Wayfair (W) 10-Q 1 Aug 19 2019 Q2 Quarterly report Financial data

Cover

Cover - shares	6 Months Ended
	Jun. 30, 2019	Jul. 24, 2019
Document Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Jun. 30, 2019
Document Transition Report	false
Entity File Number	001-36666
Entity Registrant Name	Wayfair Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	36-4791999
Entity Address, Address Line One	4 Copley Place
Entity Address, City or Town	Boston
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02116
City Area Code	617
Local Phone Number	532-6100
Title of 12(b) Security	Class A Common Stock, $0.001 par value per share
Trading Symbol	W
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Central Index Key	0001616707
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q2
Amendment Flag	false
Class A Common Stock
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		64,830,064
Class B Common Stock
Document Information [Line Items]
Entity Common Stock, Shares Outstanding		27,507,532

CONSOLIDATED AND CONDENSED BALA

CONSOLIDATED AND CONDENSED BALANCE SHEETS - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets
Cash and cash equivalents	$ 675,111	$ 849,461
Short-term investments	39,342	114,278
Accounts receivable, net of allowance of $12,644 and $9,312 at June 30, 2019 and December 31, 2018, respectively	77,295	50,603
Inventories	45,611	46,164
Prepaid expenses and other current assets	219,541	195,430
Total current assets	1,056,900	1,255,936
Property and equipment, net	480,492	606,977
Goodwill and intangible assets, net	560	2,585
Operating lease right-of-use assets	630,731
Long-term investments	0	6,526
Other noncurrent assets	13,417	18,826
Total assets	2,182,100	1,890,850
Current liabilities
Accounts payable	779,938	650,174
Accrued expenses	218,607	212,997
Unearned revenue	155,811	148,057
Other current liabilities	179,155	127,995
Total current liabilities	1,333,511	1,139,223
Lease financing obligation, net of current portion	0	183,056
Operating lease liabilities	685,489
Long-term debt	761,604	738,904
Other liabilities	6,853	160,388
Total liabilities	2,787,457	2,221,571
Commitments and contingencies (Note 7)
Convertible preferred stock, $0.001 par value per share: 10,000,000 shares authorized and none issued at June 30, 2019 and December 31, 2018	0	0
Additional paid-in capital	859,092	753,657
Accumulated deficit	(1,463,166)	(1,082,689)
Accumulated other comprehensive (loss)	(1,376)	(1,780)
Total stockholders’ deficit	(605,357)	(330,721)
Total liabilities and stockholders’ deficit	2,182,100	1,890,850
Class A common stock
Common stock	65	63
Class B common stock
Common stock	$ 28	$ 28

CONSOLIDATED AND CONDENSED BA_2

CONSOLIDATED AND CONDENSED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Accounts receivable allowance	$ 12,644	$ 9,312
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 outstanding (in shares)	64,591,843	62,329,701
Common stock, shares issued (in shares)	64,591,843	62,329,701
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 outstanding (in shares)	27,506,309	28,417,882
Common stock, shares issued (in shares)	27,506,309	28,417,882

CONSOLIDATED AND CONDENSED STAT

CONSOLIDATED AND CONDENSED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Statement [Abstract]
Net revenue	$ 2,343,251	$ 1,655,256	$ 4,288,080	$ 3,059,525
Cost of goods sold	1,783,651	1,270,249	3,258,024	2,350,694
Gross profit	559,600	385,007	1,030,056	708,831
Operating expenses:
Customer service and merchant fees	88,502	61,792	164,975	115,676
Advertising	259,166	177,582	503,135	339,228
Selling, operations, technology, general and administrative	383,109	240,972	726,757	452,335
Total operating expenses	730,777	480,346	1,394,867	907,239
Loss from operations	(171,177)	(95,339)	(364,811)	(198,408)
Interest expense, net	(10,252)	(5,796)	(19,490)	(11,203)
Other income, net	322	666	3,400	1,607
Loss before income taxes	(181,107)	(100,469)	(380,901)	(208,004)
Provision for income taxes	831	265	1,426	505
Net loss	$ (181,938)	$ (100,734)	$ (382,327)	$ (208,509)
Net loss per share, basic and diluted (in dollars per share)	$ (1.98)	$ (1.13)	$ (4.18)	$ (2.35)
Weighted average number of common stock outstanding used in computing per share amounts, basic and diluted (in shares)	91,802	89,158	91,455	88,814

CONSOLIDATED AND CONDENSED ST_2

CONSOLIDATED AND CONDENSED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement of Comprehensive Income [Abstract]
Net loss	$ (181,938)	$ (100,734)	$ (382,327)	$ (208,509)
Other comprehensive loss:
Foreign currency translation adjustments	(439)	1,252	204	492
Net unrealized gain (loss) on available-for-sale investments	47	97	200	(38)
Comprehensive loss	$ (182,330)	$ (99,385)	$ (381,923)	$ (208,055)

