelling merchandising, easy product discovery and attractive prices for over fourteen million products from over 11,000 suppliers.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	Summary of Significant Accounting Policies Principles of Consolidation The accompanying consolidated financial statements of Wayfair Inc. include its wholly owned subsidiaries including the accounts of Wayfair LLC and its wholly owned subsidiaries (collectively the "Company" or "Wayfair"). All intercompany accounts and transactions have been eliminated. Below is a summary of the wholly-owned subsidiaries of the Company with operations: Subsidiary Location Wayfair LLC U.S. Wayfair Securities Corporation U.S. SK Retail, Inc. U.S. CastleGate Logistics Inc. U.S. CastleGate Logistics Hong Kong Limited Hong Kong CastleGate Logistics Canada Inc. Canada Wayfair Maine LLC U.S. Wayfair Transportation LLC U.S. Wayfair Stores Limited Republic of Ireland Wayfair (UK) Limited United Kingdom Wayfair GmbH Germany Wayfair (BVI) Ltd. British Virgin Islands Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the U.S. ("GAAP") requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, and disclosure of contingent assets and liabilities, at the date of the financial statements, and the reported amounts of revenue and expenses during the reported period. On an ongoing basis, management evaluates these estimates and judgments, including those related to revenue recognition, capitalization of site and software development costs, stock-based compensation, and inventory. Actual results could differ from those estimates. Cash and Cash Equivalents The Company considers all highly liquid investments purchased with an original maturity (at the date of purchase) of three months or less to be the equivalent of cash for the purpose of consolidated balance sheets and statements of cash flows presentation. Cash equivalents, which consist primarily of money market accounts, are carried at cost, which approximates market value. Accounts Receivable Accounts receivable are stated net of an allowance for doubtful accounts, which is based on historical losses, existing economic conditions, and other information available at the consolidated balance sheets dates. Uncollectible amounts are written off against the allowance after all collection efforts have been exhausted. Short-Term Investments and Marketable Securities Short-term investments consist of certificates of deposits and marketable securities with original maturities of greater than three months and maturing in less than twelve months from the balance sheet date. The Company classifies its marketable securities as "available-for-sale" securities. Available-for-sale securities are classified as short-term investments and long-term investments on the consolidated balance sheets and are carried at fair value. Unrealized gains and losses on available-for-sale securities that are considered temporary are recorded, net of taxes, in the "Accumulated other comprehensive loss" caption of the Company’s consolidated balance sheets. Unrealized losses, excluding losses related to the credit rating of the security (credit losses), on available-for-sale securities that are considered other-than-temporary but relate to securities that the Company (i) does not intend to sell and (ii) will not be required to sell below cost are also recorded, net of taxes, in "Accumulated other comprehensive loss." Further, the Company does not believe it will be required to sell such securities below cost. Therefore, the only other-than-temporary losses the Company records in "Other income, net" in its consolidated statements of operations are related to credit losses. As of December 31, 2018 and 2017 , the Company’s available-for-sale securities consisted of investment securities. The maturities of the Company’s long-term marketable securities generally range from one to two years. As of December 31, 2018 and 2017 , the Company's available-for-sale securities primarily consisted of corporate bonds and other government obligations that are priced at fair value. The cost basis of a marketable security sold is determined by the Company using the specific identification method. Fair Value of Financial Instruments The Company's financial assets and liabilities are measured at fair value, which 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 measurement date (exit price). The three levels of inputs used to measure fair value are as follows: • Level 1—Unadjusted quoted prices in active markets for identical assets or liabilities • Level 2—Unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or inputs other than quoted prices that are observable or can be corroborated by observable market data for substantially the full-term of the asset or liability • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the asset or liability This hierarchy requires the Company to use observable market data, when available, and to minimize the use of unobservable inputs when determining fair value. The Company measures its cash equivalents and short-term and long-term investments at fair value. The Company classifies its cash equivalents within Level 1 because the Company values these investments using quoted market prices. The fair value of the Company's Level 1 financial assets is based on quoted market prices of the identical underlying security. The Company classifies short-term and long-term investments within Level 2 because unadjusted quoted prices for identical or similar assets in markets are not active. The Company does not have any assets or liabilities classified as Level 3 financial assets. Refer to Note 3, Fair Value Measurements , for additional detail. Concentrations of Credit Risk Financial instruments that subject the Company to credit risk consist of cash and cash equivalents, short-term and long-term investments, and accounts receivable. The risk with respect to cash and cash equivalents and short-term and long-term investments is minimized by the Company's policy of investing in financial instruments (i.e., cash equivalents) with near-term maturities issued by highly rated financial institutions. At times, these balances may exceed federally insured limits; however, to date, the Company has not incurred any losses on these investments. As of December 31, 2018 and 2017 , the Company had $129.5 million and $63.4 million , respectively, in banks located outside the U.S. The risk with respect to accounts receivable is managed by the Company through its policy of monitoring the creditworthiness of its customers to which it grants credit terms in the normal course of business. Leases The Company leases office space in several countries under non-cancelable lease agreements. The Company generally leases its office facilities under operating lease agreements. Office facilities subject to an operating lease and the related lease payments are not recorded on the balance sheet. The terms of certain lease agreements provide for rental payments on a graduated basis, however, the Company recognizes rent expense on a straight-line basis over the lease period in accordance with authoritative accounting guidance. Any lease incentives are recognized as reductions of rental expense on a straight-line basis over the term of the lease. The lease term begins on the date the Company becomes legally obligated for the rent payments or when it takes possession of the office space, whichever is earlier. The Company establishes assets and liabilities for the estimated construction costs incurred under lease arrangements where the Company is considered the owner for accounting purposes only, or build-to-suit leases, to the extent the Company is involved in the construction of structural improvements or take construction risk prior to commencement of a lease. Upon occupancy of facilities under build-to-suit leases, the Company assesses whether these arrangements qualify for sales recognition under the sale-leaseback accounting guidance. If the Company continues to be the deemed owner, the facilities are accounted for as financing leases. Refer to Note 7, Commitments and Contingencies , for additional detail. Foreign Currency Translation The functional currency of the Company is the U.S. dollar, while the functional currency of certain wholly-owned subsidiaries outside of the U.S. is as follows: Subsidiary Currency Wayfair Stores Limited Euro Wayfair (UK) Limited Pound sterling Wayfair GmbH Euro Wayfair (BVI) Ltd. Euro CastleGate Logistics Canada Inc. Canadian dollar CastleGate Logistics Hong Kong Limited Hong Kong dollar The financial statements of the Company are translated to U.S. dollars using year-end exchange rates as to assets and liabilities and average exchange rates as to revenue and expenses. Capital accounts are translated at their historical exchange rates when the capital transaction occurred. The effects of foreign currency translation are included in other comprehensive loss in the consolidated statements of comprehensive loss. Transaction gains and losses are included in the Company's consolidated statements of operations. Translation adjustments arising from the use of differing exchange rates from period to period are included in accumulated other comprehensive loss within total stockholders' deficit. Inventories Inventories consisting of finished goods are stated at the lower of cost or net realizable value, determined by the first-in, first-out (FIFO) method, and consist of product for resale. Inventory costs consist of cost of product and inbound shipping and handling costs. Inventory costs also include direct and indirect labor costs, rents and depreciation expenses associated with the Company's fulfillment centers. Inventory valuation requires the Company to make judgments, based on currently available information, about the likely method of disposition, such as through sales to individual customers, liquidations, and expected recoverable values of each disposition category. Goods In-Transit Goods in-transit to customers are recorded in prepaid expenses and other current assets. Risk of loss and the transfer of title from the supplier to the Company occur at freight on board shipping point. As of December 31, 2018 and 2017 , goods in-transit amounted to $83.7 million and $54.5 million , respectively. Property and Equipment Property and equipment are stated at cost, net of depreciation and amortization. Depreciation and amortization on property and equipment is calculated on the straight-line method over the estimated useful lives of the assets as follows: Class Range of Life (In Years) Furniture and computer equipment 3 to 7 Site and software development costs 2 Leasehold improvements The lesser of useful life or lease term Building (leased - Note 7) 30 Site and Software Development Costs The Company capitalizes certain costs associated with the development of its sites and internal-use software products after the preliminary project stage is complete and until the software is ready for its intended use. The capitalized costs are amortized over a two-year period. Costs incurred in the preliminary stages of development, after the software is ready for its intended use and for maintenance of internal-use software are expensed as incurred. Upgrade and enhancements are capitalized to the extent they will result in added functionality. Total costs capitalized, net of accumulated amortization, totaled $62.9 million and $45.4 million as of December 31, 2018 and 2017 , respectively, and are included in "Property and equipment, net" in the consolidated balance sheets. Amortization expense for the years ended December 31, 2018 , 2017 , and 2016 were $51.3 million , $34.5 million , and $21.6 million , respectively. Long-Lived Assets The Company reviews long-lived assets for impairment whenever events or changes in circumstances, such as service discontinuance or technological obsolescence, indicate that the carrying amount of the long-lived asset may not be recoverable. When such events occur, the Company compares the carrying amount of the asset to the undiscounted expected future cash flows related to the asset. If the comparison indicates that an impairment exists, the amount of the impairment is calculated as the difference between the excess of the carrying amount over the fair value of the asset. If a readily determinable market price does not exist, fair value is estimated using discounted expected cash flows attributable to the asset. For the years ended December 31, 2018 , 2017 , and 2016 , no impairment of long-lived assets or identifiable intangibles had been indicated. Contingent Liabilities The Company has certain contingent liabilities that arise in the ordinary course of business activities. The Company accrues for loss contingencies when losses become probable and are reasonably estimable. If the reasonable estimate of the loss is a range and no amount within the range is a better estimate, the minimum amount of the range is recorded as a liability. The Company does not accrue for contingent losses that, in its judgment, are considered to be reasonably possible, but not probable; however, it discloses the range of such reasonably possible losses. Revenue Recognition The Company adopted ASU No. 2014-09, "Revenue from Contracts with Customers" ("ASU 2014-09") as of January 1, 2018. The company primarily generated revenue through product sales on its five distinct sites ("Direct Retail" net revenue) and through (i) product sales on websites operated by third parties and (ii) fees earned for media solutions (collectively, "Other" net revenue). The Company recognizes net revenue on product sales through the Company's five distinct sites and third party operated websites using the gross method when the Company has concluded it controls the product before it is transferred to the customer. The Company controls products when it is the entity responsible for fulfilling the promise to the customer and takes responsibility for the acceptability of the goods, assumes inventory risk from shipment through the delivery date, has discretion in establishing prices, and selects the suppliers of products sold. The Company recognizes net revenue from sales of its products upon delivery to the customer. As the Company ships a large volume of packages through multiple carriers, actual delivery dates may not always be available and as such the Company estimates delivery dates based on historical data. Net revenue from product sales includes shipping costs charged to the customer and is recorded net of taxes collected from customers, which are remitted to governmental authorities. Cash discounts and rebates earned by customers at the time of purchase are deducted from gross revenue in determining net revenue. Allowances for sales returns are estimated and recorded based on prior returns history, recent trends, and projections for returns on sales in the current period. Allowances for sales returns at December 31, 2018 and 2017, were $35.7 million and $21.2 million , respectively. Actual returns in subsequent periods have been consistent with estimated amounts. The Company also earns revenue through third-party advertisers that pay based on the number of advertisement related clicks, actions, or impressions for advertisements placed on the Company's sites. Revenue earned under these arrangements is included in net revenue and is recognized in the period in which the click, action, or impression occurs. Net revenue from contracts with customers is disaggregated by Direct Retail and Other net revenue and by geographic region because this manner of disaggregation best depicts how the nature, amount, timing, and uncertainty of revenue and cash flows are affected by economic factors. Refer to Note 11, Segment and Geographic Information , for additional detail. The Company has three types of contractual liabilities: (i) cash collections from its customers prior to delivery of products purchased, which are initially recorded in "Deferred revenue," and are recognized as net revenue when the products are delivered, (ii) unredeemed gift cards and site credits, which are initially recorded in "Deferred revenue," and are recognized in the period they are redeemed. Subject to requirements to remit balances to governmental agencies, certain gift cards and site credits not expected to be redeemed, also known as "breakage," are recognized as net revenue based on the historical redemption pattern, which is substantially within twenty-four months from the date of issuance, and (iii) membership rewards redeemable for future purchases, which are earned by customers on purchases made with the Company's Wayfair branded, private label credit card, and are initially recorded in "Other current liabilities," and are recognized as net revenue when redeemed. Contractual liabilities included in "Deferred revenue" and "Other current liabilities" in the consolidated balance sheets were $148.1 million and $3.1 million at December 31, 2018 and $94.1 million and $2.6 million at December 31, 2017, respectively. During the year ended December 31, 2018, the Company recognized $77.3 million and $2.5 million of net revenue included in "Deferred revenue" and "Other current liabilities," respectively, at December 31, 2017. The Company adopted ASU 2014-09 as of January 1, 2018 using a modified retrospective approach and recognized a $4.7 million cumulative-effect adjustment to reduced "Accumulated deficit." The cumulative-effect adjustment to "Accumulated deficit" was due to breakage of gift cards and site credits, to the extent there is no requirement for remitting balances to governmental agencies. Prior period balances were not retrospectively adjusted. The adoption of ASU 2014-09 did not have a material impact on the Company's balance sheet and financial results for the year ended December 31, 2018. Vendor Rebates The Company earns rebates on incentive programs with its suppliers. These rebates are earned upon shipment of goods. Amounts earned and due from suppliers under these rebate programs are included in other current assets on the consolidated balance sheets and are reflected as a reduction of cost of goods sold on the consolidated statements of operations. Vendor allowances received by the Company reduce the carrying cost of inventory and are recognized in cost of goods sold when the related inventory is sold. Costs of Goods Sold Cost of goods sold consists of the cost of product sold to customers, shipping and handling costs and shipping supplies and fulfillment costs. Fulfillment costs include costs incurred in operating and staffing the fulfillment centers, such as costs attributed to receiving, inspecting, picking, packaging and preparing customer orders for shipment. Cost of goods sold also includes direct and indirect labor costs, including equity-based compensation, for fulfillment center oversight, including payroll and related benefit costs. The Company also performs logistics services for suppliers through its CastleGate and Wayfair Delivery Network solutions, which are earned upon completion of preparing customer orders for shipment and are reflected as a reduction of cost of goods sold on the consolidated statements of operations. Advertising Costs Advertising consists of direct response performance marketing costs, such as display advertising, paid search advertising, social media advertising, search engine optimization, comparison shopping engine advertising, television advertising, direct mail, catalog and print advertising. Expenditures for advertising are expensed in the period that the advertising first takes place. Advertising expense amounted to $ 774.2 million , $550.0 million , and $ 409.1 million in the years ended December 31, 2018 , 2017 , and 2016 , respectively. Included in prepaid expenses at December 31, 2018 and 2017 are approximately $0.7 million and $0.6 million , respectively, of prepaid advertising costs. Merchant Processing Fees Merchant processing fees totaling $133.4 million , $88.7 million , and $66.0 million in the years ended December 31, 2018 , 2017 , and 2016 , respectively, are included in customer service and merchant fees expense in the consolidated statements of operations. These fees are charged by third parties that provide merchant processing services for customer payments made by credit cards and debit cards. Retail Partner Fees The Company sells its products through websites owned and operated by third-party online retailers, or retail partners. The Company pays a fee for sales generated through these websites and records them as merchant processing fees and advertising costs. Retail partner fees included in merchant processing fees are $1.2 million , $1.5 million , and $1.9 million for the years ended December 31, 2018 , 2017 , and 2016 , respectively. Retail partner fees included in advertising costs are $5.5 million , $7.0 million , and $11.0 million for the years ended December 31, 2018 , 2017 , and 2016 , respectively. Equity-Based Compensation The Company accounts for its equity-based compensation awards in accordance with ASC Topic 718, Compensation—Stock Compensation ("ASC 718"). ASC 718 requires all equity-based payments to employees to be recognized as expense in the statements of operations based on their grant date fair values. The Company has granted stock options, restricted shares and restricted stock units. The Company has primarily granted restricted stock units, and to a lesser extent, restricted stock. The Company granted only restricted stock units to employees in the years ended December 31, 2018 , 2017 , and 2016 . Restricted stock values are determined based on the quoted market price of our Class A common stock on the date of grant. The Company accounts for equity awards to non-employees in accordance with FASB ASC Topic 505-50, Equity-Based Payments to Non-Employees , which requires the fair value of an award to non-employees be remeasured at fair value as the award vests. The Company adopted ASU 2016-09 "Compensation - Stock Compensation" as of January 1, 2017 using a modified retrospective approach with the option to recognize gross stock compensation expense with actual forfeitures recognized as they occur, with a cumulative-effect adjustment to retained earnings recognized as of January 1, 2017 of $8.7 million . The adoption of ASU 2016-09 also requires all income tax adjustments to be recorded in the consolidated and condensed statements of operations. Income Taxes Income taxes are accounted for under the asset and liability method. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. The Company records valuation allowances to reduce deferred income tax assets to the amount that is more likely than not to be realized. At December 31, 2018, we maintain a full valuation allowance against our net deferred tax asset in all entities with net deferred tax asset. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more likely than not that a position will be sustained, no amount of benefit attributable to the position is recognized. The tax benefit to be recognized of any tax position that meets the more likely than not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. We evaluate at the end of each reporting period whether some or all of the undistributed earnings of our foreign subsidiaries are permanently reinvested. Our position is based upon several factors including management's evaluation of the Company and its subsidiaries' financial requirements, the short term and long term operational and fiscal objectives of the Company, and the tax consequences associated with the repatriation of earnings. Net Loss Per Share The Company follows the two-class method when computing net loss per share for its two issued classes of common stock—Class A and Class B. Basic net income (loss) per share is computed by dividing the net income (loss) by the weighted average number of common stock outstanding for the period. For periods in which the Company has reported net losses, diluted net loss per share is the same as basic net loss per share, since dilutive common stock are not assumed to have been issued if their effect is anti-dilutive. Subsequent Events The Company considers events or transactions that have occurred after the balance sheet date of December 31, 2018 , but prior to the filing of the financial statements with the U.S. Securities and Exchange Commission, to provide additional evidence relative to certain estimates or to identify matters that require additional recognition or disclosure. Subsequent events have been evaluated through the filing of these financial statements. Refer to Note 16, Subsequent Event , for additional detail. Recent Accounting Pronouncements Stock Compensation In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting ("ASU 2018-07"). This ASU is intended to simplify aspects of share-based compensation issued to non-employees by making the guidance consistent with the accounting for employee share-based compensation. For public entities, ASU 2018-07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. For non-public entities, ASU 2018-07 is effective for annual periods beginning after December 15, 2019. We do not believe the adoption of ASU 2017-09 will have a material impact the Company’s consolidated financial position, results of operations or cash flows. Leases In February 2016, the FASB issued ASU No. 2016-02, "Leases" ("ASU 2016-02"). This ASU revises the accounting related to leases by requiring lessees to recognize a lease liability and a right-of-use asset for virtually all of their leases (other than leases that meet the definition of a short-term lease). The liability will be equal to the present value of the remaining lease payments. The asset will be based on the liability, subject to adjustment, such as for initial direct costs. Operating leases will result in straight-line expense (similar to current operating leases) while finance leases will result in a front-loaded expense pattern (similar to current capital leases). The FASB issued ASU 2018-11 "Leases, Targeted Improvements," which creates the optional transition expedient that allows an entity to apply the transition provisions of the new standard, including its disclosure requirements, at its adoption date instead of at the beginning of the earliest comparative period originally required by ASU 2016-02. The Company will adopt the new lease standard as of January 1, 2019 using the modified retrospective approach, applying the transition provisions of the new standard as of the adoption date. Management will elect the package of practical expedients which among other things, allows the Company to carryforward historical lease classification. The adoption of this standard will result in the derecognition of building assets and finance lease obligations for certain leases that did not pass the sale-leaseback criteria and the recognition of a lease liability and right-of-use asset for these leases in addition to operating leases with an initial term greater than twelve months that are currently not recorded on the consolidated balance sheets. Management expects the adoption of the standard will result in 1) the removal of approximately $0.2 billion of lease finance obligations and correlated property and equipment, net and 2) the addition of approximately $0.5 billion in operating lease right of use assets and $0.6 billion in operating lease right of use liabilities. We do not believe the adoption of ASU 2016-02 will have a notable impact on the Company's liquidity or a materially adverse impact on the Company's debt-covenant compliance.

Marketable Securities and Fair

Marketable Securities and Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Marketable Securities and Fair Value Measurements	Marketable Securities and Fair Value Measurements Marketable Securities As of December 31, 2018 and 2017 , all of the Company’s marketable securities were classified as available-for-sale and their estimated fair values were $120.8 million and $82.6 million , respectively. The Company periodically reviews its available-for-sale securities for other-than-temporary impairment. The Company considers factors such as the duration, severity and the reason for the decline in value, the potential recovery period, and its intent to sell. As of December 31, 2018 and 2017 , the Company’s available-for-sale securities primarily consisted of corporate bonds and other government obligations that are priced at fair value. During the years ended December 31, 2018 , 2017 , and 2016 , the Company did not recognize any other-than-temporary impairment loss. The maturities of the Company’s long-term marketable securities generally range from one to two years. The cost basis of a marketable security sold is determined by the Company using the specific identification method. During the years ended December 31, 2018 , 2017 , and 2016 , the Company did not have any realized gains or losses. The following tables present details of the Company’s marketable securities as of December 31, 2018 and 2017 (in thousands): December 31, 2018 Amortized Cost Gross Unrealized Losses Estimated Fair Value Short-term: Investment securities $ 114,402 $ (124 ) $ 114,278 Long-term: Investment securities 6,603 (77 ) 6,526 Total $ 121,005 $ (201 ) $ 120,804 December 31, 2017 Amortized Cost Gross Unrealized Losses Estimated Fair Value Short-term: Investment securities $ 61,129 $ (97 ) $ 61,032 Long-term: Investment securities 21,695 (134 ) 21,561 Total $ 82,824 $ (231 ) $ 82,593 Fair Value Measurements The following tables set forth the fair value of the Company's financial assets measured at fair value on a recurring basis as of December 31, 2018 and 2017 based on the three-tier value hierarchy described in Note 2, Summary of Significant Accounting Policies (in thousands): December 31, 2018 Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds and other funds $ 715,086 $ — $ — $ 715,086 Short-term investments: Investment securities — 114,278 — 114,278 Other non-current assets: Certificate of deposit 5,000 — — 5,000 Long-term: Investment securities — 6,526 — 6,526 Total $ 720,086 $ 120,804 $ — $ 840,890 December 31, 2017 Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 488,029 $ — $ — $ 488,029 Short-term investments: Investment securities — 61,032 — 61,032 Other non-current assets Certificate of deposit 5,000 — — 5,000 Long-term: Investment securities — 21,561 — 21,561 Total $ 493,029 $ 82,593 $ — $ 575,622

Intangible assets and Goodwill

Intangible assets and Goodwill	12 Months Ended
	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible assets and Goodwill	Intangible assets and Goodwill The following table summarizes intangible assets as of December 31, 2018 and 2017 (in thousands): Weighted-Average December 31, 2018 Amortization Period (Years) Gross Carrying Amount Accumulated Amortization Net Book Value Trademarks 5 $ 1,900 $ (1,900 ) $ — Technology 3 1,678 (1,192 ) 486 Customer relationships 5 1,300 (1,300 ) — Total $ 4,878 $ (4,392 ) $ 486 Weighted-Average December 31, 2017 Amortization Period (Years) Gross Carrying Amount Accumulated Amortization Net Book Value Trademarks 5 $ 1,900 $ (1,678 ) $ 222 Technology 3 1,453 (646 ) 807 Customer relationships 5 1,300 (1,148 ) 152 Total $ 4,653 $ (3,472 ) $ 1,181 Amortization expense related to intangible assets was $0.9 million , $1.1 million , and $0.9 million for the years ended December 31, 2018 , 2017 , and 2016 , respectively. The estimated future amortization expense of purchased intangible assets as of December 31, 2018 , is as follows (in thousands): Total 2019 $ 398 2020 75 2021 13 Thereafter — Total $ 486 Goodwill was $ 2.1 million and $1.9 million for the years ended December 31, 2018 and December 31, 2017 , respectively. In January 2019, the Company sold certain intellectual property. As of December 31, 2018, the net carrying amount of the goodwill was $1.7 million . The proceeds from the sale exceeded the carrying value of the intellectual property.

Property and Equipment, net

Property and Equipment, net	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Property and Equipment, net	Property and Equipment, net The following table summarizes property and equipment, net as of December 31, 2018 and 2017 (in thousands): December 31, 2018 2017 Furniture and computer equipment $ 339,761 $ 213,790 Site and software development costs 172,653 118,356 Leasehold improvements 109,929 82,614 Construction in progress 90,104 46,826 Building (leased - see Note 7) 184,694 83,681 897,141 545,267 Less accumulated depreciation and amortization (290,164 ) (184,126 ) Property and equipment, net $ 606,977 $ 361,141 Property and equipment depreciation and amortization expense was $122.6 million , $85.9 million , and $54.6 million for the years ended December 31, 2018 , 2017 and 2016 , respectively.

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets, Accrued Expenses, Other Current Liabilities, and Other Liabilities	12 Months Ended
	Dec. 31, 2018
Payables and Accruals [Abstract]
Prepaid Expenses and Other Current Assets, Accrued Expenses, Other Current Liabilities, and Other Liabilities	Prepaid Expenses and Other Current Assets, Accrued Expenses, Other Current Liabilities, and Other Liabilities The following table presents the components of selected balance sheet items as of December 31, 2018 and 2017 (in thousands): December 31, Prepaid expenses and other current assets: 2018 2017 Deferred costs in transit $ 83,652 $ 54,483 Supplier receivable 31,842 29,941 Supplier credits receivable 17,667 12,936 Other prepaid and other current assets 62,269 33,478 Total prepaid expenses and other current assets $ 195,430 $ 130,838 December 31, Accrued expenses: 2018 2017 Employee compensation and related benefits $ 91,914 $ 55,142 Advertising 67,259 38,888 Accrued property, plant and equipment 13,955 8,592 Credit card 16,740 4,573 Audit, legal and professional fees 1,703 1,749 Other accrued expenses 21,426 11,303 Total accrued expenses $ 212,997 $ 120,247 December 31, Other current liabilities: 2018 2017 Sales tax payable $ 49,989 $ 35,726 Sales return reserve 35,707 21,243 Other current liabilities 42,299 28,057 Total current other liabilities $ 127,995 $ 85,026 December 31, Other liabilities: 2018 2017 Deferred rent $ 99,746 $ 59,811 Construction costs under build-to-suit leases 60,089 37,545 Other liabilities 553 9,136 Total other liabilities $ 160,388 $ 106,492

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies Leases The Company leases office and warehouse spaces under non-cancelable leases. These leases expire at various dates through 2034 and include discounted rental periods and fixed escalation clauses, which are amortized straight-line over the terms of the lease. Future minimum rental commitments under non-cancelable leases with initial or remaining terms in excess of one year at December 31, 2018 were as follows (in thousands): Amount 2019 $ 106,456 2020 123,757 2021 130,017 2022 126,230 2023 122,698 Thereafter 594,464 Total $ 1,203,622 Rent expense under operating leases was $63.8 million , $45.2 million , and $33.6 million in the years ended December 31, 2018 , 2017 and 2016 , respectively. The Company has issued letters of credit for approximately $26.8 million and $15.3 million as security for these lease agreements as of December 31, 2018 and 2017 , respectively. Future lease payments have not been reduced by minimum sublease rentals of $3.4 million due to the Company in the future under non-cancelable subleases through 2020. The Company establishes assets and liabilities for the estimated construction costs incurred under lease arrangements where the Company is considered the owner for accounting purposes only, or build-to-suit leases, to the extent the Company is involved in the construction of structural improvements or takes construction risk prior to commencement of a lease. Upon occupancy of facilities under build-to-suit leases, the Company assesses whether these arrangements qualify for sales recognition under the sale-leaseback accounting guidance. If the Company continues to be the deemed owner, the facilities are accounted for as financing leases. In the year ended December 31, 2018, the construction of three warehouse lease arrangements were completed. In the first warehouse lease arrangement, the Company concluded it had a letter of credit of $2.5 million and as a result the Company did not meet the sale-leaseback criteria for derecognition of the building assets and liabilities. In both the second and third warehouse lease arrangements the Company provided non-recourse financing to the lessor and as a result the Company did not meet the sale-leaseback criteria for derecognition of the building assets and liabilities. In the year ended December 31, 2017, the construction of two warehouse lease arrangements were completed. In both lease warehouse lease arrangements, the Company concluded it had letters of credit of $0.8 million and $1.0 million and as a result the Company did not meet the sale-leaseback criteria for derecognition of the building assets and liabilities. Accordingly, these leases were accounted for as financing obligations and $101.0 million and $53.8 million were recorded in " Lease financing obligations, net of current portion " and " Property and equipment, net ," respectively, in the Company’s consolidated balance sheet as of December 31, 2018 and December 31, 2017 respectively. The monthly rent payments made to the lessor under the lease agreement are recorded in the Company’s financial statements as land lease expense and principal and interest on the financing obligation. Interest expense on the lease financing obligation reflects the portion of the Company's monthly lease payments that is allocated to interest expense. For the years ended December 31, 2018 , 2017 , and 2016 land lease expense was $2.7 million $0.9 million and $0.1 million respectively, and interest expense on lease financing obligations was $12.0 million , $6.9 million , and $1.4 million respectively. As of December 31, 2018 , future minimum commitments related to the financing obligations were $75.6 million and $22.1 million for principal and interest, respectively, through December 31, 2023 . Collection of Sales or Other Similar Taxes The Company has historically collected and remitted sales tax based on the locations of its physical operations. On June 21, 2018, the U.S. Supreme Court rendered a 5-4 majority decision in South Dakota v. Wayfair Inc., 17-494. Among other things, the Court held that a state may require an out-of-state seller with no physical presence in the state to collect and remit sales taxes on goods the seller ships to consumers in the state, overturning existing court precedent. Several states and other taxing jurisdictions have presented, or indicated that they may present, the Company with sales tax assessments. The aggregate assessments received as of December 31, 2018 are not material to the Company's business and the Company does not expect the Court's decision to have a significant impact on its business. Legal Matters On June 21, 2018, the U.S. Supreme Court rendered a 5-4 majority decision in South Dakota v. Wayfair Inc., 17-494. Among other things, the Court held that a state may require an out-of-state seller with no physical presence in the state to collect and remit sales taxes on goods the seller ships to consumers in the state, overturning existing court precedent. See Collection of Sales or Other Similar Taxes above. On January 10, 2019 and January 16, 2019, putative securities class action complaints were filed against the Company and three of its officers in the U.S. District Court for the District of Massachusetts. The two complaints allege violations of Sections 10(b) and 20(a) of the Securities Exchange Act of 1934, as amended, relating to certain prior disclosures of the Company. Each plaintiff seeks to represent a class of shareholders who purchased or acquired stock of the Company between August 2, 2018 and October 31, 2018 and seeks damages and other relief based on allegations that the defendants' conduct affected the value of such stock. The Company intends to defend these lawsuits vigorously. At this time, based on available information regarding this litigation, the Company is unable to reasonably assess the ultimate outcome of these cases or determine an estimate, or a range of estimates, of potential losses. From time to time the Company is involved in claims that arise during the ordinary course of business. Although the results of litigation and claims cannot be predicted with certainty, the Company does not currently believe that the outcome of any of these other legal matters will have a material adverse effect on the Company's results of operation or financial condition. Regardless of the outcome, litigation can be costly and time consuming, as it can divert management's attention from important business matters and initiatives, negatively impacting the Company's overall operations. In addition, the Company may also find itself at greater risk to outside party claims as it increases its operations in jurisdictions where the laws with respect to the potential liability of online retailers are uncertain, unfavorable, or unclear.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2018
Retirement Benefits [Abstract]
Employee Benefit Plans	Employee Benefit Plans The Company has a defined-contribution, incentive savings plan pursuant to Section 401(k) of the Internal Revenue Code. The plan covers all full-time employees who have reached the age of 21 years. Employees may elect to defer compensation up to a dollar limit (as allowable by the Internal Revenue Code), of which up to 4% of an employee's salary will be matched by the Company. The amounts deferred by the employee and the matching amounts contributed by the Company both vest immediately. The amount expensed under the plan totaled approximately $18.2 million , $9.0 million , and $6.3 million in the years ended December 31, 2018 , 2017 and 2016 , respectively.

Equity-Based Compensation

Equity-Based Compensation	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Equity-Based Compensation	Equity-Based Compensation The board of directors of the Company (the "Board") adopted the 2014 Incentive Award Plan ("2014 Plan") to grant cash and equity incentive awards to eligible participants in order to attract, motivate and retain talent. The 2014 Plan is administered by the Board with respect to awards to non-employee directors and by the compensation committee of the Board with respect to other participants and provides for the issuance of stock options, SARs, restricted stock, restricted stock units ("RSUs"), performance shares, stock payments, cash payments, dividend awards and other incentives. Prior to the adoption of the 2014 Plan, Wayfair LLC issued certain equity awards pursuant to the Wayfair LLC Amended and Restated Common Unit Plan (the "2010 Plan"), which was administered by the board of directors of Wayfair LLC. Awards issued under the 2010 Plan that remain outstanding currently represent Class A or Class B common stock of the Company. 8,603,066 shares of Class A common stock were initially available for issuance under awards granted pursuant to the 2014 Plan. The 2014 Plan also contains an evergreen provision whereby the shares available for future grant are increased on the first day of each calendar year beginning January 1, 2016 and ending on and including January 1, 2024. As of January 1, 2019, 6,202,378 shares of Class A common stock were available for future grant under the 2014 Plan. Shares or RSUs forfeited, withheld for minimum statutory tax obligations, and unexercised stock option lapses from the 2010 and 2014 Plans are available for future grant under the 2014 Plan. The following table presents activity relating to stock options for the year ended December 31, 2018 : Shares Weighted-Average Exercise Price Weighted-Average Remaining Contractual Term (Years) Outstanding at December 31, 2017 126,383 $ 3.02 3.5 Options exercised (46,581 ) $ 2.97 Outstanding and exercisable at December 31, 2018 79,802 $ 3.05 2.5 Intrinsic value of stock options exercised was $4.3 million and $4.8 million for the years ended December 31, 2018 and 2017 , respectively. Aggregate intrinsic value of stock options outstanding and currently exercisable is $6.9 million as of December 31, 2018 . All stock options were fully vested at December 31, 2018 . The following table presents activity relating to restricted common stock for the year ended December 31, 2018 : Shares Weighted-Average Grant Date Fair Value Unvested at December 31, 2017 40,000 $ 44.34 Restricted stock vested (20,000 ) $ 44.34 Unvested as of December 31, 2018 20,000 $ 44.34 The intrinsic value of restricted common stock vested was $1.8 million and $1.6 million for the years ended December 31, 2018 and 2017 , respectively. Aggregate intrinsic value of restricted common stock unvested is $1.8 million as of December 31, 2018 . Unrecognized equity based compensation expense related to restricted common stock expected to vest over time is $1.8 million with a weighted average remaining vesting term of 1.0 year as of December 31, 2018 . The following table presents activity relating to RSUs for the year ended December 31, 2018 : Shares Weighted-Average Grant Date Fair Value Unvested at December 31, 2017 6,853,606 $ 46.28 RSUs granted 4,834,598 $ 92.62 RSUs vested (2,523,359 ) $ 47.61 RSUs forfeited/canceled (1,193,886 ) $ 57.53 Outstanding as of December 31, 2018 7,970,959 $ 72.43 The intrinsic value of RSUs vested was $262.4 million and $137.2 million for the years ended December 31, 2018 and 2017 , respectively. Aggregate intrinsic value of RSUs unvested is $718.0 million as of December 31, 2018 . Unrecognized equity based compensation expense related to RSUs expected to vest over time is $531.5 million with a weighted average remaining vesting term of 1.6 years as of December 31, 2018 .

Stockholders' Deficit

Stockholders' Deficit	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Stockholders' Deficit	Stockholders' Deficit Preferred Stock The Company authorized 10,000,000 shares of undesignated preferred stock, $0.001 par value per share, for future issuance. As of December 31, 2018 , the Company had no shares of undesignated preferred stock issued or outstanding. Common Stock The Company authorized 500,000,000 shares of Class A common stock, $0.001 par value per share, and 164,000,000 shares of Class B common stock, $0.001 par value per share, of which 62,329,701 and 57,398,983 shares of Class A common stock and 28,417,882 and 30,809,627 shares of Class B common stock were outstanding as of December 31, 2018 and 2017 , respectively. The rights of the holders of Class A common stock and Class B common stock are identical, except with respect to voting and conversion rights. 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 may be converted into one share of Class A common stock at the option of its holder and will be automatically converted into one share of Class A common stock upon transfer thereof, subject to certain exceptions. In addition, upon the date on which the outstanding shares of Class B common stock represent less than 10% of the aggregate number of shares of the then outstanding Class A common stock and Class B common stock, or in the event of the affirmative vote or written consent of holders of at least 66 2/3% of the outstanding shares of Class B common stock, all outstanding shares of Class B common stock shall convert automatically into Class A common stock. Subject to preferences that may apply to any shares of preferred stock outstanding at the time, the holders of common stock are entitled to receive dividends out of funds legally available if the Board, in its discretion, determines to issue dividends and then only at the times and in the amounts that the Board may determine. Since the Company's initial public offering through December 31, 2018 , 53,619,863 shares of Class B common stock were converted to Class A common stock.