CONSOLIDATED AND CONDENSED ST_3

CONSOLIDATED AND CONDENSED STATEMENTS OF STOCKHOLDERS' 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
Balance (in shares) at Dec. 31, 2017		88,209
Balance at Dec. 31, 2017	$ (48,329)	$ 88	$ 537,212	$ (583,266)	$ (2,363)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(208,509)			(208,509)
Other comprehensive income	454				454
Exercise of options to purchase common stock (in shares)		25
Exercise of options to purchase common stock	74		74
Issuance of common stock upon vesting of RSUs (in shares)		1,251
Issuance of common stock upon vesting of RSUs	1	$ 1
Shares withheld related to net settlement of RSUs (in shares)		(8)
Shares withheld related to net settlement of RSUs	(635)		(635)
Equity compensation expense	56,809		56,809
Balance (in shares) at Jun. 30, 2018		89,477
Balance at Jun. 30, 2018	(195,478)	$ 89	593,460	(787,118)	(1,909)
Balance (in shares) at Mar. 31, 2018		88,854
Balance at Mar. 31, 2018	(127,204)	$ 89	562,349	(686,384)	(3,258)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(100,734)			(100,734)
Other comprehensive income	1,349				1,349
Exercise of options to purchase common stock (in shares)		15
Exercise of options to purchase common stock	45		45
Issuance of common stock upon vesting of RSUs (in shares)		611
Issuance of common stock upon vesting of RSUs	0	$ 0
Shares withheld related to net settlement of RSUs (in shares)		(3)
Shares withheld related to net settlement of RSUs	(244)		(244)
Equity compensation expense	31,310		31,310
Balance (in shares) at Jun. 30, 2018		89,477
Balance at Jun. 30, 2018	(195,478)	$ 89	593,460	(787,118)	(1,909)
Balance (in shares) at Dec. 31, 2018		90,748
Balance at Dec. 31, 2018	(330,721)	$ 91	753,657	(1,082,689)	(1,780)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(382,327)			(382,327)
Other comprehensive income	404				404
Exercise of options to purchase common stock (in shares)		25
Exercise of options to purchase common stock	80		80
Issuance of common stock upon vesting of RSUs (in shares)		1,328
Issuance of common stock upon vesting of RSUs	2	$ 2
Shares withheld related to net settlement of RSUs (in shares)		(3)
Shares withheld related to net settlement of RSUs	(424)		(424)
Equity compensation expense	105,779		105,779
Balance (in shares) at Jun. 30, 2019		92,098
Balance at Jun. 30, 2019	(605,357)	$ 93	859,092	(1,463,166)	(1,376)
Balance (in shares) at Mar. 31, 2019		91,401
Balance at Mar. 31, 2019	(479,147)	$ 92	802,973	(1,281,228)	(984)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(181,938)			(181,938)
Other comprehensive income	(392)				(392)
Exercise of options to purchase common stock (in shares)		4
Exercise of options to purchase common stock	13		13
Issuance of common stock upon vesting of RSUs (in shares)		695
Issuance of common stock upon vesting of RSUs	1	$ 1
Shares withheld related to net settlement of RSUs (in shares)		(2)
Shares withheld related to net settlement of RSUs	(259)		(259)
Equity compensation expense	56,365		56,365
Balance (in shares) at Jun. 30, 2019		92,098
Balance at Jun. 30, 2019	$ (605,357)	$ 93	$ 859,092	$ (1,463,166)	$ (1,376)

CONSOLIDATED AND CONDENSED ST_4

CONSOLIDATED AND CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities
Net loss	$ (382,327)	$ (208,509)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	83,922	54,882
Equity based compensation	100,247	54,213
Amortization of discount and issuance costs on convertible notes	23,015	9,353
Other non-cash adjustments	(1,595)	133
Changes in operating assets and liabilities:
Accounts receivable	(27,653)	3,058
Inventories	540	(2,741)
Prepaid expenses and other current assets	(22,432)	(22,354)
Accounts payable and accrued expenses	128,611	95,743
Unearned revenue and other liabilities	14,914	52,211
Other assets	(1,324)	(1,462)
Net cash (used in) provided by operating activities	(84,082)	34,527
Cash flows from investing activities
Sale and maturities of short-term investments	82,164	26,646
Purchase of property and equipment	(115,340)	(61,093)
Site and software development costs	(58,866)	(28,573)
Other investing activities	2,773	(267)
Net cash used in investing activities	(89,269)	(63,287)
Cash flows from financing activities
Taxes paid related to net share settlement of equity awards	(424)	(635)
Deferred financing costs	(791)	0
Net proceeds from exercise of stock options	80	74
Net cash used in financing activities	(1,135)	(561)
Effect of exchange rate changes on cash and cash equivalents	136	(187)
Net decrease in cash and cash equivalents	(174,350)	(29,508)
Cash and cash equivalents
Beginning of period	849,461	558,960
End of period	675,111	529,452
Supplemental cash flow information
Cash paid for interest on long-term debt	3,720	746
Purchase of property and equipment included in accounts payable and accrued expenses and in other liabilities	5,259	15,463
Construction costs capitalized under finance lease obligation and other leases	$ 0	$ 54,143