Segment and Geographic Informat

Segment and Geographic Information	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Segment and Geographic Information	Segment and Geographic Information Operating segments are defined as components of an enterprise for which separate financial information is available that is evaluated on a regular basis by the Chief Operating Decision Maker ("CODM") in deciding how to allocate resources to an individual segment and in assessing performance. The Company’s CODM is its Chief Executive Officer. The Company's operating and reportable segments are U.S. and International. These segments reflect the way the CODM allocates resources and evaluates financial performance, which is based upon each segment's Adjusted EBITDA. Adjusted EBITDA is defined as loss before depreciation and amortization, equity-based compensation and related taxes, interest and other income and expense, provision for income taxes, and non-recurring items. These charges are excluded from evaluation of segment performance because it facilitates reportable segment performance comparisons on a period-to-period basis. The accounting policies of the segments are the same as those described in Note 2, Summary of Significant Accounting Policies. The Company allocates certain operating expenses to the operating and reportable segments, including " Customer service and merchant fees " and " Selling, operations, technology, general and administrative " based on the usage and relative contribution provided to the segments. It excludes from the allocations certain operating expense lines, including "Depreciation and amortization, "Equity based compensation and related taxes," " Interest (expense) income, net ," " Other (expense) income, net ," and " Provision for income taxes ." There are no revenue transactions between the Company's reportable segments. U.S. The U.S. segment primarily consists of amounts earned through product sales through the Company's five distinct sites in the U.S. and through websites operated by third parties in the U.S. International The International segment primarily consists of amounts earned through product sales through the Company's international sites. Revenue from external customers for each group of similar products and services are not reported to the CODM. Separate identification of this information for purposes of segment disclosure is impractical, as it is not readily available and the cost to develop it would be excessive. No individual country outside of the U.S. provided greater than 10% of total revenue. The following tables present Direct Retail and Other net revenues and Adjusted EBITDA attributable to the Company’s reportable segments for the periods presented (in thousands): Year Ended December 31, 2018 2017 2016 U.S. Direct Retail $ 5,751,975 $ 4,075,405 $ 2,993,365 U.S. Other 61,095 77,652 117,132 U.S. segment net revenue 5,813,070 4,153,057 3,110,497 International Direct Retail 966,104 567,838 265,544 International Other — — 4,319 International segment net revenue 966,104 567,838 269,863 Total $ 6,779,174 $ 4,720,895 $ 3,380,360 Year Ended December 31, 2018 2017 2016 Adjusted EBITDA: U.S. $ (19,049 ) $ 35,888 $ 176 International (195,937 ) (102,921 ) (88,868 ) Total reportable segments Adjusted EBITDA (214,986 ) (67,033 ) (88,692 ) Less: reconciling items (1) (289,094 ) (177,581 ) (105,683 ) Net loss $ (504,080 ) $ (244,614 ) $ (194,375 ) (1) Adjustments are made to reconcile total reportable segments Adjusted EBITDA to consolidated net loss including the following (in thousands): Year Ended December 31, 2018 2017 2016 Depreciation and amortization (1) $ 123,542 $ 87,020 $ 55,572 Equity based compensation and related taxes 136,415 72,626 51,953 Interest expense (income), net 28,560 9,433 (694 ) Other expense (income), net 204 (758 ) (1,756 ) Provision for income taxes 2,037 486 608 Other (1) (1,664 ) 8,774 — Total reconciling items $ 289,094 $ 177,581 $ 105,683 (1) The Company recorded $9.6 million of one-time charges in the year ended December 31, 2017 in " Selling, operations, technology, general and administrative " in the consolidated statements of operations related to a warehouse the Company vacated in July 2017. Of the $9.6 million charges, $8.8 million was included in "Other" and related primarily to the excess of the Company's estimated future remaining lease commitments through 2023 over its expected sublease income over the same period, and $0.8 million was included in "Depreciation and amortization" related to accelerated depreciation of leasehold improvements in the warehouse. In the year ended December 31, 2018, the Company terminated the lease and recognized in "Other" a 1.7 million one-time gain related to the difference in the expected future net lease commitments and the actual costs incurred to terminate the lease. The gain was recognized in " Selling, operations, technology, general and administrative " in the consolidated statements of operations. The following table presents the activity related to the Company’s net revenue from Direct Retail sales derived through the Company’s sites and Other sales derived through websites operated by third parties and fees from third-party advertising distribution providers (in thousands): Year Ended December 31, 2018 2017 2016 Net revenue Direct Retail $ 6,718,079 $ 4,643,243 $ 3,258,909 Other 61,095 77,652 121,451 Net revenue $ 6,779,174 $ 4,720,895 $ 3,380,360 The following table presents long-lived assets by segment (in thousands): Year Ended December 31, 2018 2017 Geographic long-lived assets: U.S. $ 577,615 $ 353,414 International 29,362 7,727 Total $ 606,977 $ 361,141

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes Income tax expense (benefit) from continuing operations for the years ended December 31, 2018 , 2017 and 2016 is presented below (in thousands): Year ended December 31, 2018 Current Deferred Total Federal $ — $ (86 ) $ (86 ) State 744 (24 ) 720 Foreign 78 1,325 1,403 $ 822 $ 1,215 $ 2,037 Year ended December 31, 2017 Current Deferred Total Federal $ — $ (31 ) $ (31 ) State 540 8 548 Foreign 942 (973 ) (31 ) $ 1,482 $ (996 ) $ 486 Year ended December 31, 2016 Current Deferred Total Federal $ — $ 32 $ 32 State 329 5 334 Foreign 285 (43 ) 242 614 (6 ) 608 The actual income tax expense (benefit) differs from the expected income tax expense (benefit) computed at the U.S. Federal statutory tax rate of 21% due to the following (in thousands): Year Ended December 31, 2018 2017 2016 Tax expense (benefit) at federal statutory rate $ (105,429 ) $ (85,445 ) $ (67,819 ) State income tax expense, net of federal benefit (22,584 ) (11,432 ) (5,225 ) Foreign tax rate differential 14,976 23,179 17,109 Non-deductible equity based compensation expense 3,267 1,080 2,321 Windfall benefits from equity based compensation (29,003 ) (24,168 ) — Change in valuation allowance 119,370 24,209 53,467 Change in tax rate 197 71,919 — Limitation on officer's compensation 5,283 — — Debt integration termination 9,236 — — Other 6,724 1,144 755 Net income tax expense $ 2,037 $ 486 $ 608 We recorded an income tax expense of $2.0 million , representing an effective tax rate of almost zero . The effective tax rate differs from the U.S. statutory rate of 21% primarily as a result of the valuation allowance maintained against our worldwide net deferred tax asset. The components of income before income tax expense (benefit) determined by tax jurisdiction, are as follows (in thousands): Year Ended December 31, 2018 2017 2016 U.S. $ (327,356 ) $ (143,800 ) $ (118,851 ) Foreign (174,687 ) (100,328 ) (74,916 ) Total $ (502,043 ) $ (244,128 ) $ (193,767 ) The tax effects of temporary differences that give rise to significant portions of deferred tax assets and liabilities for the periods presented are as follows (in thousands): December 31, 2018 2017 Deferred tax assets: Accounts receivable $ 2,400 $ 1,814 Inventories 495 391 Operating loss carry-forwards 262,481 146,666 Equity based compensation expense 9,733 9,087 Intangibles 12,526 13,862 Accrued payroll 13,900 8,582 Accrued expenses and reserves 12,339 10,933 Charitable contributions 657 284 Deferred rent 90,868 48,936 Gross deferred tax assets 405,399 240,555 Less: Valuation allowance (247,454 ) (178,488 ) Net deferred tax assets 157,945 62,067 Deferred tax liabilities: Prepaid expenses (3,030 ) (1,825 ) Capitalized technology (15,427 ) (11,339 ) Property and equipment (73,189 ) (34,986 ) Goodwill (133 ) (110 ) Convertible debt (66,166 ) (12,580 ) Other — (12 ) Total deferred tax liabilities (157,945 ) (60,852 ) Net deferred tax assets (liabilities) — 1,215 Non-current net deferred tax assets (liabilities) $ — $ 1,215 The valuation allowance increased by $69.0 million during 2018. The increase in valuation allowance is the result of establishing a valuation allowance primarily against the current year operating losses of our U.S. and Irish entities, partially offset by a decrease in valuation allowance through equity as a result of the deferred tax liability recorded related to the Company’s convertible debt issuance. In determining the need for a valuation allowance, the Company has given consideration to the cumulative book income and loss positions of each of its entities as well as its worldwide cumulative loss position. The Company has assessed, on a jurisdictional basis, the available means of recovering deferred tax assets, including the ability to carry-back net operating losses, the existence of reversing temporary differences, the availability of tax planning strategies and available sources of future taxable income. At December 31, 2018, we maintain a full valuation allowance against our worldwide net deferred tax asset. As of December 31, 2018 , the Company had federal net operating loss carryforwards available to offset future federal taxable income of $787.8 million . In addition, the Company had state net operating loss carryforwards available in the amount of $711.8 million which are available to offset future state taxable income. Of the federal net operating loss carryforwards, $427.4 million begin to expire in the year ending December 31, 2034. The remaining $360.4 million of federal net operating loss carryforwards do not expire. The state net operating loss carryforwards begin to expire in the year ending December 31, 2022. The Company's ability to utilize these federal and state net operating loss carry-forwards may be limited in the future if the Company experiences an ownership change pursuant to Internal Revenue Code Section 382. An ownership change occurs when the ownership percentages of 5% or greater stockholders change by more than 50% over a three-year period. The Company also had foreign net operating loss carry-forwards available to offset future foreign income of $424.4 million . The Canadian net operating loss of $4.8 million will expire in the year ending December 31, 2038. The remaining foreign net operating loss carryforwards do not expire. As of December 31, 2018 , the Company has not provided for U.S. deferred income taxes on undistributed earnings of its foreign subsidiaries of approximately $5.2 million since these earnings are deemed to be indefinitely reinvested. Upon distribution of those earnings in the form of dividends or otherwise, the Company could be subject to income taxes as well as withholding taxes. The amount of taxes attributable to the undistributed earnings is immaterial. The Company establishes reserves for uncertain tax positions based on management's assessment of exposures associated with tax deductions, permanent tax differences and tax credits. The tax reserves are analyzed periodically and adjustments are made as events occur to warrant adjustment to the reserve. The Company's policy is to recognize interest and penalties related to unrecognized tax benefits as a component of income tax expense. Reserves for uncertain tax positions as of December 31, 2018 and 2017 are not material and would not impact the effective tax rate if recognized as a result of the valuation allowance maintained against our net deferred tax assets. The Company's policy is to recognize interest and penalties related to unrecognized tax benefits as a component of income tax expense. Related to the unrecognized tax benefits noted above, the Company did not accrue any penalties and interest during 2018 , 2017 , and 2016 , respectively, because it believes that such additional interest and penalties would be immaterial. The Company's tax jurisdictions include the U.S., the UK, Germany, Ireland, Canada, Hong Kong and the British Virgin Islands. The statute of limitations with respect to the Company's U.S. federal income taxes has expired for years prior to 2015. The relevant state statutes vary. The statute of limitations with respect to the Company's foreign income taxes varies, but has expired for years prior to 2014. However, preceding years remain open to examination by U.S. federal and state and foreign taxing authorities to the extent of future utilization of net operating losses generated in each preceding year. In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (SAB 118), which allowed the Company to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. The Company’s provisional estimate associated with the reduction in the U.S. federal corporate tax rate from 35% to 21% impacted the change in valuation allowance and change in tax rate component of the Company’s effective tax rate reconciliation as well as its ending deferred tax assets, deferred tax liabilities and valuation allowance in the 2017 deferred tax footnote disclosure. In the fourth quarter of 2018, we completed our analysis to determine the effect of the Tax Act and recorded no adjustments during the year ended December 31, 2018.

Credit Agreement

Credit Agreement	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Credit Agreement	Credit Agreement Subsequent Event: On February 21, 2019, the Company, as guarantor, and Wayfair LLC, a wholly-owned subsidiary of the Company, as borrower, entered into an Amended and Restated Credit Agreement (the “Amended Credit Agreement”) with Citibank, N.A. ("Citibank"), in its capacity as administrative agent, swing line lender and letter of credit issuer, and certain other lenders party thereto. The Amended Credit Agreement replaced the Company's existing credit facility with Citibank. Refer to Note 16, Subsequent Event , for additional detail. On February 22, 2017, the Company entered into a $40 million credit card program and a credit agreement consisting of a $100 million secured revolving credit facility (the "Old Revolver") with Citibank. This Citibank credit facility replaced the Company's then existing credit facility with Bank of America, N.A. ("Bank of America"), which was terminated on February 22, 2017. This Citibank credit agreement was amended four times. Amendment No. 1, dated September 11, 2017, added a new letter of credit sublimit and made clarifying edits to the mandatory prepayment provisions of the credit agreement. Amendment No. 2, dated April 12, 2018, provided for, among other changes: (i) an increase in the letter of credit sublimit to $65 million and (ii) modifications to certain baskets in the exceptions to the negative covenants, including, without limitation, the restricted payments, investments and indebtedness covenants. Amendment No. 3, dated December 7, 2018 made clarifying edits to the letter of credit procedure provisions. Amendment No. 4, dated December 20, 2018, updated certain financial definitions and enhanced the Company's ability to conduct asset dispositions. As amended, the Old Revolver had a $65 million letter of credit sublimit and a $10 million swing line sublimit, and a final maturity date of February 21, 2020. Wayfair LLC was the borrower (the "Borrower") under the Citibank credit agreement. Subject to certain conditions, the Borrower had the right to increase the Old Revolver by $25 million . Borrowings under the Old Revolver would bear interest through maturity at a variable rate based upon, at the Borrower’s option, either the Eurodollar rate or the base rate (which is the highest of (x) Citibank's prime rate, (y) one-half of 1.00% in excess of the federal funds effective rate, and (z) 1.00% in excess of the one-month Eurodollar rate), plus, in each case an applicable margin. From closing until September 30, 2019, the applicable margin for Eurodollar rate loans was 1.75% per annum and the applicable margin for base rate loans was 0.75% per annum. After September 30, 2019, the applicable margin was subject to specified changes depending on the applicable consolidated leverage ratio. Any amounts outstanding under the Old Revolver were due at maturity. In addition, subject to the terms and conditions set forth in the credit agreement, the Borrower was required to make certain mandatory prepayments prior to maturity. This Citibank credit agreement contained affirmative and negative covenants customarily applicable to senior secured credit facilities, including covenants that, among other things, limited or restricted the ability of the Company and its subsidiaries, subject to negotiated exceptions, to incur additional indebtedness and additional liens on their assets, engage in mergers or acquisitions or dispose of assets, pay dividends or make other distributions, voluntarily prepay other indebtedness, enter into transactions with affiliated persons, make investments, and change the nature of their businesses. In addition, this Citibank credit agreement required the Company to maintain certain financial ratios. As of December 31, 2018 , the Company was in compliance with its covenants under the Old Revolver. The Company did not borrow any amounts under the Old Revolver or the Bank of America credit agreement during the years ended December 31, 2018 and 2017 . Convertible Debt On September 15, 2017, the Company issued $431.25 million aggregate principal amount of 0.375% Convertible Senior Notes due 2022 (the "2017 Notes"), which includes the exercise in full of a $56.25 million over-allotment option, to certain financial institutions as the initial purchasers of the 2017 Notes (the "2017 Initial Purchasers"). On September 11, 2017, in connection with the pricing of the 2017 Notes, the Company entered into privately negotiated capped call transactions (the "2017 Base Capped Call Transactions") with two of the 2017 Initial Purchasers and certain other financial institutions (the "2017 Option Counterparties") and, in connection with the exercise in full of the over-allotment option by the 2017 Initial Purchasers, on September 14, 2017, entered into additional capped call transactions (such additional capped call transactions, the "2017 Additional Capped Call Transactions” and, together with the 2017 Base Capped Call Transactions, the "2017 Capped Call Transactions") with the 2017 Option Counterparties. Collectively, the 2017 Capped Call Transactions covered, initially, the number of shares of the Company’s Class A common stock underlying the 2017 Notes, subject to anti-dilution adjustments substantially similar to those applicable to the 2017 Notes. The cost of the 2017 Capped Call Transactions was approximately $44.2 million . On November 15, 2018, the Company amended and restated the 2017 Capped Call Transactions (the "Restated 2017 Capped Call Transactions") with each of the 2017 Option Counterparties in order to, among other things, provide that the options underlying the Restated 2017 Capped Call Transactions can, at the Company’s option, remain outstanding until September 1, 2022, which is the maturity date for the 2017 Notes, even if all or a portion of the 2017 Notes are converted, repurchased or redeemed prior to such date. In November 2018, the Company issued $575.0 million aggregate principal amount of 1.125% Convertible Senior Notes due 2024 (the "2018 Notes" and together with the 2017 Notes, the "Notes"), which includes the exercise in full of a $75.0 million option granted to the initial purchasers, to certain financial institutions as the initial purchasers of the 2018 Notes (the "2018 Initial Purchasers"). The issuance of $500.0 million of 2018 Notes closed on November 19, 2018 and the additional $75.0 million of additional 2018 Notes, which were issued pursuant to the exercise of the 2018 Initial Purchasers' option to purchase such additional 2018 Notes, closed on November 29, 2018. On November 14, 2018, in connection with the pricing of the 2018 Notes, the Company entered into privately negotiated capped call transactions (the "2018 Base Capped Call Transactions") with one of the 2018 Initial Purchasers and certain other financial institutions (the "2018 Option Counterparties") and, in connection with the exercise in full of the over-allotment option by the 2018 Initial Purchasers, on November 27, 2018, entered into additional capped call transactions (such additional capped call transactions, the "2018 Additional Capped Call Transactions" and, together with the 2018 Base Capped Call Transactions, the "2018 Capped Call Transactions") with the 2018 Option Counterparties. Collectively, the 2018 Capped Call Transactions cover, initially, the number of shares of the Company’s Class A common stock underlying the 2018 Notes, subject to anti-dilution adjustments substantially similar to those applicable to the 2018 Notes. The cost of the 2018 Capped Call Transactions was approximately $93.4 million . The net proceeds from the sale of the 2017 Notes and 2018 Notes were approximately $420.4 million and $562.0 million , respectively, after deducting the initial purchasers’ discounts and the offering expenses payable by the Company. The Company used approximately $44.2 million and $93.4 million , respectively, of the net proceeds from the 2017 Notes and 2018 Notes to pay the cost of the 2017 Capped Call Transactions and the 2018 Capped Call Transactions, respectively. The Company intends to use the remainder of the net proceeds from the Notes for working capital and general corporate purposes. The Notes are general unsecured obligations of the Company. The Notes rank senior in right of payment to any of the Company’s future indebtedness that is expressly subordinated in right of payment to the Notes; rank equal in right of payment to the Company’s existing and future unsecured indebtedness that is not so subordinated; are effectively subordinated in right of payment to any of the Company’s secured indebtedness to the extent of the value of the assets securing such indebtedness; and are structurally subordinated to all existing and future indebtedness and liabilities of the Company’s subsidiaries. In accounting for the issuance of the Notes, the Company separated the Notes into liability and equity components. The carrying amount of the liability component was calculated by measuring the fair value of a similar liability that does not have an associated convertible feature. The carrying amount of the equity component, representing the conversion option, which does not meet the criteria for separate accounting as a derivative as it is indexed to the Company's own stock, was determined by deducting the fair value of the liability component from the par value of the Notes. The difference between the principal amount of the Notes and the liability component represents the debt discount, which is recorded as a direct deduction from the related debt liability in the consolidated and condensed balance sheet and amortized to interest expense using the effective interest method over the term of the Notes. The effective interest rate of the 2017 Notes and 2018 Notes is 6.0% and 8.1% respectively. The equity component of the 2017 Notes and 2018 Notes of approximately $95.8 million and $181.5 million , respectively, is included in additional paid-in capital in the consolidated and condensed balance sheet and is not remeasured as long as it continues to meet the conditions for equity classification. The Company allocated transaction costs related to the Notes using the same proportions as the proceeds from the Notes. Transaction costs attributable to the liability component were recorded as a direct deduction from the related debt liability in the consolidated and condensed balance sheet and amortized to interest expense over the term of the Notes, and transaction costs attributable to the equity component were netted with the equity component in shareholders’ equity. Interest expense related to the Notes for the year ended December 31, 2018 was $24.2 million , which is also comprised of the amortization of debt discount and debt issuance costs and the contractual coupon interest. The estimated fair value of the 2017 Notes was $465.0 million and $452.5 million as of December 31, 2018 and December 31, 2017 , respectively. The estimated fair value of the 2018 Notes was $580.2 million as of December 31, 2018 . The estimated fair value of the Notes was determined through consideration of quoted market prices. The fair value is classified as Level 2, as defined in Note 3, Marketable Securities and Fair Value Measurements . The if-converted value of both the 2017 and 2018 Notes did not exceed the principal value as of December 31, 2018. 2017 Notes The 2017 Notes were issued pursuant to an indenture, dated September 15, 2017 (the "2017 Indenture"), between the Company and U.S. Bank National Association, as trustee. The Company pays interest on the 2017 Notes semiannually in arrears at a rate of 0.375% per annum on March 1 and September 1 of each year. The 2017 Notes are convertible based upon an initial conversion rate of 9.61 shares of the Company’s Class A common stock per $1,000 principal amount of 2017 Notes (equivalent to a conversion price of approximately $104.06 per share of the Company’s Class A common stock). The conversion rate will be subject to adjustment upon the occurrence of certain specified events, including certain distributions and dividends to all or substantially all of the holders of the Company’s Class A common stock, but will not be adjusted for accrued and unpaid interest. The Company will settle any conversions of the 2017 Notes in cash, shares of the Company’s Class A common stock or a combination thereof, with the form of consideration determined at the Company’s election. The 2017 Notes will mature on September 1, 2022, unless earlier purchased, redeemed or converted. Prior to June 1, 2022, holders may convert all or a portion of their 2017 Notes only under the following circumstances: (1) during any calendar quarter (and only during such calendar quarter), if the last reported sale price of the Company’s Class A common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day; (2) during the 5 business day period after any 10 consecutive trading day period (the "2017 Notes measurement period") in which the trading price per $1,000 principal amount of 2017 Notes for each trading day of the 2017 Notes measurement period was less than 98% of the product of the last reported sale price of the Company’s Class A common stock and the conversion rate on each such trading day; (3) with respect to any 2017 Notes called for redemption by the Company, at any time prior to the close of business on the second scheduled trading day immediately preceding the redemption date; or (4) upon the occurrence of specified corporate events. On and after June 1, 2022 until the close of business on the second scheduled trading day immediately preceding the maturity date, holders may convert their 2017 Notes at any time, regardless of the foregoing circumstances. Holders of 2017 Notes who convert their 2017 Notes in connection with a notice of a redemption or a make-whole fundamental change (each as defined in the 2017 Indenture) may be entitled to a premium in the form of an increase in the conversion rate of the 2017 Notes. The Company may not redeem the 2017 Notes prior to September 8, 2020. On or after September 8, 2020, the Company may redeem for cash all or part of the 2017 Notes if the last reported sale price of the Company’s Class A common stock equals or exceeds 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive), including at least one of the five trading days immediately preceding the date on which the Company provides notice of redemption, during any 30 consecutive trading days ending on, and including the trading day immediately preceding the date on which the Company provides notice of the redemption. The redemption price will be 100% of the principal amount of the 2017 Notes to be redeemed, plus accrued and unpaid interest, if any. Upon the occurrence of a fundamental change (as defined in the 2017 Indenture), holders may require the Company to repurchase all or a portion of their 2017 Notes for cash at a price equal to 100% of the principal amount of the 2017 Notes to be repurchased plus any accrued but unpaid interest to, but excluding, the fundamental change repurchase date. The 2017 Indenture contains customary terms and covenants, including that upon certain events of default occurring and continuing, either the Trustee or the holders of not less than 25% in aggregate principal amount of the 2017 Notes then outstanding may declare the entire principal amount of all the 2017 Notes plus accrued interest, if any, to be immediately due and payable. 2018 Notes The 2018 Notes were issued pursuant to an indenture, dated November 19, 2018 (the "2018 Indenture"), between the Company and U.S. Bank National Association, as trustee. The Company will pay interest on the 2018 Notes semiannually in arrears at a rate of 1.125% per annum on May 1 and November 1 of each year commencing on May 1, 2019. The 2018 Notes are convertible based upon an initial conversion rate of 8.5910 shares of the Company’s Class A common stock per $1,000 principal amount of 2018 Notes (equivalent to a conversion price of approximately $116.40 per share of the Company’s Class A common stock). The conversion rate will be subject to adjustment upon the occurrence of certain specified events, including certain distributions and dividends to all or substantially all of the holders of the Company’s Class A common stock, but will not be adjusted for accrued and unpaid interest. The Company will settle any conversions of the 2018 Notes in cash, shares of the Company’s Class A common stock or a combination thereof, with the form of consideration determined at the Company’s election. The 2018 Notes will mature on November 1, 2024, unless earlier purchased, redeemed or converted. Prior to August 1, 2024, holders may convert all or a portion of their 2018 Notes only under the following circumstances: (1) during any calendar quarter (and only during such calendar quarter), if the last reported sale price of the Company’s Class A common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day; (2) during the five business day period after any ten consecutive trading day period (the "2018 Notes measurement period") in which the trading price per $1,000 principal amount of 2018 Notes for each trading day of the 2018 Notes measurement period was less than 98% of the product of the last reported sale price of the Company’s Class A common stock and the conversion rate on each such trading day; (3) with respect to any 2018 Notes called for redemption by the Company, at any time prior to the close of business on the second scheduled trading day immediately preceding the redemption date; or (4) upon the occurrence of specified corporate events. On and after August 1, 2024 until the close of business on the second scheduled trading day immediately preceding the maturity date, holders may convert their 2018 Notes at any time, regardless of the foregoing circumstances. Holders of 2018 Notes who convert their 2018 Notes in connection with a make-whole fundamental change or a notice of redemption (each as defined in the 2018 Indenture) may be entitled to a premium in the form of an increase in the conversion rate of the 2018 Notes. The Company may not redeem the 2018 Notes prior to May 8, 2022. On or after May 8, 2022, the Company may redeem for cash all or part of the 2018 Notes if the last reported sale price of the Company’s Class A common stock equals or exceeds 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive), including at least one of the five trading days immediately preceding the date on which the Company provides notice of redemption, during any 30 consecutive trading days ending on, and including the trading day immediately preceding the date on which the Company provides notice of the redemption. The redemption price will be 100% of the principal amount of the 2018 Notes to be redeemed, plus accrued and unpaid interest, if any. Upon the occurrence of a fundamental change (as defined in the 2018 Indenture), holders may require the Company to repurchase all or a portion of their 2018 Notes for cash at a price equal to 100% of the principal amount of the 2018 Notes to be repurchased plus any accrued but unpaid interest to, but excluding, the fundamental change repurchase date. The 2018 Indenture contains customary terms and covenants, including that upon certain events of default occurring and continuing, either the Trustee or the holders of not less than 25% in aggregate principal amount of the 2018 Notes then outstanding may declare the entire principal amount of all the 2018 Notes plus accrued interest, if any, to be immediately due and payable. Capped Call Transactions The Restated 2017 Capped Call Transactions and 2018 Capped Call Transactions (collectively, the "Capped Call Transactions") are expected generally to reduce the potential dilution and/or offset the cash payments the Company is required to make in excess of the principal amount of the Notes upon conversion of the Notes in the event that the market price per share of the Company’s Class A common stock is greater than the strike price of the Capped Call Transactions (which initially corresponds to the initial conversion price of the Notes and is subject to certain adjustments under the terms of the Capped Call Transactions), with such reduction and/or offset subject to a cap based on the cap price of the Capped Call Transactions. The 2017 Capped Call Transactions have an initial cap price of $154.16 per share of the Company’s Class A common stock, which represents a premium of 100% over the last reported sale price of the Company’s Class A common stock on September 11, 2017, and is subject to certain adjustments under the terms of the Restated 2017 Capped Call Transactions. The 2018 Capped Call Transactions have an initial cap price of $219.63 per share of the Company’s Class A common stock, which represents a premium of 150% over the last reported sale price of the Company’s Class A common stock on November 14, 2018, and is subject to certain adjustments under the terms of the 2018 Capped Call Transactions. Collectively, the Capped Call Transactions cover, initially, the number of shares of the Company’s Class A common stock underlying the Notes, subject to anti-dilution adjustments substantially similar to those applicable to the Notes. The Capped Call Transactions are separate transactions, in each case, entered into by the Company with the 2017 Option Counterparties and 2018 Option Counterparties, and are not part of the terms of the Notes and will not affect any holder’s rights under the Notes. Holders of the Notes will not have any rights with respect to the Capped Call Transactions. The Capped Call Transactions do not meet the criteria for separate accounting as a derivative as they are indexed to the Company's stock. The premiums paid for the Capped Call Transactions have been included as a net reduction to additional paid-in capital within shareholders’ equity.

Convertible Debt

Convertible Debt	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Convertible Debt	Credit Agreement Subsequent Event: On February 21, 2019, the Company, as guarantor, and Wayfair LLC, a wholly-owned subsidiary of the Company, as borrower, entered into an Amended and Restated Credit Agreement (the “Amended Credit Agreement”) with Citibank, N.A. ("Citibank"), in its capacity as administrative agent, swing line lender and letter of credit issuer, and certain other lenders party thereto. The Amended Credit Agreement replaced the Company's existing credit facility with Citibank. Refer to Note 16, Subsequent Event , for additional detail. On February 22, 2017, the Company entered into a $40 million credit card program and a credit agreement consisting of a $100 million secured revolving credit facility (the "Old Revolver") with Citibank. This Citibank credit facility replaced the Company's then existing credit facility with Bank of America, N.A. ("Bank of America"), which was terminated on February 22, 2017. This Citibank credit agreement was amended four times. Amendment No. 1, dated September 11, 2017, added a new letter of credit sublimit and made clarifying edits to the mandatory prepayment provisions of the credit agreement. Amendment No. 2, dated April 12, 2018, provided for, among other changes: (i) an increase in the letter of credit sublimit to $65 million and (ii) modifications to certain baskets in the exceptions to the negative covenants, including, without limitation, the restricted payments, investments and indebtedness covenants. Amendment No. 3, dated December 7, 2018 made clarifying edits to the letter of credit procedure provisions. Amendment No. 4, dated December 20, 2018, updated certain financial definitions and enhanced the Company's ability to conduct asset dispositions. As amended, the Old Revolver had a $65 million letter of credit sublimit and a $10 million swing line sublimit, and a final maturity date of February 21, 2020. Wayfair LLC was the borrower (the "Borrower") under the Citibank credit agreement. Subject to certain conditions, the Borrower had the right to increase the Old Revolver by $25 million . Borrowings under the Old Revolver would bear interest through maturity at a variable rate based upon, at the Borrower’s option, either the Eurodollar rate or the base rate (which is the highest of (x) Citibank's prime rate, (y) one-half of 1.00% in excess of the federal funds effective rate, and (z) 1.00% in excess of the one-month Eurodollar rate), plus, in each case an applicable margin. From closing until September 30, 2019, the applicable margin for Eurodollar rate loans was 1.75% per annum and the applicable margin for base rate loans was 0.75% per annum. After September 30, 2019, the applicable margin was subject to specified changes depending on the applicable consolidated leverage ratio. Any amounts outstanding under the Old Revolver were due at maturity. In addition, subject to the terms and conditions set forth in the credit agreement, the Borrower was required to make certain mandatory prepayments prior to maturity. This Citibank credit agreement contained affirmative and negative covenants customarily applicable to senior secured credit facilities, including covenants that, among other things, limited or restricted the ability of the Company and its subsidiaries, subject to negotiated exceptions, to incur additional indebtedness and additional liens on their assets, engage in mergers or acquisitions or dispose of assets, pay dividends or make other distributions, voluntarily prepay other indebtedness, enter into transactions with affiliated persons, make investments, and change the nature of their businesses. In addition, this Citibank credit agreement required the Company to maintain certain financial ratios. As of December 31, 2018 , the Company was in compliance with its covenants under the Old Revolver. The Company did not borrow any amounts under the Old Revolver or the Bank of America credit agreement during the years ended December 31, 2018 and 2017 . Convertible Debt On September 15, 2017, the Company issued $431.25 million aggregate principal amount of 0.375% Convertible Senior Notes due 2022 (the "2017 Notes"), which includes the exercise in full of a $56.25 million over-allotment option, to certain financial institutions as the initial purchasers of the 2017 Notes (the "2017 Initial Purchasers"). On September 11, 2017, in connection with the pricing of the 2017 Notes, the Company entered into privately negotiated capped call transactions (the "2017 Base Capped Call Transactions") with two of the 2017 Initial Purchasers and certain other financial institutions (the "2017 Option Counterparties") and, in connection with the exercise in full of the over-allotment option by the 2017 Initial Purchasers, on September 14, 2017, entered into additional capped call transactions (such additional capped call transactions, the "2017 Additional Capped Call Transactions” and, together with the 2017 Base Capped Call Transactions, the "2017 Capped Call Transactions") with the 2017 Option Counterparties. Collectively, the 2017 Capped Call Transactions covered, initially, the number of shares of the Company’s Class A common stock underlying the 2017 Notes, subject to anti-dilution adjustments substantially similar to those applicable to the 2017 Notes. The cost of the 2017 Capped Call Transactions was approximately $44.2 million . On November 15, 2018, the Company amended and restated the 2017 Capped Call Transactions (the "Restated 2017 Capped Call Transactions") with each of the 2017 Option Counterparties in order to, among other things, provide that the options underlying the Restated 2017 Capped Call Transactions can, at the Company’s option, remain outstanding until September 1, 2022, which is the maturity date for the 2017 Notes, even if all or a portion of the 2017 Notes are converted, repurchased or redeemed prior to such date. In November 2018, the Company issued $575.0 million aggregate principal amount of 1.125% Convertible Senior Notes due 2024 (the "2018 Notes" and together with the 2017 Notes, the "Notes"), which includes the exercise in full of a $75.0 million option granted to the initial purchasers, to certain financial institutions as the initial purchasers of the 2018 Notes (the "2018 Initial Purchasers"). The issuance of $500.0 million of 2018 Notes closed on November 19, 2018 and the additional $75.0 million of additional 2018 Notes, which were issued pursuant to the exercise of the 2018 Initial Purchasers' option to purchase such additional 2018 Notes, closed on November 29, 2018. On November 14, 2018, in connection with the pricing of the 2018 Notes, the Company entered into privately negotiated capped call transactions (the "2018 Base Capped Call Transactions") with one of the 2018 Initial Purchasers and certain other financial institutions (the "2018 Option Counterparties") and, in connection with the exercise in full of the over-allotment option by the 2018 Initial Purchasers, on November 27, 2018, entered into additional capped call transactions (such additional capped call transactions, the "2018 Additional Capped Call Transactions" and, together with the 2018 Base Capped Call Transactions, the "2018 Capped Call Transactions") with the 2018 Option Counterparties. Collectively, the 2018 Capped Call Transactions cover, initially, the number of shares of the Company’s Class A common stock underlying the 2018 Notes, subject to anti-dilution adjustments substantially similar to those applicable to the 2018 Notes. The cost of the 2018 Capped Call Transactions was approximately $93.4 million . The net proceeds from the sale of the 2017 Notes and 2018 Notes were approximately $420.4 million and $562.0 million , respectively, after deducting the initial purchasers’ discounts and the offering expenses payable by the Company. The Company used approximately $44.2 million and $93.4 million , respectively, of the net proceeds from the 2017 Notes and 2018 Notes to pay the cost of the 2017 Capped Call Transactions and the 2018 Capped Call Transactions, respectively. The Company intends to use the remainder of the net proceeds from the Notes for working capital and general corporate purposes. The Notes are general unsecured obligations of the Company. The Notes rank senior in right of payment to any of the Company’s future indebtedness that is expressly subordinated in right of payment to the Notes; rank equal in right of payment to the Company’s existing and future unsecured indebtedness that is not so subordinated; are effectively subordinated in right of payment to any of the Company’s secured indebtedness to the extent of the value of the assets securing such indebtedness; and are structurally subordinated to all existing and future indebtedness and liabilities of the Company’s subsidiaries. In accounting for the issuance of the Notes, the Company separated the Notes into liability and equity components. The carrying amount of the liability component was calculated by measuring the fair value of a similar liability that does not have an associated convertible feature. The carrying amount of the equity component, representing the conversion option, which does not meet the criteria for separate accounting as a derivative as it is indexed to the Company's own stock, was determined by deducting the fair value of the liability component from the par value of the Notes. The difference between the principal amount of the Notes and the liability component represents the debt discount, which is recorded as a direct deduction from the related debt liability in the consolidated and condensed balance sheet and amortized to interest expense using the effective interest method over the term of the Notes. The effective interest rate of the 2017 Notes and 2018 Notes is 6.0% and 8.1% respectively. The equity component of the 2017 Notes and 2018 Notes of approximately $95.8 million and $181.5 million , respectively, is included in additional paid-in capital in the consolidated and condensed balance sheet and is not remeasured as long as it continues to meet the conditions for equity classification. The Company allocated transaction costs related to the Notes using the same proportions as the proceeds from the Notes. Transaction costs attributable to the liability component were recorded as a direct deduction from the related debt liability in the consolidated and condensed balance sheet and amortized to interest expense over the term of the Notes, and transaction costs attributable to the equity component were netted with the equity component in shareholders’ equity. Interest expense related to the Notes for the year ended December 31, 2018 was $24.2 million , which is also comprised of the amortization of debt discount and debt issuance costs and the contractual coupon interest. The estimated fair value of the 2017 Notes was $465.0 million and $452.5 million as of December 31, 2018 and December 31, 2017 , respectively. The estimated fair value of the 2018 Notes was $580.2 million as of December 31, 2018 . The estimated fair value of the Notes was determined through consideration of quoted market prices. The fair value is classified as Level 2, as defined in Note 3, Marketable Securities and Fair Value Measurements . The if-converted value of both the 2017 and 2018 Notes did not exceed the principal value as of December 31, 2018. 2017 Notes The 2017 Notes were issued pursuant to an indenture, dated September 15, 2017 (the "2017 Indenture"), between the Company and U.S. Bank National Association, as trustee. The Company pays interest on the 2017 Notes semiannually in arrears at a rate of 0.375% per annum on March 1 and September 1 of each year. The 2017 Notes are convertible based upon an initial conversion rate of 9.61 shares of the Company’s Class A common stock per $1,000 principal amount of 2017 Notes (equivalent to a conversion price of approximately $104.06 per share of the Company’s Class A common stock). The conversion rate will be subject to adjustment upon the occurrence of certain specified events, including certain distributions and dividends to all or substantially all of the holders of the Company’s Class A common stock, but will not be adjusted for accrued and unpaid interest. The Company will settle any conversions of the 2017 Notes in cash, shares of the Company’s Class A common stock or a combination thereof, with the form of consideration determined at the Company’s election. The 2017 Notes will mature on September 1, 2022, unless earlier purchased, redeemed or converted. Prior to June 1, 2022, holders may convert all or a portion of their 2017 Notes only under the following circumstances: (1) during any calendar quarter (and only during such calendar quarter), if the last reported sale price of the Company’s Class A common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day; (2) during the 5 business day period after any 10 consecutive trading day period (the "2017 Notes measurement period") in which the trading price per $1,000 principal amount of 2017 Notes for each trading day of the 2017 Notes measurement period was less than 98% of the product of the last reported sale price of the Company’s Class A common stock and the conversion rate on each such trading day; (3) with respect to any 2017 Notes called for redemption by the Company, at any time prior to the close of business on the second scheduled trading day immediately preceding the redemption date; or (4) upon the occurrence of specified corporate events. On and after June 1, 2022 until the close of business on the second scheduled trading day immediately preceding the maturity date, holders may convert their 2017 Notes at any time, regardless of the foregoing circumstances. Holders of 2017 Notes who convert their 2017 Notes in connection with a notice of a redemption or a make-whole fundamental change (each as defined in the 2017 Indenture) may be entitled to a premium in the form of an increase in the conversion rate of the 2017 Notes. The Company may not redeem the 2017 Notes prior to September 8, 2020. On or after September 8, 2020, the Company may redeem for cash all or part of the 2017 Notes if the last reported sale price of the Company’s Class A common stock equals or exceeds 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive), including at least one of the five trading days immediately preceding the date on which the Company provides notice of redemption, during any 30 consecutive trading days ending on, and including the trading day immediately preceding the date on which the Company provides notice of the redemption. The redemption price will be 100% of the principal amount of the 2017 Notes to be redeemed, plus accrued and unpaid interest, if any. Upon the occurrence of a fundamental change (as defined in the 2017 Indenture), holders may require the Company to repurchase all or a portion of their 2017 Notes for cash at a price equal to 100% of the principal amount of the 2017 Notes to be repurchased plus any accrued but unpaid interest to, but excluding, the fundamental change repurchase date. The 2017 Indenture contains customary terms and covenants, including that upon certain events of default occurring and continuing, either the Trustee or the holders of not less than 25% in aggregate principal amount of the 2017 Notes then outstanding may declare the entire principal amount of all the 2017 Notes plus accrued interest, if any, to be immediately due and payable. 2018 Notes The 2018 Notes were issued pursuant to an indenture, dated November 19, 2018 (the "2018 Indenture"), between the Company and U.S. Bank National Association, as trustee. The Company will pay interest on the 2018 Notes semiannually in arrears at a rate of 1.125% per annum on May 1 and November 1 of each year commencing on May 1, 2019. The 2018 Notes are convertible based upon an initial conversion rate of 8.5910 shares of the Company’s Class A common stock per $1,000 principal amount of 2018 Notes (equivalent to a conversion price of approximately $116.40 per share of the Company’s Class A common stock). The conversion rate will be subject to adjustment upon the occurrence of certain specified events, including certain distributions and dividends to all or substantially all of the holders of the Company’s Class A common stock, but will not be adjusted for accrued and unpaid interest. The Company will settle any conversions of the 2018 Notes in cash, shares of the Company’s Class A common stock or a combination thereof, with the form of consideration determined at the Company’s election. The 2018 Notes will mature on November 1, 2024, unless earlier purchased, redeemed or converted. Prior to August 1, 2024, holders may convert all or a portion of their 2018 Notes only under the following circumstances: (1) during any calendar quarter (and only during such calendar quarter), if the last reported sale price of the Company’s Class A common stock for at least 20 trading days (whether or not consecutive) during a period of 30 consecutive trading days ending on, and including, the last trading day of the immediately preceding calendar quarter is greater than or equal to 130% of the conversion price on each applicable trading day; (2) during the five business day period after any ten consecutive trading day period (the "2018 Notes measurement period") in which the trading price per $1,000 principal amount of 2018 Notes for each trading day of the 2018 Notes measurement period was less than 98% of the product of the last reported sale price of the Company’s Class A common stock and the conversion rate on each such trading day; (3) with respect to any 2018 Notes called for redemption by the Company, at any time prior to the close of business on the second scheduled trading day immediately preceding the redemption date; or (4) upon the occurrence of specified corporate events. On and after August 1, 2024 until the close of business on the second scheduled trading day immediately preceding the maturity date, holders may convert their 2018 Notes at any time, regardless of the foregoing circumstances. Holders of 2018 Notes who convert their 2018 Notes in connection with a make-whole fundamental change or a notice of redemption (each as defined in the 2018 Indenture) may be entitled to a premium in the form of an increase in the conversion rate of the 2018 Notes. The Company may not redeem the 2018 Notes prior to May 8, 2022. On or after May 8, 2022, the Company may redeem for cash all or part of the 2018 Notes if the last reported sale price of the Company’s Class A common stock equals or exceeds 130% of the conversion price then in effect for at least 20 trading days (whether or not consecutive), including at least one of the five trading days immediately preceding the date on which the Company provides notice of redemption, during any 30 consecutive trading days ending on, and including the trading day immediately preceding the date on which the Company provides notice of the redemption. The redemption price will be 100% of the principal amount of the 2018 Notes to be redeemed, plus accrued and unpaid interest, if any. Upon the occurrence of a fundamental change (as defined in the 2018 Indenture), holders may require the Company to repurchase all or a portion of their 2018 Notes for cash at a price equal to 100% of the principal amount of the 2018 Notes to be repurchased plus any accrued but unpaid interest to, but excluding, the fundamental change repurchase date. The 2018 Indenture contains customary terms and covenants, including that upon certain events of default occurring and continuing, either the Trustee or the holders of not less than 25% in aggregate principal amount of the 2018 Notes then outstanding may declare the entire principal amount of all the 2018 Notes plus accrued interest, if any, to be immediately due and payable. Capped Call Transactions The Restated 2017 Capped Call Transactions and 2018 Capped Call Transactions (collectively, the "Capped Call Transactions") are expected generally to reduce the potential dilution and/or offset the cash payments the Company is required to make in excess of the principal amount of the Notes upon conversion of the Notes in the event that the market price per share of the Company’s Class A common stock is greater than the strike price of the Capped Call Transactions (which initially corresponds to the initial conversion price of the Notes and is subject to certain adjustments under the terms of the Capped Call Transactions), with such reduction and/or offset subject to a cap based on the cap price of the Capped Call Transactions. The 2017 Capped Call Transactions have an initial cap price of $154.16 per share of the Company’s Class A common stock, which represents a premium of 100% over the last reported sale price of the Company’s Class A common stock on September 11, 2017, and is subject to certain adjustments under the terms of the Restated 2017 Capped Call Transactions. The 2018 Capped Call Transactions have an initial cap price of $219.63 per share of the Company’s Class A common stock, which represents a premium of 150% over the last reported sale price of the Company’s Class A common stock on November 14, 2018, and is subject to certain adjustments under the terms of the 2018 Capped Call Transactions. Collectively, the Capped Call Transactions cover, initially, the number of shares of the Company’s Class A common stock underlying the Notes, subject to anti-dilution adjustments substantially similar to those applicable to the Notes. The Capped Call Transactions are separate transactions, in each case, entered into by the Company with the 2017 Option Counterparties and 2018 Option Counterparties, and are not part of the terms of the Notes and will not affect any holder’s rights under the Notes. Holders of the Notes will not have any rights with respect to the Capped Call Transactions. The Capped Call Transactions do not meet the criteria for separate accounting as a derivative as they are indexed to the Company's stock. The premiums paid for the Capped Call Transactions have been included as a net reduction to additional paid-in capital within shareholders’ equity.