Basis of Presentation

Basis of Presentation	6 Months Ended
	Jun. 30, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	1. 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. The consolidated and condensed financial statements and other disclosures contained in this Quarterly Report on Form 10-Q are those of the Company. The consolidated and condensed balance sheet data as of December 31, 2018 was derived from audited financial statements. The accompanying consolidated and condensed balance sheet as of June 30, 2019 , the consolidated and condensed statements of operations, consolidated and condensed statements of comprehensive loss, consolidated and condensed statements of stockholders' deficit, and consolidated and condensed statements of cash flows for the periods ended June 30, 2019 and 2018 are unaudited. The unaudited interim financial statements have been prepared on the same basis as the audited consolidated financial statements and in the opinion of management, reflect all adjustments, consisting of only normal recurring adjustments, necessary for the fair statement of the Company’s financial position as of June 30, 2019 and statements of operations, comprehensive loss, statements of stockholders' deficit, and cash flows for the periods ended June 30, 2019 and 2018 . The financial data and the other information disclosed in these notes to the consolidated and condensed financial statements related to these periods are unaudited. The consolidated and condensed statements of operations, comprehensive loss, stockholders' deficit, and cash flows for the period ended June 30, 2019 are not necessarily indicative of the results of operations and cash flows that may be expected for the year ending December 31, 2019

Summary of Significant Accounti

Summary of Significant Accounting Policies	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies The Company has identified the significant accounting policies that are critical to understanding its business and results of operations. Leases The Company adopted ASU No. 2016-02, "Leases" ("ASU 2016-02") on January 1, 2019 using the modified retrospective approach. The Company also elected the package of practical expedients, which among other things, allowed the Company to carryforward historical lease classification. The adoption of the standard resulted in (1) the derecognition of building assets and finance lease obligations for certain leases that did not pass the sale-leaseback criteria, (2) the derecognition of construction in progress assets and other long-term liabilities for certain lease arrangements whereby the Company is no longer considered the deemed construction owner of the construction projects, and (3) the recognition of operating lease right-of-use ("ROU") assets and lease liabilities for lease arrangements with an initial term greater than twelve months. Adoption of ASU 2016-02 resulted in the following adjustments to the balance sheet (in thousands): December 31, 2018 ASU 2016-02 Adjustments January 1, 2019 Balance sheet line item Property and equipment, net $ 606,977 $ (227,709 ) $ 379,268 Operating lease right-of-use assets $ — $ 524,082 $ 524,082 Other noncurrent assets $ 18,826 $ (6,814 ) $ 12,012 Other liabilities $ 160,388 $ (155,996 ) $ 4,392 Other current liabilities $ 127,995 $ (9,624 ) $ 118,371 Lease financing obligation, net of current portion $ 183,056 $ (183,056 ) $ — Operating lease liabilities $ — $ 636,385 $ 636,385 Accumulated deficit $ (1,082,689 ) $ 1,850 $ (1,080,839 ) The adoption of the standard did not have a notable impact on the Company's liquidity or a materially adverse impact on the Company's debt covenant compliance. The Company determines if an arrangement is a lease at inception. Operating leases are included in " Operating lease right-of-use assets ," " Other current liabilities ," and " Operating lease liabilities " on our consolidated and condensed balance sheets. As of June 30, 2019 , the Company has not entered into material financing lease arrangements. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. The Company's lease terms may include options to extend or terminate the lease when it is reasonably certain that management will exercise that option. The Company has lease arrangements with lease and non-lease components. For the Company's warehouse and fulfillment center lease arrangements, the Company accounts for lease and non-lease components separately. For all other lease arrangements, the Company accounts for lease and non-lease components as a single lease component. Operating lease ROU assets and operating lease liabilities are recognized based on the present value of the future minimum lease payments over the lease term at commencement date. As most of the Company's leases do not provide an implicit rate, the Company uses its estimated incremental borrowing rate based on the information available at commencement date in determining the present value of future payments. The determination of the Company’s incremental borrowing rate requires judgment. The incremental borrowing rate for each lease is primarily based on publicly-available information for companies within the same industry and with similar credit profiles. The rate is then adjusted for the impact of collateralization, the lease term and other specific terms included in the Company’s lease arrangements. The incremental borrowing rate was determined at lease commencement, or as of January 1, 2019 for operating leases existing upon adoption of ASC 842. The incremental borrowing rate is subsequently reassessed upon a modification to the lease arrangement. The operating lease ROU asset also includes any lease payments made prior to commencement date and excludes lease incentives and initial direct costs incurred. ROU assets are subsequently assessed for impairment in accordance with the Company’s accounting policy for long-lived assets. Stock Compensation The Company adopted ASU No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting ("ASU 2018-07") on January 1, 2019. The ASU simplifies aspects of share-based compensation issued to non-employees by making the guidance consistent with the accounting for employee share-based compensation. The adoption of ASU 2018-07 did not have a material impact the Company’s consolidated financial position, results of operations, comprehensive loss, stockholders' deficit or cash flows. Hosting Arrangements In August 2018, the FASB issued guidance related to a customer’s accounting for implementation costs incurred in hosting arrangements. The guidance aligns the requirements for capitalizing implementation costs incurred in cloud computing arrangements with the requirements for capitalizing costs to develop or obtain internal-use software. This guidance is effective for interim and annual reporting periods beginning after December 15, 2019, and early adoption is permitted. This guidance may be applied either retrospectively or prospectively. The Company early adopted the guidance on a prospective basis as of January 1, 2019. The adoption of the guidance did not have a material impact on the Company's consolidated financial position, results of operations, comprehensive loss, stockholders' deficit or cash flows. The Company believes that other than the implementation of ASU 2016-02 and ASU 2018-07, there have been no significant changes during the six months ended June 30, 2019 to the items disclosed in Note 2, Summary of Significant Accounting Policies , included in Part II, Item 8, Financial Statements and Supplementary Data , of the Company’s Annual Report on Form 10-K for the year ended December 31, 2018 .