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Loss per Share	Net Loss per Share Basic and diluted net loss per share is presented using the two-class method required for participating securities: Class A and Class B common stock. The rights of the holders of Class A and Class B common stock are identical, except with respect to voting and conversion. For more information on the rights of Class A and Class B common stockholders, see Note 10, Stockholders' Equity (Deficit) . Basic net loss per share is computed using the weighted-average number of shares of common stock outstanding during the period. Diluted net loss per share is computed using the weighted-average number of shares of common stock and, if dilutive, common stock equivalents outstanding during the period. The Company's common stock equivalents consist of shares issuable upon the release of restricted stock units, and to a lesser extent, the incremental shares of common stock issuable upon the exercise of stock options and unvested restricted stock. The dilutive effect of these common stock equivalents is reflected in diluted earnings per share by application of the treasury stock method. The Company's basic and diluted net loss per share are the same because the Company has generated net loss and common stock equivalents are excluded from diluted net loss per share because they have an antidilutive impact. The Company allocates undistributed earnings between the classes on a one -to- one basis when computing net loss per share. As a result, basic and diluted net loss per Class A and Class B shares are equivalent. The following table presents the calculation of basic and diluted net loss per share (in thousands, except per share data): Year Ended December 31, 2018 2017 2016 Net loss $ (504,080 ) $ (244,614 ) $ (194,375 ) Weighted average common shares used for basic and diluted net loss per share computation 89,472 86,983 84,977 Net loss per common share: Basic and Diluted $ (5.63 ) $ (2.81 ) $ (2.29 ) Dilutive common stock equivalents, representing potentially dilutive common stock options, restricted stock and restricted stock units, of 8.1 million , 7.0 million , and 7.3 million for 2018 , 2017 , and 2016 , respectively, were excluded from diluted earnings per share calculations for these periods because of their anti-dilutive effect. Furthermore, the shares of Class A common stock that would be issuable if the Company elects to settle the Notes in shares were excluded from the diluted earnings per share calculation (using the if-converted method) for the year ended December 31, 2018 because their effect would have been anti-dilutive. The Company may settle the conversions of the Notes in cash, shares of the Company's Class A common stock or any combination thereof at its election. For the 2017 Notes, the number of shares of the Company's Class A common stock issuable at the conversion price of $104.06 per share is expected to be 4.1 million shares and for the 2018 Notes, the number of shares of the Company's Class A common stock issuable at the conversion price of $116.40 is expected to be 4.9 million shares. However, the Capped Call Transactions are expected generally to reduce the potential dilution of the Company's Class A common stock upon any conversion of Notes and/or offset the cash payments the Company is required to make in excess of the principal amount of the Notes. Under the Restated 2017 Capped Call Transactions, the number of shares of Class A common stock issuable at the conversion price of $154.16 is expected to be 2.8 million shares and under the 2018 Capped Call Transactions, the number of shares of Class A common stock issuable at the conversion price of $ 219.63 is expected to be 2.6 million shares. For more information on the Notes and the Capped Call Transactions, see Note 14, Convertible Debt .

Subsequent Event

Subsequent Event	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Event	Subsequent Event On February 21, 2019 (the "Closing Date"), the Company, as guarantor, and Wayfair LLC, a wholly-owned subsidiary of the Company, as borrower (the “Borrower”) entered into an Amended and Restated Credit Agreement (the “Amended Credit Agreement”) with Citibank, in its capacity as administrative agent, swing line lender and letter of credit issuer, and certain other lenders party thereto. The Amended Credit Agreement replaced the Company's existing credit facility with Citibank. The Amended Credit Agreement consists of: • A secured revolving credit facility under which the Borrower may borrow up to $165 million , subject to certain sublimits, with a final maturity date of February 21, 2022 (the “Revolver”). The Revolver may be increased to up to $200 million in the aggregate within 90 days following the Closing Date. • The Borrower also has the right, subject to certain customary conditions, to increase the Revolver by $50 million . • The Revolver has the following sublimits: ◦ a $100 million letter of credit sublimit; and ◦ a $15 million swing line sublimit. The Borrower’s obligations under the Amended Credit Agreement are guaranteed by the Company and certain of its subsidiaries (together, the “Guarantors”). The obligations of the Borrower and the Guarantors are secured by first-priority liens on substantially all of the assets of the Borrower and the Guarantors, including, with certain exceptions, all of the capital stock of the Company’s domestic subsidiaries and 65% of the capital stock of the Company’s first-tier foreign subsidiaries. The proceeds of the Revolver may be used to finance working capital, to refinance certain existing indebtedness and to provide funds for permitted acquisitions, repurchases of equity interests and other general corporate purposes. Borrowings under the Revolver will bear interest through maturity at a variable rate based upon, at the Borrower’s option, either the Eurodollar rate or the base rate (which is the highest of (x) Citibank’s prime rate, (y) one-half of 1.00% in excess of the federal funds effective rate, and (z) 1.00% in excess of the one-month Eurodollar rate), plus, in each case an applicable margin. As of the Closing Date, the applicable margin for Eurodollar rate loans is 1.75% per annum and the applicable margin for base rate loans is 0.75% per annum. The applicable margin is subject to specified changes depending on the Liquidity (as defined in the Amended Credit Agreement) of the Company. Any amounts outstanding under the Revolver are due at maturity. In addition, subject to the terms and conditions set forth in the Amended Credit Agreement, the Borrower is required to make certain mandatory prepayments prior to maturity. The Amended Credit Agreement contains affirmative and negative covenants customarily applicable to senior secured credit facilities, including covenants that, among other things, will limit or restrict the ability of the Borrower and the Guarantors, subject to negotiated exceptions, to incur additional indebtedness and additional liens on their assets, engage in mergers or acquisitions or dispose of assets, pay dividends or make other distributions, voluntarily prepay other indebtedness, enter into transactions with affiliated persons, make investments, and change the nature of their businesses. The Amended Credit Agreement also contains customary events of default, subject to thresholds and grace periods, including, among others, payment default, covenant default, cross default to other material indebtedness, and judgment default. In addition, the Amended Credit Agreement requires the Company to maintain certain levels of Free Cash Flow (as defined in the Amended Credit Agreement).