Marketable Securities and Fair

Marketable Securities and Fair Value Measurements	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
Marketable Securities and Fair Value Measurements	3. Marketable Securities and Fair Value Measurements Marketable Securities As of June 30, 2019 and December 31, 2018 , all of the Company’s marketable securities were classified as available-for-sale and their estimated fair values were $39.3 million and $120.8 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 June 30, 2019 , the Company’s available-for-sale securities primarily consisted of corporate bonds and other government obligations that are priced at fair value. During the three and six months ended June 30, 2019 and 2018 , the Company did not recognize any other-than-temporary impairment losses. 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 three and six months ended June 30, 2019 and 2018 , the Company did not have any realized gains or losses. The following tables present details of the Company’s marketable securities as of June 30, 2019 and December 31, 2018 (in thousands): June 30, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Short-term: Investment securities $ 39,343 $ 20 $ (21 ) $ 39,342 Total $ 39,343 $ 20 $ (21 ) $ 39,342 December 31, 2018 Amortized Cost Gross Unrealized Gains 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 Fair Value Measurements 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 and restricted cash 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. The following tables set forth the fair value of the Company’s financial assets measured at fair value on a recurring basis as of June 30, 2019 and December 31, 2018 based on the three-tier value hierarchy (in thousands): June 30, 2019 Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds and other funds $ 618,520 $ — $ — $ 618,520 Short-term investments: Investment securities — 39,342 — 39,342 Other non-current assets: Certificate of deposit 5,076 — — 5,076 Total $ 623,596 $ 39,342 $ — $ 662,938 December 31, 2018 Level 1 Level 2 Level 3 Total Cash equivalents: Money market 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

Intangible Assets and Goodwill

Intangible Assets and Goodwill	6 Months Ended
	Jun. 30, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets and Goodwill	4. Intangible Assets and Goodwill As of June 30, 2019 and December 31, 2018 , the Company had $0.2 million and $0.5 million of intangible assets respectively. Amortization expense related to intangible assets was $0.1 million and $0.3 million for the three months ended June 30, 2019 and 2018 , respectively, and $0.3 million and $0.6 million for the six months ended June 30, 2019 and June 30, 2018 , respectively. In January 2019, the Company sold certain intellectual property. The proceeds from the sale exceeded the $1.7 million goodwill carrying value. Goodwill was $0.4 million and $2.1 million as of June 30, 2019 and December 31, 2018 , respectively.

Property and Equipment, net

Property and Equipment, net	6 Months Ended
	Jun. 30, 2019
Property, Plant and Equipment [Abstract]
Property and Equipment, net	5. Property and Equipment, net The following table summarizes property and equipment, net as of June 30, 2019 and December 31, 2018 (in thousands): June 30, December 31, Furniture and computer equipment $ 425,499 $ 339,761 Site and software development costs 228,624 172,653 Leasehold improvements 156,405 109,929 Construction in progress 27,010 90,104 Buildings (leased - Note 6) — 184,694 837,538 897,141 Less accumulated depreciation and amortization (357,046 ) (290,164 ) Property and equipment, net $ 480,492 $ 606,977 Property and equipment depreciation and amortization expense was $44.2 million and $28.6 million for the three months ended June 30, 2019 and 2018 , respectively, and $83.6 million and $54.3 million for the six months ended June 30, 2019 and 2018 , respectively.