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The accompanying consolidated financial statements of Wayfair Inc. include its wholly owned subsidiaries including the accounts of Wayfair LLC and its wholly owned subsidiaries (collectively the "Company" or "Wayfair"). All intercompany accounts and transactions have been eliminated. Below is a summary of the wholly-owned subsidiaries of the Company with operations: Subsidiary Location Wayfair LLC U.S. Wayfair Securities Corporation U.S. SK Retail, Inc. U.S. CastleGate Logistics Inc. U.S. CastleGate Logistics Hong Kong Limited Hong Kong CastleGate Logistics Canada Inc. Canada Wayfair Maine LLC U.S. Wayfair Transportation LLC U.S. Wayfair Stores Limited Republic of Ireland Wayfair (UK) Limited United Kingdom Wayfair GmbH Germany Wayfair (BVI) Ltd. British Virgin Islands
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with accounting principles generally accepted in the U.S. ("GAAP") requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, and disclosure of contingent assets and liabilities, at the date of the financial statements, and the reported amounts of revenue and expenses during the reported period. On an ongoing basis, management evaluates these estimates and judgments, including those related to revenue recognition, capitalization of site and software development costs, stock-based compensation, and inventory. Actual results could differ from those estimates.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments purchased with an original maturity (at the date of purchase) of three months or less to be the equivalent of cash for the purpose of consolidated balance sheets and statements of cash flows presentation. Cash equivalents, which consist primarily of money market accounts, are carried at cost, which approximates market value.
Accounts Receivable	Accounts Receivable Accounts receivable are stated net of an allowance for doubtful accounts, which is based on historical losses, existing economic conditions, and other information available at the consolidated balance sheets dates. Uncollectible amounts are written off against the allowance after all collection efforts have been exhausted.
Short-Term Investments and Marketable Securities	Short-Term Investments and Marketable Securities Short-term investments consist of certificates of deposits and marketable securities with original maturities of greater than three months and maturing in less than twelve months from the balance sheet date. The Company classifies its marketable securities as "available-for-sale" securities. Available-for-sale securities are classified as short-term investments and long-term investments on the consolidated balance sheets and are carried at fair value. Unrealized gains and losses on available-for-sale securities that are considered temporary are recorded, net of taxes, in the "Accumulated other comprehensive loss" caption of the Company’s consolidated balance sheets. Unrealized losses, excluding losses related to the credit rating of the security (credit losses), on available-for-sale securities that are considered other-than-temporary but relate to securities that the Company (i) does not intend to sell and (ii) will not be required to sell below cost are also recorded, net of taxes, in "Accumulated other comprehensive loss." Further, the Company does not believe it will be required to sell such securities below cost. Therefore, the only other-than-temporary losses the Company records in "Other income, net" in its consolidated statements of operations are related to credit losses. As of December 31, 2018 and 2017 , the Company’s available-for-sale securities consisted of investment securities. The maturities of the Company’s long-term marketable securities generally range from one to two years. As of December 31, 2018 and 2017 , the Company's available-for-sale securities primarily consisted of corporate bonds and other government obligations that are priced at fair value. The cost basis of a marketable security sold is determined by the Company using the specific identification method.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company's financial assets and liabilities are measured at fair value, which 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 measurement date (exit price). The three levels of inputs used to measure fair value are as follows: • Level 1—Unadjusted quoted prices in active markets for identical assets or liabilities • Level 2—Unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or inputs other than quoted prices that are observable or can be corroborated by observable market data for substantially the full-term of the asset or liability • Level 3—Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the asset or liability This hierarchy requires the Company to use observable market data, when available, and to minimize the use of unobservable inputs when determining fair value. The Company measures its cash equivalents and short-term and long-term investments at fair value. The Company classifies its cash equivalents within Level 1 because the Company values these investments using quoted market prices. The fair value of the Company's Level 1 financial assets is based on quoted market prices of the identical underlying security. The Company classifies short-term and long-term investments within Level 2 because unadjusted quoted prices for identical or similar assets in markets are not active. The Company does not have any assets or liabilities classified as Level 3 financial assets. Refer to Note 3, Fair Value Measurements , for additional detail.
Concentrations of Credit Risk	Concentrations of Credit Risk Financial instruments that subject the Company to credit risk consist of cash and cash equivalents, short-term and long-term investments, and accounts receivable. The risk with respect to cash and cash equivalents and short-term and long-term investments is minimized by the Company's policy of investing in financial instruments (i.e., cash equivalents) with near-term maturities issued by highly rated financial institutions. At times, these balances may exceed federally insured limits; however, to date, the Company has not incurred any losses on these investments. As of December 31, 2018 and 2017 , the Company had $129.5 million and $63.4 million , respectively, in banks located outside the U.S. The risk with respect to accounts receivable is managed by the Company through its policy of monitoring the creditworthiness of its customers to which it grants credit terms in the normal course of business.
Leases	Leases The Company leases office space in several countries under non-cancelable lease agreements. The Company generally leases its office facilities under operating lease agreements. Office facilities subject to an operating lease and the related lease payments are not recorded on the balance sheet. The terms of certain lease agreements provide for rental payments on a graduated basis, however, the Company recognizes rent expense on a straight-line basis over the lease period in accordance with authoritative accounting guidance. Any lease incentives are recognized as reductions of rental expense on a straight-line basis over the term of the lease. The lease term begins on the date the Company becomes legally obligated for the rent payments or when it takes possession of the office space, whichever is earlier. The Company establishes assets and liabilities for the estimated construction costs incurred under lease arrangements where the Company is considered the owner for accounting purposes only, or build-to-suit leases, to the extent the Company is involved in the construction of structural improvements or take construction risk prior to commencement of a lease. Upon occupancy of facilities under build-to-suit leases, the Company assesses whether these arrangements qualify for sales recognition under the sale-leaseback accounting guidance. If the Company continues to be the deemed owner, the facilities are accounted for as financing leases.
Foreign Currency Translation	Foreign Currency Translation The functional currency of the Company is the U.S. dollar, while the functional currency of certain wholly-owned subsidiaries outside of the U.S. is as follows: Subsidiary Currency Wayfair Stores Limited Euro Wayfair (UK) Limited Pound sterling Wayfair GmbH Euro Wayfair (BVI) Ltd. Euro CastleGate Logistics Canada Inc. Canadian dollar CastleGate Logistics Hong Kong Limited Hong Kong dollar The financial statements of the Company are translated to U.S. dollars using year-end exchange rates as to assets and liabilities and average exchange rates as to revenue and expenses. Capital accounts are translated at their historical exchange rates when the capital transaction occurred. The effects of foreign currency translation are included in other comprehensive loss in the consolidated statements of comprehensive loss. Transaction gains and losses are included in the Company's consolidated statements of operations. Translation adjustments arising from the use of differing exchange rates from period to period are included in accumulated other comprehensive loss within total stockholders' deficit.
Inventories	Inventories Inventories consisting of finished goods are stated at the lower of cost or net realizable value, determined by the first-in, first-out (FIFO) method, and consist of product for resale. Inventory costs consist of cost of product and inbound shipping and handling costs. Inventory costs also include direct and indirect labor costs, rents and depreciation expenses associated with the Company's fulfillment centers. Inventory valuation requires the Company to make judgments, based on currently available information, about the likely method of disposition, such as through sales to individual customers, liquidations, and expected recoverable values of each disposition category.
Goods In-Transit	Goods In-Transit Goods in-transit to customers are recorded in prepaid expenses and other current assets. Risk of loss and the transfer of title from the supplier to the Company occur at freight on board shipping point.
Property and Equipment	Property and Equipment Property and equipment are stated at cost, net of depreciation and amortization. Depreciation and amortization on property and equipment is calculated on the straight-line method over the estimated useful lives of the assets as follows: Class Range of Life (In Years) Furniture and computer equipment 3 to 7 Site and software development costs 2 Leasehold improvements The lesser of useful life or lease term Building (leased - Note 7) 30
Site and Software Development Costs	Site and Software Development Costs The Company capitalizes certain costs associated with the development of its sites and internal-use software products after the preliminary project stage is complete and until the software is ready for its intended use. The capitalized costs are amortized over a two-year period. Costs incurred in the preliminary stages of development, after the software is ready for its intended use and for maintenance of internal-use software are expensed as incurred. Upgrade and enhancements are capitalized to the extent they will result in added functionality.
Long-Lived Assets	Long-Lived Assets The Company reviews long-lived assets for impairment whenever events or changes in circumstances, such as service discontinuance or technological obsolescence, indicate that the carrying amount of the long-lived asset may not be recoverable. When such events occur, the Company compares the carrying amount of the asset to the undiscounted expected future cash flows related to the asset. If the comparison indicates that an impairment exists, the amount of the impairment is calculated as the difference between the excess of the carrying amount over the fair value of the asset. If a readily determinable market price does not exist, fair value is estimated using discounted expected cash flows attributable to the asset.
Contingent Liabilities	Contingent Liabilities The Company has certain contingent liabilities that arise in the ordinary course of business activities. The Company accrues for loss contingencies when losses become probable and are reasonably estimable. If the reasonable estimate of the loss is a range and no amount within the range is a better estimate, the minimum amount of the range is recorded as a liability. The Company does not accrue for contingent losses that, in its judgment, are considered to be reasonably possible, but not probable; however, it discloses the range of such reasonably possible losses.
Revenue Recognition	Revenue Recognition The Company adopted ASU No. 2014-09, "Revenue from Contracts with Customers" ("ASU 2014-09") as of January 1, 2018. The company primarily generated revenue through product sales on its five distinct sites ("Direct Retail" net revenue) and through (i) product sales on websites operated by third parties and (ii) fees earned for media solutions (collectively, "Other" net revenue). The Company recognizes net revenue on product sales through the Company's five distinct sites and third party operated websites using the gross method when the Company has concluded it controls the product before it is transferred to the customer. The Company controls products when it is the entity responsible for fulfilling the promise to the customer and takes responsibility for the acceptability of the goods, assumes inventory risk from shipment through the delivery date, has discretion in establishing prices, and selects the suppliers of products sold. The Company recognizes net revenue from sales of its products upon delivery to the customer. As the Company ships a large volume of packages through multiple carriers, actual delivery dates may not always be available and as such the Company estimates delivery dates based on historical data. Net revenue from product sales includes shipping costs charged to the customer and is recorded net of taxes collected from customers, which are remitted to governmental authorities. Cash discounts and rebates earned by customers at the time of purchase are deducted from gross revenue in determining net revenue. Allowances for sales returns are estimated and recorded based on prior returns history, recent trends, and projections for returns on sales in the current period. Allowances for sales returns at December 31, 2018 and 2017, were $35.7 million and $21.2 million , respectively. Actual returns in subsequent periods have been consistent with estimated amounts. The Company also earns revenue through third-party advertisers that pay based on the number of advertisement related clicks, actions, or impressions for advertisements placed on the Company's sites. Revenue earned under these arrangements is included in net revenue and is recognized in the period in which the click, action, or impression occurs. Net revenue from contracts with customers is disaggregated by Direct Retail and Other net revenue and by geographic region because this manner of disaggregation best depicts how the nature, amount, timing, and uncertainty of revenue and cash flows are affected by economic factors. Refer to Note 11, Segment and Geographic Information , for additional detail. The Company has three types of contractual liabilities: (i) cash collections from its customers prior to delivery of products purchased, which are initially recorded in "Deferred revenue," and are recognized as net revenue when the products are delivered, (ii) unredeemed gift cards and site credits, which are initially recorded in "Deferred revenue," and are recognized in the period they are redeemed. Subject to requirements to remit balances to governmental agencies, certain gift cards and site credits not expected to be redeemed, also known as "breakage," are recognized as net revenue based on the historical redemption pattern, which is substantially within twenty-four months from the date of issuance, and (iii) membership rewards redeemable for future purchases, which are earned by customers on purchases made with the Company's Wayfair branded, private label credit card, and are initially recorded in "Other current liabilities," and are recognized as net revenue when redeemed. Contractual liabilities included in "Deferred revenue" and "Other current liabilities" in the consolidated balance sheets were $148.1 million and $3.1 million at December 31, 2018 and $94.1 million and $2.6 million at December 31, 2017, respectively. During the year ended December 31, 2018, the Company recognized $77.3 million and $2.5 million of net revenue included in "Deferred revenue" and "Other current liabilities," respectively, at December 31, 2017. The Company adopted ASU 2014-09 as of January 1, 2018 using a modified retrospective approach and recognized a $4.7 million cumulative-effect adjustment to reduced "Accumulated deficit." The cumulative-effect adjustment to "Accumulated deficit" was due to breakage of gift cards and site credits, to the extent there is no requirement for remitting balances to governmental agencies. Prior period balances were not retrospectively adjusted. The adoption of ASU 2014-09 did not have a material impact on the Company's balance sheet and financial results for the year ended December 31, 2018.
Vendor Rebates	Vendor Rebates The Company earns rebates on incentive programs with its suppliers. These rebates are earned upon shipment of goods. Amounts earned and due from suppliers under these rebate programs are included in other current assets on the consolidated balance sheets and are reflected as a reduction of cost of goods sold on the consolidated statements of operations. Vendor allowances received by the Company reduce the carrying cost of inventory and are recognized in cost of goods sold when the related inventory is sold.
Costs of Goods Sold	Costs of Goods Sold Cost of goods sold consists of the cost of product sold to customers, shipping and handling costs and shipping supplies and fulfillment costs. Fulfillment costs include costs incurred in operating and staffing the fulfillment centers, such as costs attributed to receiving, inspecting, picking, packaging and preparing customer orders for shipment. Cost of goods sold also includes direct and indirect labor costs, including equity-based compensation, for fulfillment center oversight, including payroll and related benefit costs. The Company also performs logistics services for suppliers through its CastleGate and Wayfair Delivery Network solutions, which are earned upon completion of preparing customer orders for shipment and are reflected as a reduction of cost of goods sold on the consolidated statements of operations.
Advertising Costs	Advertising Costs Advertising consists of direct response performance marketing costs, such as display advertising, paid search advertising, social media advertising, search engine optimization, comparison shopping engine advertising, television advertising, direct mail, catalog and print advertising. Expenditures for advertising are expensed in the period that the advertising first takes place.
Merchant Processing Fees	Merchant Processing Fees Merchant processing fees totaling $133.4 million , $88.7 million , and $66.0 million in the years ended December 31, 2018 , 2017 , and 2016 , respectively, are included in customer service and merchant fees expense in the consolidated statements of operations. These fees are charged by third parties that provide merchant processing services for customer payments made by credit cards and debit cards.
Retail Partner Fees	Retail Partner Fees The Company sells its products through websites owned and operated by third-party online retailers, or retail partners. The Company pays a fee for sales generated through these websites and records them as merchant processing fees and advertising costs.
Equity-Based Compensation	Equity-Based Compensation The Company accounts for its equity-based compensation awards in accordance with ASC Topic 718, Compensation—Stock Compensation ("ASC 718"). ASC 718 requires all equity-based payments to employees to be recognized as expense in the statements of operations based on their grant date fair values. The Company has granted stock options, restricted shares and restricted stock units. The Company has primarily granted restricted stock units, and to a lesser extent, restricted stock. The Company granted only restricted stock units to employees in the years ended December 31, 2018 , 2017 , and 2016 . Restricted stock values are determined based on the quoted market price of our Class A common stock on the date of grant. The Company accounts for equity awards to non-employees in accordance with FASB ASC Topic 505-50, Equity-Based Payments to Non-Employees , which requires the fair value of an award to non-employees be remeasured at fair value as the award vests. The Company adopted ASU 2016-09 "Compensation - Stock Compensation" as of January 1, 2017 using a modified retrospective approach with the option to recognize gross stock compensation expense with actual forfeitures recognized as they occur, with a cumulative-effect adjustment to retained earnings recognized as of January 1, 2017 of $8.7 million . The adoption of ASU 2016-09 also requires all income tax adjustments to be recorded in the consolidated and condensed statements of operations.
Income Taxes	Income Taxes Income taxes are accounted for under the asset and liability method. Under the asset and liability method, deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. The Company records valuation allowances to reduce deferred income tax assets to the amount that is more likely than not to be realized. At December 31, 2018, we maintain a full valuation allowance against our net deferred tax asset in all entities with net deferred tax asset. The Company determines whether it is more likely than not that a tax position will be sustained upon examination. If it is not more likely than not that a position will be sustained, no amount of benefit attributable to the position is recognized. The tax benefit to be recognized of any tax position that meets the more likely than not recognition threshold is calculated as the largest amount that is more than 50% likely of being realized upon resolution of the contingency. We evaluate at the end of each reporting period whether some or all of the undistributed earnings of our foreign subsidiaries are permanently reinvested. Our position is based upon several factors including management's evaluation of the Company and its subsidiaries' financial requirements, the short term and long term operational and fiscal objectives of the Company, and the tax consequences associated with the repatriation of earnings.
Net Loss Per Share	Net Loss Per Share The Company follows the two-class method when computing net loss per share for its two issued classes of common stock—Class A and Class B. Basic net income (loss) per share is computed by dividing the net income (loss) by the weighted average number of common stock outstanding for the period. For periods in which the Company has reported net losses, diluted net loss per share is the same as basic net loss per share, since dilutive common stock are not assumed to have been issued if their effect is anti-dilutive.
Subsequent Events	Subsequent Events The Company considers events or transactions that have occurred after the balance sheet date of December 31, 2018 , but prior to the filing of the financial statements with the U.S. Securities and Exchange Commission, to provide additional evidence relative to certain estimates or to identify matters that require additional recognition or disclosure. Subsequent events have been evaluated through the filing of these financial statements.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Stock Compensation In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting ("ASU 2018-07"). This ASU is intended to simplify aspects of share-based compensation issued to non-employees by making the guidance consistent with the accounting for employee share-based compensation. For public entities, ASU 2018-07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. For non-public entities, ASU 2018-07 is effective for annual periods beginning after December 15, 2019. We do not believe the adoption of ASU 2017-09 will have a material impact the Company’s consolidated financial position, results of operations or cash flows. Leases In February 2016, the FASB issued ASU No. 2016-02, "Leases" ("ASU 2016-02"). This ASU revises the accounting related to leases by requiring lessees to recognize a lease liability and a right-of-use asset for virtually all of their leases (other than leases that meet the definition of a short-term lease). The liability will be equal to the present value of the remaining lease payments. The asset will be based on the liability, subject to adjustment, such as for initial direct costs. Operating leases will result in straight-line expense (similar to current operating leases) while finance leases will result in a front-loaded expense pattern (similar to current capital leases). The FASB issued ASU 2018-11 "Leases, Targeted Improvements," which creates the optional transition expedient that allows an entity to apply the transition provisions of the new standard, including its disclosure requirements, at its adoption date instead of at the beginning of the earliest comparative period originally required by ASU 2016-02. The Company will adopt the new lease standard as of January 1, 2019 using the modified retrospective approach, applying the transition provisions of the new standard as of the adoption date. Management will elect the package of practical expedients which among other things, allows the Company to carryforward historical lease classification. The adoption of this standard will result in the derecognition of building assets and finance lease obligations for certain leases that did not pass the sale-leaseback criteria and the recognition of a lease liability and right-of-use asset for these leases in addition to operating leases with an initial term greater than twelve months that are currently not recorded on the consolidated balance sheets. Management expects the adoption of the standard will result in 1) the removal of approximately $0.2 billion of lease finance obligations and correlated property and equipment, net and 2) the addition of approximately $0.5 billion in operating lease right of use assets and $0.6 billion in operating lease right of use liabilities. We do not believe the adoption of ASU 2016-02 will have a notable impact on the Company's liquidity or a materially adverse impact on the Company's debt-covenant compliance.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of the Wholly-Owned Subsidiaries of the Company	Below is a summary of the wholly-owned subsidiaries of the Company with operations: Subsidiary Location Wayfair LLC U.S. Wayfair Securities Corporation U.S. SK Retail, Inc. U.S. CastleGate Logistics Inc. U.S. CastleGate Logistics Hong Kong Limited Hong Kong CastleGate Logistics Canada Inc. Canada Wayfair Maine LLC U.S. Wayfair Transportation LLC U.S. Wayfair Stores Limited Republic of Ireland Wayfair (UK) Limited United Kingdom Wayfair GmbH Germany Wayfair (BVI) Ltd. British Virgin Islands
Summary of Subsidiaries' Functional Currencies	The functional currency of the Company is the U.S. dollar, while the functional currency of certain wholly-owned subsidiaries outside of the U.S. is as follows: Subsidiary Currency Wayfair Stores Limited Euro Wayfair (UK) Limited Pound sterling Wayfair GmbH Euro Wayfair (BVI) Ltd. Euro CastleGate Logistics Canada Inc. Canadian dollar CastleGate Logistics Hong Kong Limited Hong Kong dollar
Schedule of Estimated Useful Lives of Plant, Property and Equipment	Depreciation and amortization on property and equipment is calculated on the straight-line method over the estimated useful lives of the assets as follows: Class Range of Life (In Years) Furniture and computer equipment 3 to 7 Site and software development costs 2 Leasehold improvements The lesser of useful life or lease term Building (leased - Note 7) 30

Marketable Securities and Fai_2

Marketable Securities and Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Schedule of Marketable Securities	The following tables present details of the Company’s marketable securities as of December 31, 2018 and 2017 (in thousands): December 31, 2018 Amortized Cost Gross Unrealized Losses Estimated Fair Value Short-term: Investment securities $ 114,402 $ (124 ) $ 114,278 Long-term: Investment securities 6,603 (77 ) 6,526 Total $ 121,005 $ (201 ) $ 120,804 December 31, 2017 Amortized Cost Gross Unrealized Losses Estimated Fair Value Short-term: Investment securities $ 61,129 $ (97 ) $ 61,032 Long-term: Investment securities 21,695 (134 ) 21,561 Total $ 82,824 $ (231 ) $ 82,593
Schedule of the Fair Value of the Company's Financial Assets Measured at Fair Value on a Recurring Basis Based on the Three-tier Value Hierarchy	The following tables set forth the fair value of the Company's financial assets measured at fair value on a recurring basis as of December 31, 2018 and 2017 based on the three-tier value hierarchy described in Note 2, Summary of Significant Accounting Policies (in thousands): December 31, 2018 Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds and other funds $ 715,086 $ — $ — $ 715,086 Short-term investments: Investment securities — 114,278 — 114,278 Other non-current assets: Certificate of deposit 5,000 — — 5,000 Long-term: Investment securities — 6,526 — 6,526 Total $ 720,086 $ 120,804 $ — $ 840,890 December 31, 2017 Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds $ 488,029 $ — $ — $ 488,029 Short-term investments: Investment securities — 61,032 — 61,032 Other non-current assets Certificate of deposit 5,000 — — 5,000 Long-term: Investment securities — 21,561 — 21,561 Total $ 493,029 $ 82,593 $ — $ 575,622

Intangible assets and Goodwill

Intangible assets and Goodwill (Tables)	12 Months Ended
	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Schedule of Intangible Assets	The following table summarizes intangible assets as of December 31, 2018 and 2017 (in thousands): Weighted-Average December 31, 2018 Amortization Period (Years) Gross Carrying Amount Accumulated Amortization Net Book Value Trademarks 5 $ 1,900 $ (1,900 ) $ — Technology 3 1,678 (1,192 ) 486 Customer relationships 5 1,300 (1,300 ) — Total $ 4,878 $ (4,392 ) $ 486 Weighted-Average December 31, 2017 Amortization Period (Years) Gross Carrying Amount Accumulated Amortization Net Book Value Trademarks 5 $ 1,900 $ (1,678 ) $ 222 Technology 3 1,453 (646 ) 807 Customer relationships 5 1,300 (1,148 ) 152 Total $ 4,653 $ (3,472 ) $ 1,181
Summary of the Future Amortization Expense of Purchased Intangible Assets	The estimated future amortization expense of purchased intangible assets as of December 31, 2018 , is as follows (in thousands): Total 2019 $ 398 2020 75 2021 13 Thereafter — Total $ 486

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	12 Months Ended
	Dec. 31, 2018
Property, Plant and Equipment [Abstract]
Schedule of property and equipment, net	The following table summarizes property and equipment, net as of December 31, 2018 and 2017 (in thousands): December 31, 2018 2017 Furniture and computer equipment $ 339,761 $ 213,790 Site and software development costs 172,653 118,356 Leasehold improvements 109,929 82,614 Construction in progress 90,104 46,826 Building (leased - see Note 7) 184,694 83,681 897,141 545,267 Less accumulated depreciation and amortization (290,164 ) (184,126 ) Property and equipment, net $ 606,977 $ 361,141

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets, Accrued Expenses, Other Current Liabilities, and Other Liabilities (Tables)	12 Months Ended
	Dec. 31, 2018
Payables and Accruals [Abstract]
Components of Prepaid Expenses and Other Current Assets	The following table presents the components of selected balance sheet items as of December 31, 2018 and 2017 (in thousands): December 31, Prepaid expenses and other current assets: 2018 2017 Deferred costs in transit $ 83,652 $ 54,483 Supplier receivable 31,842 29,941 Supplier credits receivable 17,667 12,936 Other prepaid and other current assets 62,269 33,478 Total prepaid expenses and other current assets $ 195,430 $ 130,838
Components of Accrued Expenses	December 31, Accrued expenses: 2018 2017 Employee compensation and related benefits $ 91,914 $ 55,142 Advertising 67,259 38,888 Accrued property, plant and equipment 13,955 8,592 Credit card 16,740 4,573 Audit, legal and professional fees 1,703 1,749 Other accrued expenses 21,426 11,303 Total accrued expenses $ 212,997 $ 120,247
Components of Other Liabilities	December 31, Other current liabilities: 2018 2017 Sales tax payable $ 49,989 $ 35,726 Sales return reserve 35,707 21,243 Other current liabilities 42,299 28,057 Total current other liabilities $ 127,995 $ 85,026 December 31, Other liabilities: 2018 2017 Deferred rent $ 99,746 $ 59,811 Construction costs under build-to-suit leases 60,089 37,545 Other liabilities 553 9,136 Total other liabilities $ 160,388 $ 106,492

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Schedule of future minimum lease commitments under non-cancelable operating leases	Future minimum rental commitments under non-cancelable leases with initial or remaining terms in excess of one year at December 31, 2018 were as follows (in thousands): Amount 2019 $ 106,456 2020 123,757 2021 130,017 2022 126,230 2023 122,698 Thereafter 594,464 Total $ 1,203,622