Leases

Leases	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Leases	6. Leases After Adoption of ASU 2016-02 The Company has lease arrangements for warehouse, fulfillment center, office, and data center spaces. These leases expire at various dates through 2035 . Operating lease expense was $29.4 million and $16.1 million in the three months ended June 30, 2019 and 2018 , respectively, and $55.0 million and $30.6 million in the six months ended June 30, 2019 and 2018 , respectively. The following table presents other information related to leases (in thousands): Six months ended June 30, 2019 Supplemental cash flows information Cash payments included in operating cash flows from lease arrangements $ 50,387 Right-of-use assets obtained in exchange for lease obligations $ 138,064 June 30, 2019 Additional lease information Weighted average remaining lease term 10 years Weighted average discount rate 6.9 % Future minimum lease payments under non-cancellable leases as of June 30, 2019 were as follows (in thousands): Amount 2019 (excluding the six months ended June 30, 2019) $ 57,089 2020 121,836 2021 123,559 2022 117,310 2023 113,546 Thereafter 507,024 Total future minimum lease payments 1,040,364 Less: Imputed interest (285,661 ) Total $ 754,703 The following table presents total operating leases (in thousands): June 30, 2019 Balance sheet line item Other current liabilities $ 69,214 Operating lease liabilities 685,489 Total operating leases $ 754,703 As of June 30, 2019 , the Company has entered into $498.7 million of additional operating leases, primarily related to build-to-suit warehouse leases that have not yet commenced. As the Company does not control the underlying assets during the construction period, the Company is not considered the owner of the construction projects for accounting purposes. These operating leases will commence between 2019 and 2021 with lease terms of 2 to 15 years. Before Adoption of ASU 2016-02 The Company established assets and liabilities for the estimated construction costs incurred under lease arrangements where the Company is considered the owner for accounting purposes only to the extent the Company is involved in the construction of structural improvements or takes construction risk prior to commencement of a lease. These are referred to as "build-to-suit leases". 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 efforts related to 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. Accordingly, these leases were accounted for as financing leases and $101.0 million was recorded in " Lease financing obligation, net of current portion " and " Property and equipment, net
Leases	6. Leases After Adoption of ASU 2016-02 The Company has lease arrangements for warehouse, fulfillment center, office, and data center spaces. These leases expire at various dates through 2035 . Operating lease expense was $29.4 million and $16.1 million in the three months ended June 30, 2019 and 2018 , respectively, and $55.0 million and $30.6 million in the six months ended June 30, 2019 and 2018 , respectively. The following table presents other information related to leases (in thousands): Six months ended June 30, 2019 Supplemental cash flows information Cash payments included in operating cash flows from lease arrangements $ 50,387 Right-of-use assets obtained in exchange for lease obligations $ 138,064 June 30, 2019 Additional lease information Weighted average remaining lease term 10 years Weighted average discount rate 6.9 % Future minimum lease payments under non-cancellable leases as of June 30, 2019 were as follows (in thousands): Amount 2019 (excluding the six months ended June 30, 2019) $ 57,089 2020 121,836 2021 123,559 2022 117,310 2023 113,546 Thereafter 507,024 Total future minimum lease payments 1,040,364 Less: Imputed interest (285,661 ) Total $ 754,703 The following table presents total operating leases (in thousands): June 30, 2019 Balance sheet line item Other current liabilities $ 69,214 Operating lease liabilities 685,489 Total operating leases $ 754,703 As of June 30, 2019 , the Company has entered into $498.7 million of additional operating leases, primarily related to build-to-suit warehouse leases that have not yet commenced. As the Company does not control the underlying assets during the construction period, the Company is not considered the owner of the construction projects for accounting purposes. These operating leases will commence between 2019 and 2021 with lease terms of 2 to 15 years. Before Adoption of ASU 2016-02 The Company established assets and liabilities for the estimated construction costs incurred under lease arrangements where the Company is considered the owner for accounting purposes only to the extent the Company is involved in the construction of structural improvements or takes construction risk prior to commencement of a lease. These are referred to as "build-to-suit leases". 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 efforts related to 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. Accordingly, these leases were accounted for as financing leases and $101.0 million was recorded in " Lease financing obligation, net of current portion " and " Property and equipment, net

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Jun. 30, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	7. Commitments and Contingencies Letters of Credit The Company has issued letters of credit for approximately $45.7 million and $26.8 million , primarily as security for certain lease agreements as of June 30, 2019 and December 31, 2018 , respectively. Legal Matters 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.

Equity-Based Compensation

Equity-Based Compensation	6 Months Ended
	Jun. 30, 2019
Share-based Payment Arrangement [Abstract]
Equity-Based Compensation	8. 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, stock appreciation rights ("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 six months ended June 30, 2019 : Options Weighted-Average Exercise Price Weighted-Average Remaining Contractual Term (Years) Outstanding at December 31, 2018 79,802 $ 3.05 2.5 Options exercised (25,179 ) $ 3.17 Outstanding and exercisable at June 30, 2019 54,623 $ 3.00 2.0 Intrinsic value of stock options exercised was $3.7 million for the six months ended June 30, 2019 . Aggregate intrinsic value of stock options outstanding and currently exercisable is $7.8 million . All stock options were fully vested at June 30, 2019 . The following table presents activity relating to restricted common stock for the six months ended June 30, 2019 : Shares Weighted- Average Grant Date Fair Value Unvested at December 31, 2018 20,000 $ 44.34 Unvested as of June 30, 2019 20,000 $ 44.34 Aggregate intrinsic value of unvested restricted common stock is $2.9 million as of June 30, 2019 . Unrecognized equity based compensation expense related to unvested restricted common stock is $0.9 million with a weighted average remaining vesting term of 0.5 years as of June 30, 2019 . The following table presents activity relating to RSUs for the six months ended June 30, 2019 : Shares Weighted- Average Grant Date Fair Value Outstanding at December 31, 2018 7,970,959 $ 72.43 RSUs granted 1,939,275 $ 151.00 RSUs vested (1,330,455 ) $ 65.87 RSUs forfeited/canceled (581,240 ) $ 83.66 Outstanding as of June 30, 2019 7,998,539 $ 91.75 The intrinsic value of RSUs vested was $187.3 million for the six mon ths ended June 30, 2019 . Aggregate intrinsic value of RSUs unvested is $1.2 billion as of June 30, 2019 . Unrecognized equity based compensation expense related to outstanding RSUs is $671.8 million with a weighted average remaining ve sting term of 1.5 years at June 30, 2019 .

Unearned Revenue

Unearned Revenue	6 Months Ended
	Jun. 30, 2019
Revenue from Contract with Customer [Abstract]
Unearned Revenue	9. Unearned Revenue 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 " Unearned revenue ," and are recognized as net revenue when the products are delivered, (ii) unredeemed gift cards and store credits, which are initially recorded in " Unearned revenue ," and are recognized in the period they are redeemed, 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. The portion of gift cards and store credits not expected to be redeemed are recognized as net revenue based on historical redemption pattern, which is substantially within twenty-four months from the date of issuance. Contractual liabilities included in " Unearned revenue " and " Other current liabilities " in the consolidated and condensed balance sheets were $155.8 million and $3.1 million at June 30, 2019 and $148.1 million and $3.1 million at December 31, 2018 , respectively. During the six months ended June 30, 2019 , the Company recognized $121.9 million and $1.5 million of net revenue included in " Unearned revenue " and " Other current liabilities ," respectively, at December 31, 2018 . 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 10, Segment and Geographic Information , for additional detail.

Segment and Geographic Informat

Segment and Geographic Information	6 Months Ended
	Jun. 30, 2019
Segment Reporting [Abstract]
Segment and Geographic Information	10. 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 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, net ," " Other (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): Three months ended June 30, Six months ended June 30, 2019 2018 2019 2018 U.S. Direct Retail $ 1,989,026 $ 1,397,009 $ 3,633,076 $ 2,583,214 U.S. Other 11,492 14,335 25,140 29,714 U.S. segment net revenue 2,000,518 1,411,344 3,658,216 2,612,928 International Direct Retail 342,733 243,912 629,864 446,597 International segment net revenue 342,733 243,912 629,864 446,597 Total net revenue $ 2,343,251 $ 1,655,256 $ 4,288,080 $ 3,059,525 Three months ended June 30, Six months ended June 30, 2019 2018 2019 2018 Adjusted EBITDA: U.S. $ (342 ) $ 7,200 $ (28,124 ) $ (738 ) International (69,641 ) (42,009 ) (144,077 ) (84,031 ) Total reportable segments Adjusted EBITDA (69,983 ) (34,809 ) (172,201 ) (84,769 ) Less: reconciling items (1) (111,955 ) (65,925 ) (210,126 ) (123,740 ) Net loss $ (181,938 ) $ (100,734 ) $ (382,327 ) $ (208,509 ) (1) Adjustments are made to reconcile total reportable segments Adjusted EBITDA to consolidated net loss including the following (in thousands): Three months ended June 30, Six months ended June 30, 2019 2018 2019 2018 Depreciation and amortization $ 44,339 $ 28,920 $ 83,922 $ 54,882 Equity based compensation and related taxes 56,855 31,610 108,688 58,757 Interest expense, net 10,252 5,796 19,490 11,203 Other (income), net (322 ) (666 ) (3,400 ) (1,607 ) Provision for income taxes 831 265 1,426 505 Total reconciling items $ 111,955 $ 65,925 $ 210,126 $ 123,740 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 our media solutions business (in thousands): Three months ended June 30, Six months ended June 30, 2019 2018 2019 2018 Net revenue Direct Retail $ 2,331,759 $ 1,640,921 $ 4,262,940 $ 3,029,811 Other 11,492 14,335 25,140 29,714 Net revenue $ 2,343,251 $ 1,655,256 $ 4,288,080 $ 3,059,525 The following table presents total assets attributable to the Company's reportable segments reconciled to consolidated amounts (in thousands): June 30, 2019 December 31, 2018 Assets by segment U.S. $ 822,954 $ 670,344 International 179,303 56,177 Total reportable segments assets 1,002,257 726,521 Plus: reconciling corporate assets 1,179,843 1,164,329 Total assets $ 2,182,100 $ 1,890,850 U.S. and International segment assets consist primarily of "Accounts receivable," " Inventories ," " Prepaid expenses and other current assets ," " Property and equipment, net " and " Operating lease right-of-use assets ." Corporate assets include " Cash and cash equivalents ," marketable securities, " Long-term investments ," corporate facilities, " Goodwill and intangible assets, net ," and capitalized internal-use software and website development costs.