Equity-Based Compensation (Tabl

Equity-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Activity relating to stock options	The following table presents activity relating to stock options for the year ended December 31, 2018 : Shares Weighted-Average Exercise Price Weighted-Average Remaining Contractual Term (Years) Outstanding at December 31, 2017 126,383 $ 3.02 3.5 Options exercised (46,581 ) $ 2.97 Outstanding and exercisable at December 31, 2018 79,802 $ 3.05 2.5 The following table presents activity relating to restricted common stock for the year ended December 31, 2018 : Shares Weighted-Average Grant Date Fair Value Unvested at December 31, 2017 40,000 $ 44.34 Restricted stock vested (20,000 ) $ 44.34 Unvested as of December 31, 2018 20,000 $ 44.34
Activity relating to restricted stock units	The following table presents activity relating to RSUs for the year ended December 31, 2018 : Shares Weighted-Average Grant Date Fair Value Unvested at December 31, 2017 6,853,606 $ 46.28 RSUs granted 4,834,598 $ 92.62 RSUs vested (2,523,359 ) $ 47.61 RSUs forfeited/canceled (1,193,886 ) $ 57.53 Outstanding as of December 31, 2018 7,970,959 $ 72.43

Segment and Geographic Inform_2

Segment and Geographic Information (Tables)	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Activity Related to Net Revenue and Adjusted EBITDA by Segment	The following tables present Direct Retail and Other net revenues and Adjusted EBITDA attributable to the Company’s reportable segments for the periods presented (in thousands): Year Ended December 31, 2018 2017 2016 U.S. Direct Retail $ 5,751,975 $ 4,075,405 $ 2,993,365 U.S. Other 61,095 77,652 117,132 U.S. segment net revenue 5,813,070 4,153,057 3,110,497 International Direct Retail 966,104 567,838 265,544 International Other — — 4,319 International segment net revenue 966,104 567,838 269,863 Total $ 6,779,174 $ 4,720,895 $ 3,380,360 Year Ended December 31, 2018 2017 2016 Adjusted EBITDA: U.S. $ (19,049 ) $ 35,888 $ 176 International (195,937 ) (102,921 ) (88,868 ) Total reportable segments Adjusted EBITDA (214,986 ) (67,033 ) (88,692 ) Less: reconciling items (1) (289,094 ) (177,581 ) (105,683 ) Net loss $ (504,080 ) $ (244,614 ) $ (194,375 ) (1) Adjustments are made to reconcile total reportable segments Adjusted EBITDA to consolidated net loss including the following (in thousands): Year Ended December 31, 2018 2017 2016 Depreciation and amortization (1) $ 123,542 $ 87,020 $ 55,572 Equity based compensation and related taxes 136,415 72,626 51,953 Interest expense (income), net 28,560 9,433 (694 ) Other expense (income), net 204 (758 ) (1,756 ) Provision for income taxes 2,037 486 608 Other (1) (1,664 ) 8,774 — Total reconciling items $ 289,094 $ 177,581 $ 105,683 (1) The Company recorded $9.6 million of one-time charges in the year ended December 31, 2017 in " Selling, operations, technology, general and administrative " in the consolidated statements of operations related to a warehouse the Company vacated in July 2017. Of the $9.6 million charges, $8.8 million was included in "Other" and related primarily to the excess of the Company's estimated future remaining lease commitments through 2023 over its expected sublease income over the same period, and $0.8 million was included in "Depreciation and amortization" related to accelerated depreciation of leasehold improvements in the warehouse. In the year ended December 31, 2018, the Company terminated the lease and recognized in "Other" a 1.7 million one-time gain related to the difference in the expected future net lease commitments and the actual costs incurred to terminate the lease. The gain was recognized in " Selling, operations, technology, general and administrative " in the consolidated statements of operations. The following table presents the activity related to the Company’s net revenue from Direct Retail sales derived through the Company’s sites and Other sales derived through websites operated by third parties and fees from third-party advertising distribution providers (in thousands): Year Ended December 31, 2018 2017 2016 Net revenue Direct Retail $ 6,718,079 $ 4,643,243 $ 3,258,909 Other 61,095 77,652 121,451 Net revenue $ 6,779,174 $ 4,720,895 $ 3,380,360
Schedule of Long-lived Assets by Geographic Area	The following table presents long-lived assets by segment (in thousands): Year Ended December 31, 2018 2017 Geographic long-lived assets: U.S. $ 577,615 $ 353,414 International 29,362 7,727 Total $ 606,977 $ 361,141

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Schedule of Income Tax Expense (Benefit) Attributable to Income (Loss) From Operation	Income tax expense (benefit) from continuing operations for the years ended December 31, 2018 , 2017 and 2016 is presented below (in thousands): Year ended December 31, 2018 Current Deferred Total Federal $ — $ (86 ) $ (86 ) State 744 (24 ) 720 Foreign 78 1,325 1,403 $ 822 $ 1,215 $ 2,037 Year ended December 31, 2017 Current Deferred Total Federal $ — $ (31 ) $ (31 ) State 540 8 548 Foreign 942 (973 ) (31 ) $ 1,482 $ (996 ) $ 486 Year ended December 31, 2016 Current Deferred Total Federal $ — $ 32 $ 32 State 329 5 334 Foreign 285 (43 ) 242 614 (6 ) 608
Schedule of Difference in the Company's Expected Tax Expense (Benefit) as Computed by Applying the U.S. Federal Corporate Rate to Income Before Tax Expense (Benefit), and Actual Tax is Reconciled	The actual income tax expense (benefit) differs from the expected income tax expense (benefit) computed at the U.S. Federal statutory tax rate of 21% due to the following (in thousands): Year Ended December 31, 2018 2017 2016 Tax expense (benefit) at federal statutory rate $ (105,429 ) $ (85,445 ) $ (67,819 ) State income tax expense, net of federal benefit (22,584 ) (11,432 ) (5,225 ) Foreign tax rate differential 14,976 23,179 17,109 Non-deductible equity based compensation expense 3,267 1,080 2,321 Windfall benefits from equity based compensation (29,003 ) (24,168 ) — Change in valuation allowance 119,370 24,209 53,467 Change in tax rate 197 71,919 — Limitation on officer's compensation 5,283 — — Debt integration termination 9,236 — — Other 6,724 1,144 755 Net income tax expense $ 2,037 $ 486 $ 608
Schedule of Components of Income Tax Expense (Benefit) Determined by Tax Jurisdiction	The components of income before income tax expense (benefit) determined by tax jurisdiction, are as follows (in thousands): Year Ended December 31, 2018 2017 2016 U.S. $ (327,356 ) $ (143,800 ) $ (118,851 ) Foreign (174,687 ) (100,328 ) (74,916 ) Total $ (502,043 ) $ (244,128 ) $ (193,767 )
Schedule of Tax Effects of Temporary Differences that Give Rise to Significant Portions of Deferred Tax Assets and Liabilities	The tax effects of temporary differences that give rise to significant portions of deferred tax assets and liabilities for the periods presented are as follows (in thousands): December 31, 2018 2017 Deferred tax assets: Accounts receivable $ 2,400 $ 1,814 Inventories 495 391 Operating loss carry-forwards 262,481 146,666 Equity based compensation expense 9,733 9,087 Intangibles 12,526 13,862 Accrued payroll 13,900 8,582 Accrued expenses and reserves 12,339 10,933 Charitable contributions 657 284 Deferred rent 90,868 48,936 Gross deferred tax assets 405,399 240,555 Less: Valuation allowance (247,454 ) (178,488 ) Net deferred tax assets 157,945 62,067 Deferred tax liabilities: Prepaid expenses (3,030 ) (1,825 ) Capitalized technology (15,427 ) (11,339 ) Property and equipment (73,189 ) (34,986 ) Goodwill (133 ) (110 ) Convertible debt (66,166 ) (12,580 ) Other — (12 ) Total deferred tax liabilities (157,945 ) (60,852 ) Net deferred tax assets (liabilities) — 1,215 Non-current net deferred tax assets (liabilities) $ — $ 1,215

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Schedule of Calculation of Basic and Diluted Net Loss Per Share	The following table presents the calculation of basic and diluted net loss per share (in thousands, except per share data): Year Ended December 31, 2018 2017 2016 Net loss $ (504,080 ) $ (244,614 ) $ (194,375 ) Weighted average common shares used for basic and diluted net loss per share computation 89,472 86,983 84,977 Net loss per common share: Basic and Diluted $ (5.63 ) $ (2.81 ) $ (2.29 )

Basis of Presentation (Details)

Basis of Presentation (Details) supplier in Thousands, product in Millions	12 Months Ended
	Dec. 31, 2018productsupplier
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Minimum number of products offered | product	14
Minimum number of suppliers providing products offered | supplier	11

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Details)	12 Months Ended
	Dec. 31, 2018USD ($)common_stock_classbrand	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Jan. 01, 2018USD ($)	Jan. 01, 2017USD ($)
Concentrations of Credit Risk
Cash and cash equivalents and short-term investments held in banks located outside the U.S.	$ 129,500,000	$ 63,400,000
Goods In-Transit
Goods in-transit	83,700,000	54,500,000
Property, plant and equipment
Property and equipment, net	606,977,000	361,141,000
Long-Lived Assets
Impairment of long-lived assets or identifiable intangibles	$ 0	0	$ 0
Revenue Recognition
Number of distinct sites | brand	5
Allowance for sales returns	$ 35,700,000	21,200,000
Contract liabilities	148,057,000	94,116,000
Cumulative-effect adjustment to reduce Accumulated deficit	1,082,689,000	583,266,000
Advertising Costs
Advertising expense	774,189,000	549,959,000	409,125,000
Prepaid advertising costs	700,000	600,000
Merchant Processing Fees
Merchant processing fees	133,400,000	88,700,000	66,000,000
Retail Partner Fees
Retail partner fees included in merchant processing fees	1,200,000	1,500,000	1,900,000
Retail partner fees included in advertising costs	$ 5,500,000	7,000,000	11,000,000
Earnings Per Share
Number of classes of common stock | common_stock_class	2
ASU 2014-09
Equity-based Compensation
Cumulative-effect adjustment for adoption of ASU	$ 4,657,000
ASU 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Revenue Recognition
Cumulative-effect adjustment to reduce Accumulated deficit				$ 4,700,000
ASU 2014-09 | Retained Earnings
Equity-based Compensation
Cumulative-effect adjustment for adoption of ASU	4,657,000
ASU 2016-09
Equity-based Compensation
Cumulative-effect adjustment for adoption of ASU		0
ASU 2016-09 | Retained Earnings
Equity-based Compensation
Cumulative-effect adjustment for adoption of ASU		(8,712,000)			$ 8,700,000
Deferred Revenue
Revenue Recognition
Contract liabilities	148,100,000	94,100,000
Revenue recognized	77,300,000
Other Current Liabilities
Revenue Recognition
Contract liabilities	$ 3,100,000	2,600,000
Revenue recognized		2,500,000
Other Long-term Investments | Minimum
Marketable Securities [Abstract]
Marketable securities, term	1 year
Other Long-term Investments | Maximum
Marketable Securities [Abstract]
Marketable securities, term	2 years
Furniture and computer equipment | Minimum
Property, plant and equipment
Estimated useful lives	3 years
Furniture and computer equipment | Maximum
Property, plant and equipment
Estimated useful lives	7 years
Site and software development costs
Property, plant and equipment
Estimated useful lives	2 years
Property and equipment, net	$ 62,900,000	45,400,000
Amortization expense	$ 51,300,000	$ 34,500,000	$ 21,600,000
Building
Property, plant and equipment
Estimated useful lives	30 years

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - Scenario, Forecast [Member] - ASU 2016-02 $ in Billions	Jan. 01, 2019USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Finance Lease, Liability	$ (0.2)
Operating lease, right-of-use asset	0.5
Operating lease, liabilities	$ 0.6

Marketable Securities and Fai_3

Marketable Securities and Fair Value Measurements - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities, available-for-sale	$ 120,804,000	$ 82,593,000
Other-than-temporary impairment loss	0	0	$ 0
Realized gains (losses)	0	0	$ 0
Other Long-term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Securities, available-for-sale	$ 6,526,000	$ 21,561,000
Minimum | Other Long-term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities, term	1 year
Maximum | Other Long-term Investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities, term	2 years

Marketable Securities and Fai_4

Marketable Securities and Fair Value Measurements - Schedule of Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair value measurements
Amortized Cost	$ 121,005	$ 82,824
Gross Unrealized Losses	(201)	(231)
Securities, available-for-sale	120,804	82,593
Investment securities
Fair value measurements
Amortized Cost	114,402	61,129
Gross Unrealized Losses	(124)	(97)
Securities, available-for-sale	114,278	61,032
Investment securities
Fair value measurements
Amortized Cost	6,603	21,695
Gross Unrealized Losses	(77)	(134)
Securities, available-for-sale	$ 6,526	$ 21,561

Marketable Securities and Fai_5

Marketable Securities and Fair Value Measurements - Schedule of Financial Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair value measurements
Total	$ 840,890
Cash equivalents
Fair value measurements
Money market funds and other funds	715,086	$ 488,029
Short-term investments
Fair value measurements
Investment securities	114,278	61,032
Other non-current assets
Fair value measurements
Certificate of deposit	5,000	5,000
Other Long-term Investments
Fair value measurements
Investment securities	6,526	21,561
Level 1
Fair value measurements
Total	720,086
Level 1 | Cash equivalents
Fair value measurements
Money market funds and other funds	715,086	488,029
Level 1 | Short-term investments
Fair value measurements
Investment securities	0	0
Level 1 | Other non-current assets
Fair value measurements
Certificate of deposit	5,000	5,000
Level 1 | Other Long-term Investments
Fair value measurements
Investment securities	0	0
Level 2
Fair value measurements
Total	120,804
Level 2 | Cash equivalents
Fair value measurements
Money market funds and other funds	0	0
Level 2 | Short-term investments
Fair value measurements
Investment securities	114,278	61,032
Level 2 | Other non-current assets
Fair value measurements
Certificate of deposit	0	0
Level 2 | Other Long-term Investments
Fair value measurements
Investment securities	6,526	21,561
Level 3
Fair value measurements
Total	0
Level 3 | Cash equivalents
Fair value measurements
Money market funds and other funds	0	0
Level 3 | Short-term investments
Fair value measurements
Investment securities	0	0
Level 3 | Other non-current assets
Fair value measurements
Certificate of deposit	0	0
Level 3 | Other Long-term Investments
Fair value measurements
Investment securities	$ 0	0
Recurring basis
Fair value measurements
Total		575,622
Recurring basis | Level 1
Fair value measurements
Total		493,029
Recurring basis | Level 2
Fair value measurements
Total		82,593
Recurring basis | Level 3
Fair value measurements
Total		$ 0

Intangible assets and Goodwil_2

Intangible assets and Goodwill - Summary of Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Intangible assets
Gross Carrying Amount	$ 4,878	$ 4,653
Accumulated Amortization	(4,392)	(3,472)
Net Book Value	486	1,181
Amortization expense related to intangible assets	$ 900	$ 1,100	$ 900
Trademarks
Intangible assets
Weighted Average Amortization Period (Years)	5 years	5 years
Gross Carrying Amount	$ 1,900	$ 1,900
Accumulated Amortization	(1,900)	(1,678)
Net Book Value	$ 0	$ 222
Technology
Intangible assets
Weighted Average Amortization Period (Years)	3 years	3 years
Gross Carrying Amount	$ 1,678	$ 1,453
Accumulated Amortization	(1,192)	(646)
Net Book Value	$ 486	$ 807
Customer relationships
Intangible assets
Weighted Average Amortization Period (Years)	5 years	5 years
Gross Carrying Amount	$ 1,300	$ 1,300
Accumulated Amortization	(1,300)	(1,148)
Net Book Value	$ 0	$ 152

Intangible assets and Goodwil_3

Intangible assets and Goodwill - Estimated Future Amortization Expense (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
2,019	$ 398
2,020	75
2,021	13
Thereafter	0
Net Book Value	486	$ 1,181
Goodwill [Line Items]
Goodwill	2,100	$ 1,900
Intellectual Property
Goodwill [Line Items]
Goodwill	$ 1,700

Property and Equipment, net (De

Property and Equipment, net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 897,141	$ 545,267
Less accumulated depreciation and amortization	(290,164)	(184,126)
Property and equipment, net	606,977	361,141
Depreciation and amortization	122,600	85,900	$ 54,600
Furniture and computer equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	339,761	213,790
Site and software development costs
Property, Plant and Equipment [Line Items]
Property and equipment, gross	172,653	118,356
Property and equipment, net	62,900	45,400
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	109,929	82,614
Construction in progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	90,104	46,826
Building
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 184,694	$ 83,681

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets, Accrued Expenses, Other Current Liabilities, and Other Liabilities - Components of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Prepaid expenses and other current assets:
Deferred costs in transit	$ 83,652	$ 54,483
Supplier receivable	31,842	29,941
Supplier credits receivable	17,667	12,936
Other prepaid and other current assets	62,269	33,478
Total prepaid expenses and other current assets	$ 195,430	$ 130,838

Prepaid Expenses and Other Cu_4

Prepaid Expenses and Other Current Assets, Accrued Expenses, Other Current Liabilities, and Other Liabilities - Components of Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued expenses:
Employee compensation and related benefits	$ 91,914	$ 55,142
Advertising	67,259	38,888
Accrued property, plant and equipment	13,955	8,592
Credit card	16,740	4,573
Audit, legal and professional fees	1,703	1,749
Other accrued expenses	21,426	11,303
Total accrued expenses	$ 212,997	$ 120,247

Prepaid Expenses and Other Cu_5

Prepaid Expenses and Other Current Assets, Accrued Expenses, Other Current Liabilities, and Other Liabilities - Components of Other Current Liabilities and Other Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Other current liabilities
Sales tax payable	$ 49,989	$ 35,726
Sales return reserve	35,707	21,243
Other current liabilities	42,299	28,057
Other current liabilities	127,995	85,026
Other liabilities:
Deferred rent	99,746	59,811
Construction costs under build-to-suit leases	60,089	37,545
Other liabilities	553	9,136
Total other liabilities	$ 160,388	$ 106,492

Commitments and Contingencies -

Commitments and Contingencies - Future Minimum Rental Commitments Under Non-cancelable Leases (Details) $ in Thousands	Dec. 31, 2018USD ($)
Future minimum lease commitments under non cancelable operating leases
2,019	$ 106,456
2,020	123,757
2,021	130,017
2,022	126,230
2,023	122,698
Thereafter	594,464
Total	$ 1,203,622

Commitments and Contingencies_2

Commitments and Contingencies - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)wharehouse	Dec. 31, 2017USD ($)wharehouse	Dec. 31, 2016USD ($)
Other Commitments [Line Items]
Rent expense under operating leases	$ 63,800	$ 45,200	$ 33,600
Letters of credit outstanding, amount	26,800	15,300
Minimum sublease rentals	3,400
Lease financing obligations, net of current portion	183,056	82,580
Land lease expense	2,700	900	100
Financing obligation, interest expense	12,000	$ 6,900	$ 1,400
Financing obligation, future minimum commitments due, principal	75,600
Financing obligation, future minimum commitments due, interest	$ 22,100
Warehouse Lease Agreement
Other Commitments [Line Items]
Warehouse lease arrangements | wharehouse	3	2
Warehouse Lease Agreement | Warehouse Lease Arrangement, One, 2018
Other Commitments [Line Items]
Letters of credit outstanding, amount	$ 2,500
Warehouse Lease Agreement | Warehouse Lease Arrangement One, 2017
Other Commitments [Line Items]
Letters of credit outstanding, amount		$ 800
Warehouse Lease Agreement | Warehouse Lease Arrangement, Two, 2017
Other Commitments [Line Items]
Letters of credit outstanding, amount		1,000
Warehouse Lease Agreement | Warehouse Lease Arrangement One and Two, 2017
Other Commitments [Line Items]
Lease financing obligations, net of current portion	$ 101,000	$ 53,800

Employee Benefit Plans - Employ

Employee Benefit Plans - Employee Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Benefit Plans
Age of the full-time employees qualified to participate in the defined contribution plan	21 years
Expense related to savings plan recognized	$ 18.2	$ 9	$ 6.3
Maximum
Employee Benefit Plans
Matching contribution by employer as a percentage of employee's considered compensation	4.00%

Equity-Based Compensation - Nar

Equity-Based Compensation - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Stock options
Equity based compensation
Aggregate intrinsic value of stock options exercised	$ 4.3	$ 4.8
Aggregate intrinsic value of stock options outstanding	6.9
Aggregate intrinsic value of stock options exercisable	6.9
Restricted stock
Equity based compensation
Intrinsic value of stock vested and repurchased (less than $0.1 million)	1.8	1.6
Aggregate intrinsic value of nonvested restricted common stock	$ 1.8
Weighted average remaining vesting term for options expected to vest	1 year
Restricted stock | Maximum
Equity based compensation
Unrecognized share-based compensation	$ 1.8
Restricted stock units
Equity based compensation
Aggregate intrinsic value of nonvested restricted common stock	718
Unrecognized share-based compensation	$ 531.5
Weighted average remaining vesting term for options expected to vest	1 year 7 months 4 days
Intrinsic value, of stock vested	$ 262.4	$ 137.2
2014 Plan
Equity based compensation
Number of shares authorized	8,603,066
2014 Plan | Subsequent Event
Equity based compensation
Number of shares available for future grant			6,202,378

Equity-Based Compensation - Sum

Equity-Based Compensation - Summary of Activity Related to Stock Options (Details ) - Stock options - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Shares
Outstanding at the beginning of the period (in shares)	126,383
Options exercised (in shares)	(46,581)
Outstanding and exercisable at the end of the period (in shares)	79,802	126,383
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 3.02
Options exercised (in dollars per share)	2.97
Outstanding and exercisable at the end of the period (in dollars per share)	$ 3.05	$ 3.02
Weighted-Average Remaining Contractual Term (Years)
Outstanding		3 years 6 months
Exercisable at the end of the period	2 years 5 months 20 days

Equity-Based Compensation - S_2

Equity-Based Compensation - Summary of Activity Related to Restricted Common Stock (Details) - Restricted stock	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Restricted common stock
Unvested at the beginning of the period (in shares) | shares	40,000
Restricted stock vested (in shares) | shares	(20,000)
Unvested at the end of the period (in shares) | shares	20,000
Weighted-Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share) | $ / shares	$ 44.34
Restricted stock vested (in dollars per share) | $ / shares	44.34
Unvested at the end of the period (in dollars per share) | $ / shares	$ 44.34

Equity-Based Compensation - S_3

Equity-Based Compensation - Summary of Activity Relating to RSU's (Details) - Restricted stock units	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Shares
Unvested at the beginning of the period (in shares) | shares	6,853,606
RSU's granted (in shares) | shares	4,834,598
RSUs vested (in shares) | shares	(2,523,359)
RSUs forfeited/cancelled (in shares) | shares	(1,193,886)
Unvested at the end of the period (in shares) | shares	7,970,959
Weighted-Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share) | $ / shares	$ 46.28
RSUs granted (in dollars per share) | $ / shares	92.62
RSUs vested (in dollars per share) | $ / shares	47.61
RSUs forfeited/cancelled (in dollars per share) | $ / shares	57.53
Unvested at the end of the period (in dollars per share) | $ / shares	$ 72.43

Stockholders' Deficit (Details)

Stockholders' Deficit (Details)	12 Months Ended
	Dec. 31, 2018vote$ / sharesshares	Dec. 31, 2017$ / sharesshares	Oct. 31, 2014$ / sharesshares
Stockholders' Equity
Preferred stock, shares authorized			10,000,000
Preferred stock, par value (in dollars per share) | $ / shares			$ 0.001
Preferred stock, shares issued	0
Preferred stock, shares outstanding	0
Class A common stock
Common stock
Common stock, shares authorized (in shares)	500,000,000	500,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0.001	$ 0.001
Common stock, shares outstanding (in shares)	62,329,701	57,398,983
Number of votes each holder is entitled | vote	1
Class B common stock
Common stock
Common stock, shares authorized (in shares)	164,000,000	164,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0.001	$ 0.001
Common stock, shares outstanding (in shares)	28,417,882	30,809,627
Number of votes each holder is entitled | vote	10
Conversion ratio	1
Conversion ratio upon transfer	1
Number of shares converted into Class A shares	53,619,863
Class B common stock | Minimum
Common stock
Percentage of outstanding shares of class B common stock that shall convert automatically in event of affirmative vote or written consent of holders for automatic conversion	66.67%
Class B common stock | Maximum
Common stock
Aggregate number of shares outstanding Class A common stock and Class B common stock that will automatically convert (less than)	10.00%

Segment and Geographic Inform_3

Segment and Geographic Information (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total net revenue		$ 6,779,174	$ 4,720,895	$ 3,380,360
Adjusted EBITDA		(214,986)	(67,033)	(88,692)
Less: reconciling items		(289,094)	(177,581)	(105,683)
Net loss		(504,080)	(244,614)	(194,375)
Depreciation and amortization		123,542	87,020	55,572
Equity based compensation and related taxes		136,415	72,626	51,953
Interest expense (income), net		28,560	9,433	(694)
Other expense (income), net		204	(758)	(1,756)
Provision for income taxes		2,037	486	608
Other			8,774	0
Total reconciling items		289,094	177,581	105,683
Selling, operations, technology, general and administrative		1,025,767	634,801	467,020
Leasehold improvements
Segment Reporting Information [Line Items]
Depreciation and amortization			800
Warehouse vacated
Segment Reporting Information [Line Items]
Other	$ 1,700	(1,664)	8,800
Selling, operations, technology, general and administrative			9,600
U.S.
Segment Reporting Information [Line Items]
Total net revenue		5,813,070	4,153,057	3,110,497
Adjusted EBITDA		(19,049)	35,888	176
International
Segment Reporting Information [Line Items]
Total net revenue		966,104	567,838	269,863
Adjusted EBITDA		(195,937)	(102,921)	(88,868)
Direct Retail
Segment Reporting Information [Line Items]
Total net revenue		6,718,079	4,643,243	3,258,909
Direct Retail | U.S.
Segment Reporting Information [Line Items]
Total net revenue		5,751,975	4,075,405	2,993,365
Direct Retail | International
Segment Reporting Information [Line Items]
Total net revenue		966,104	567,838	265,544
Other
Segment Reporting Information [Line Items]
Total net revenue		61,095	77,652	121,451
Other | U.S.
Segment Reporting Information [Line Items]
Total net revenue		61,095	77,652	117,132
Other | International
Segment Reporting Information [Line Items]
Total net revenue		$ 0	$ 0	$ 4,319

Segment and Geographic Inform_4

Segment and Geographic Information - Long Lived Assets by Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Geographic analysis, revenue and long-lived assets
Total net revenue	$ 6,779,174	$ 4,720,895	$ 3,380,360
Geographic long-lived assets	606,977	361,141
U.S.
Geographic analysis, revenue and long-lived assets
Total net revenue	5,813,070	4,153,057	3,110,497
Geographic long-lived assets	577,615	353,414
International
Geographic analysis, revenue and long-lived assets
Total net revenue	966,104	567,838	269,863
Geographic long-lived assets	29,362	7,727
Direct Retail
Geographic analysis, revenue and long-lived assets
Total net revenue	6,718,079	4,643,243	3,258,909
Direct Retail | U.S.
Geographic analysis, revenue and long-lived assets
Total net revenue	5,751,975	4,075,405	2,993,365
Direct Retail | International
Geographic analysis, revenue and long-lived assets
Total net revenue	966,104	567,838	265,544
Other
Geographic analysis, revenue and long-lived assets
Total net revenue	61,095	77,652	121,451
Other | U.S.
Geographic analysis, revenue and long-lived assets
Total net revenue	61,095	77,652	117,132
Other | International
Geographic analysis, revenue and long-lived assets
Total net revenue	$ 0	$ 0	$ 4,319

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current income tax expense (benefit)
Current federal tax expense (benefit)	$ 0	$ 0	$ 0
Current state income tax expense (benefit)	744	540	329
Current foreign income tax expense (benefit)	78	942	285
Current income tax expense (benefit)	822	1,482	614
Deferred income tax expense (benefit)
Deferred federal income tax expense (benefit)	(86)	(31)	32
Deferred state income tax expense (benefit)	(24)	8	5
Deferred foreign income tax expense (benefit)	1,325	(973)	(43)
Deferred income tax expense (benefit)	1,215	(996)	(6)
Income tax expense (benefit)
Federal income tax expense (benefit), Total	(86)	(31)	32
State income tax expense (benefit), Total	720	548	334
Foreign income tax expense (benefit), Total	1,403	(31)	242
Net income tax expense	2,037	486	608
Reconciliation of the U.S. federal corporate rate to to income before tax expense (benefit), and actual tax
Tax expense (benefit) at federal statutory rate	(105,429)	(85,445)	(67,819)
Decrease in income taxes resulting from:
State income tax expense, net of federal benefit	(22,584)	(11,432)	(5,225)
Foreign tax rate differential	14,976	23,179	17,109
Non-deductible equity based compensation expense	3,267	1,080	2,321
Windfall benefits from equity based compensation	(29,003)	(24,168)	0
Change in valuation allowance	119,370	24,209	53,467
Change in tax rate	197	71,919	0
Limitation on officer's compensation	5,283	0	0
Debt integration termination	9,236	0	0
Other	6,724	1,144	755
Net income tax expense	$ 2,037	486	608
Income (loss) from continuing operations:
Effective tax rate (almost), percent	0.00%
U.S.	$ (327,356)	(143,800)	(118,851)
Foreign	(174,687)	(100,328)	(74,916)
Loss before income taxes	(502,043)	(244,128)	$ (193,767)
Deferred tax assets:
Accounts receivable	2,400	1,814
Inventories	495	391
Operating loss carry-forwards	262,481	146,666
Equity based compensation expense	9,733	9,087
Intangibles	12,526	13,862
Accrued payroll	13,900	8,582
Accrued expenses and reserves	12,339	10,933
Charitable contributions	657	284
Deferred rent	90,868	48,936
Gross deferred tax assets	405,399	240,555
Less: Valuation allowance	(247,454)	(178,488)
Net deferred tax assets	157,945	62,067
Deferred tax liabilities:
Prepaid expenses	(3,030)	(1,825)
Capitalized technology	(15,427)	(11,339)
Property and equipment	(73,189)	(34,986)
Goodwill	(133)	(110)
Convertible debt	(66,166)	(12,580)
Other	0	(12)
Total deferred tax liabilities	(157,945)	(60,852)
Deferred tax assets	0	$ 1,215
Valuation allowance	69,000
Federal net operating loss carryforwards	787,800
State net operating loss carryforwards	711,800
Foreign operating loss carryforwards	424,400
Operating loss carryforwards, subject to expiration	4,800
Undistributed earnings of the entity's foreign subsidiaries	$ 5,200

Credit Agreement (Details)

Credit Agreement (Details) - USD ($)	Apr. 12, 2018	Feb. 22, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Credit agreement
Borrowing under credit agreement			$ 0	$ 0	$ 0
Credit Card Program
Credit agreement
Maximum borrowing capacity		$ 40,000,000
Revolving Credit Facility
Credit agreement
Maximum borrowing capacity		$ 100,000,000
Right to increase	$ 25,000,000
Revolving Credit Facility | Eurodollar
Credit agreement
Basis spread	1.00%
Applicable margin	0.0175
Revolving Credit Facility | Base Rate
Credit agreement
Applicable margin	0.0075
Revolving Credit Facility | Federal Funds Effective Swap Rate
Credit agreement
Basis spread		0.50%
Revolving Credit Facility | Letter of Credit Sublimit
Credit agreement
Maximum borrowing capacity	$ 65,000,000
Revolving Credit Facility | Revolver Swing Line Supplement
Credit agreement
Maximum borrowing capacity	$ 10,000,000

Convertible Debt (Details)

Convertible Debt (Details)	Nov. 19, 2018USD ($)day$ / shares	Sep. 15, 2017USD ($)day$ / shares	Sep. 11, 2017institution$ / shares	Nov. 30, 2018USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Debt Instrument [Line Items]
Number of Initial Purchasers | institution			2
Proceeds from issuance of convertible notes, net of issuance costs					$ 562,047,000	$ 420,449,000	$ 0
Capped call transactions paid from net proceeds				$ 93,400,000	93,438,000	44,160,000	$ 0
Convertible Debt | 3.75% Senior Note Due 2022
Debt Instrument [Line Items]
Debt, aggregate principal amount		$ 431,250,000
Interest rate, stated percentage		0.375%
Proceeds from issuance of convertible notes, net of issuance costs		$ 420,400,000
Capped call transactions paid from net proceeds		44,200,000
Equity component of Notes		$ 95,800,000
Interest expense					(24,200,000)
Fair value of Notes					465,000,000	452,500,000
Conversion ratio		0.00961
Conversion price (in dollars per share) | $ / shares		$ 104.06
Trading days (weather or not consecutively) | day		20
Trading days (consecutive) | day		30
Percentage of conversion stock price		130.00%
During number of business day period		5 days
Consecutive trading day period (after any)		10 days
Principal amount of Notes		$ 1,000
Measurement period percentage (less than)		98.00%
Minimum number of trading days immediately preceding the date on which notice or redemption provided | day		1
Trading days immediately preceding date on which notice of redemption provided | day		5
Redemption price, percentage of principal amount to be redeemed		100.00%
Default percentage of aggregate principal amount, of notes outstanding (not less than)		25.00%
Convertible Debt | 3.75% Senior Note Due 2022 | Class A common stock
Debt Instrument [Line Items]
Conversion price (in dollars per share) | $ / shares		$ 104.06
Cap price per share (in usd per share) | $ / shares			$ 154.16
Premium over last reported sale price, percentage			100.00%
Convertible Debt | 3.75% Senior Note Due 2022 | Over-Allotment Option
Debt Instrument [Line Items]
Over-allotment option included in issuance of notes		$ 56,250,000
Effective interest rate, percentage		6.00%
Convertible Debt | 1.125% 2018 Senior Note Due 2024
Debt Instrument [Line Items]
Debt, aggregate principal amount	$ 500,000,000			$ 575,000,000
Interest rate, stated percentage				1.125%
Over-allotment option included in issuance of notes	$ 75,000,000			$ 0
Proceeds from issuance of convertible notes, net of issuance costs				$ 562,000,000
Capped call transactions paid from net proceeds						$ 93,400,000
Effective interest rate, percentage				8.10%
Equity component of Notes				$ 181,500,000
Fair value of Notes					$ 580,200,000
Conversion ratio	0.008591
Trading days (weather or not consecutively) | day	20
Trading days (consecutive) | day	30
Percentage of conversion stock price	130.00%
Principal amount of Notes	$ 1,000
Measurement period percentage (less than)	98.00%
Redemption price, percentage of principal amount to be redeemed	100.00%
Default percentage of aggregate principal amount, of notes outstanding (not less than)	25.00%
Convertible Debt | 1.125% 2018 Senior Note Due 2024 | Class A common stock
Debt Instrument [Line Items]
Interest rate, stated percentage	1.125%
Conversion price (in dollars per share) | $ / shares	$ 116.40
Cap price per share (in usd per share) | $ / shares	$ 219.63
Premium over last reported sale price, percentage	150.00%

Net Loss per Share (Details)

Net Loss per Share (Details) $ / shares in Units, shares in Thousands, $ in Thousands	Nov. 19, 2018$ / sharesshares	Sep. 15, 2017USD ($)$ / sharesshares	Sep. 11, 2017$ / sharesshares	Nov. 30, 2018USD ($)	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)$ / sharesshares
Anti-dilutive securities
Capped call transactions paid from net proceeds | $				$ 93,400	$ 93,438	$ 44,160	$ 0
Allocation of undistributed earnings between stock classes, conversion ratio					1
Numerator:
Net loss | $					$ (504,080)	$ (244,614)	$ (194,375)
Denominator:
Weighted average units used for basic and diluted net loss per unit computation (in units) | shares					89,472	86,983	84,977
Net loss per common unit attributable to common unit holders:
Basic and Diluted (in dollars per unit) | $ / shares					$ (5.63)	$ (2.81)	$ (2.29)
3.75% Senior Note Due 2022 | Convertible Debt
Anti-dilutive securities
Capped call transactions paid from net proceeds | $		$ 44,200
Net loss per common unit attributable to common unit holders:
Conversion price (in dollars per share) | $ / shares		$ 104.06
3.75% Senior Note Due 2022 | Convertible Debt | Class A common stock
Net loss per common unit attributable to common unit holders:
Conversion price (in dollars per share) | $ / shares		$ 104.06
Conversion of stock, shares Issued (in shares) | shares		4,100
Cap price per share (in usd per share) | $ / shares			$ 154.16
Capped Call Transaction, shares issued (in shares) | shares			2,800
1.125% 2018 Senior Note Due 2024 | Convertible Debt
Anti-dilutive securities
Capped call transactions paid from net proceeds | $						$ 93,400
1.125% 2018 Senior Note Due 2024 | Convertible Debt | Class A common stock
Net loss per common unit attributable to common unit holders:
Conversion price (in dollars per share) | $ / shares	$ 116.40
Conversion of stock, shares Issued (in shares) | shares	4,900
Cap price per share (in usd per share) | $ / shares	$ 219.63
Capped Call Transaction, shares issued (in shares) | shares	2,600
Common Stock Options, Restricted Stock Options And Restricted Stock Units
Net loss per common unit attributable to common unit holders:
Common stock outstanding that have been excluded from the computation of diluted net loss per share | shares					8,100	7,000	7,300

Subsequent Event (Details)

Subsequent Event (Details) - Revolving Credit Facility - USD ($)	Feb. 19, 2019	Apr. 12, 2018	Feb. 22, 2017	Feb. 21, 2019
Subsequent Event [Line Items]
Maximum borrowing capacity			$ 100,000,000
Federal Funds Effective Swap Rate
Subsequent Event [Line Items]
Basis spread			0.50%
Eurodollar
Subsequent Event [Line Items]
Basis spread		1.00%
Applicable margin		0.0175
Base Rate
Subsequent Event [Line Items]
Applicable margin		0.0075
Letter of Credit Sublimit
Subsequent Event [Line Items]
Maximum borrowing capacity		$ 65,000,000
Revolver Swing Line Supplement
Subsequent Event [Line Items]
Maximum borrowing capacity		$ 10,000,000
Subsequent Event
Subsequent Event [Line Items]
Maximum borrowing capacity				$ 165,000,000
Optional borrowing capacity				200,000,000
Right to increase the Revolver	$ 50,000,000
Borrower Obligations, capital stock of first-tier foreign subsidiaries, percent	65.00%
Subsequent Event | Federal Funds Effective Swap Rate
Subsequent Event [Line Items]
Basis spread	0.50%
Subsequent Event | Eurodollar
Subsequent Event [Line Items]
Basis spread	1.00%
Applicable margin	0.0175
Subsequent Event | Base Rate
Subsequent Event [Line Items]
Applicable margin	0.0075
Subsequent Event | Letter of Credit Sublimit
Subsequent Event [Line Items]
Maximum borrowing capacity				100,000,000
Subsequent Event | Revolver Swing Line Supplement
Subsequent Event [Line Items]
Maximum borrowing capacity				$ 15,000,000