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2019
Income Tax Disclosure [Abstract]
Income Taxes	11. Income Taxes Income tax expense was $0.8 million and $0.3 million for the three months ended June 30, 2019 and 2018 , respectively, and $1.4 million and $0.5 million for the six months ended June 30, 2019 and 2018 , respectively. The income tax expense recorded in the three and six months ended June 30, 2019 and 2018 is primarily related to income earned in certain foreign jurisdictions and U.S. state income taxes. Deferred income taxes reflect the impact of temporary differences between the amounts of assets and liabilities for financial reporting purposes and such amounts as measured by tax laws. The Company has deferred tax assets related to its net operating loss carryforwards accumulated since the fourth quarter of 2014 and related to net operating loss carryforwards of certain of its foreign subsidiaries. A valuation allowance against net deferred tax assets is required if, based upon the available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized. The Company reassesses the valuation allowance on a quarterly basis and has provided a valuation allowance on substantially all of its worldwide net deferred tax assets. The Company had no material unrecognized tax benefits as of June 30, 2019 and December 31, 2018

Stockholders' Deficit

Stockholders' Deficit	6 Months Ended
	Jun. 30, 2019
Equity [Abstract]
Stockholders' Deficit	12. 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 June 30, 2019 , 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 64,591,843 and 62,329,701 shares of Class A common stock and 27,506,309 and 28,417,882 shares of Class B common stock were outstanding as of June 30, 2019 and December 31, 2018 , 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 June 30, 2019 , 54,532,689

Credit Agreement

Credit Agreement	6 Months Ended
	Jun. 30, 2019
Debt Disclosure [Abstract]
Credit Agreement	13. Credit Agreement 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 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 was 1.75% per annum and the applicable margin for base rate loans was 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). The Company did not borrow any amounts under its credit agreement during the six months ended June 30, 2019 and the year ended December 31, 2018 . 14. 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. 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 2018 Initial Purchasers' option to purchase such additional 2018 Notes, 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 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. The following table presents the outstanding principal amount and carrying value of the 2017 Notes and 2018 Notes as of the date presented (in thousands): June 30, 2019 December 31, 2018 2017 Notes 2018 Notes 2017 Notes 2018 Notes Principal amounts: Principal $ 431,250 $ 575,000 $ 431,250 $ 575,000 Unamortized debt discount (70,021 ) (174,625 ) (79,911 ) (187,435 ) Net carrying amount $ 361,229 $ 400,375 $ 351,339 $ 387,565 The following table presents total interest expense recognized related to the 2017 Notes and 2018 Notes (in thousands): Three Months Ended June 30, 2019 2018 2017 Notes 2018 Notes 2017 Notes Contractual interest expense $ 404 $ 1,617 $ 404 Interest cost related to amortization of the debt discount $ 4,981 $ 6,467 $ 4,691 Six Months Ended June 30, 2019 2018 2017 Notes 2018 Notes 2017 Notes Contractual interest expense $ 809 $ 3,234 $ 809 Interest cost related to amortization of the debt discount $ 9,890 $ 12,811 $ 9,313 The estimated fair value of the 2017 Notes was $650.7 million as of June 30, 2019 . The estimated fair value of the 2018 Notes was $842.9 million as of June 30, 2019 . 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 . As of June 30, 2019 , the if-converted value of the 2017 Notes and 2018 Notes exceeded the principal amount by $173.8 million and $146.2 million , respectively. 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. For more than 20 trading days during the 30 consecutive trading days ended March 31, 2019 and June 30, 2019, the last reported sale price of the Company’s Class A common stock exceeded 130% of the conversion price of the 2017 Notes. As a result, the 2017 Notes became convertible at the option of the holders on April 1, 2019, were convertible during the second quarter of 2019, and will continue to be convertible during the third quarter of 2019. From April 1, 2019 through the date of this filing, none of the 2017 Notes has been converted. 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. For more than 20 trading days during the 30 consecutive trading days ended March 31, 2019, the last reported sale price of the Company’s Class A common stock exceeded 130% of the conversion price of the 2018 Notes. As a result, the 2018 Notes became convertible at the option of the holders on April 1, 2019 and were convertible during the second quarter of 2019. From April 1, 2019 through June 30, 2019, none of the 2018 Notes has been converted. The 2018 Notes are not convertible during the third quarter of 2019 pursuant to the last reported sale price condition. 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	6 Months Ended
	Jun. 30, 2019
Debt Disclosure [Abstract]
Convertible Debt	13. Credit Agreement 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 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 was 1.75% per annum and the applicable margin for base rate loans was 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). The Company did not borrow any amounts under its credit agreement during the six months ended June 30, 2019 and the year ended December 31, 2018 . 14. 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. 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 2018 Initial Purchasers' option to purchase such additional 2018 Notes, 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 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. The following table presents the outstanding principal amount and carrying value of the 2017 Notes and 2018 Notes as of the date presented (in thousands): June 30, 2019 December 31, 2018 2017 Notes 2018 Notes 2017 Notes 2018 Notes Principal amounts: Principal $ 431,250 $ 575,000 $ 431,250 $ 575,000 Unamortized debt discount (70,021 ) (174,625 ) (79,911 ) (187,435 ) Net carrying amount $ 361,229 $ 400,375 $ 351,339 $ 387,565 The following table presents total interest expense recognized related to the 2017 Notes and 2018 Notes (in thousands): Three Months Ended June 30, 2019 2018 2017 Notes 2018 Notes 2017 Notes Contractual interest expense $ 404 $ 1,617 $ 404 Interest cost related to amortization of the debt discount $ 4,981 $ 6,467 $ 4,691 Six Months Ended June 30, 2019 2018 2017 Notes 2018 Notes 2017 Notes Contractual interest expense $ 809 $ 3,234 $ 809 Interest cost related to amortization of the debt discount $ 9,890 $ 12,811 $ 9,313 The estimated fair value of the 2017 Notes was $650.7 million as of June 30, 2019 . The estimated fair value of the 2018 Notes was $842.9 million as of June 30, 2019 . 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 . As of June 30, 2019 , the if-converted value of the 2017 Notes and 2018 Notes exceeded the principal amount by $173.8 million and $146.2 million , respectively. 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. For more than 20 trading days during the 30 consecutive trading days ended March 31, 2019 and June 30, 2019, the last reported sale price of the Company’s Class A common stock exceeded 130% of the conversion price of the 2017 Notes. As a result, the 2017 Notes became convertible at the option of the holders on April 1, 2019, were convertible during the second quarter of 2019, and will continue to be convertible during the third quarter of 2019. From April 1, 2019 through the date of this filing, none of the 2017 Notes has been converted. 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. For more than 20 trading days during the 30 consecutive trading days ended March 31, 2019, the last reported sale price of the Company’s Class A common stock exceeded 130% of the conversion price of the 2018 Notes. As a result, the 2018 Notes became convertible at the option of the holders on April 1, 2019 and were convertible during the second quarter of 2019. From April 1, 2019 through June 30, 2019, none of the 2018 Notes has been converted. The 2018 Notes are not convertible during the third quarter of 2019 pursuant to the last reported sale price condition. 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	6 Months Ended
	Jun. 30, 2019
Earnings Per Share [Abstract]
Net Loss per Share	15. 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 12, Stockholders’ 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): Three months ended June 30, Six months ended June 30, 2019 2018 2019 2018 Net loss $ (181,938 ) $ (100,734 ) $ (382,327 ) $ (208,509 ) Weighted average common shares used for basic and diluted net loss per share computation 91,802 89,158 91,455 88,814 Net loss per common share: Basic and Diluted $ (1.98 ) $ (1.13 ) $ (4.18 ) $ (2.35 ) Dilutive common stock equivalents, representing potentially dilutive common stock options, restricted stock and restricted stock units, of 8.1 million and 7.7 million for the three and six months ended June 30, 2019 and 2018 , 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 six month period ended June 30, 2019 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 .

Recent Accounting Pronouncement

Recent Accounting Pronouncements	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Recent Accounting Pronouncements	16. Recent Accounting Pronouncements Credit Impairment In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments ("ASU 2016-13"). This ASU revises how entities account for credit losses for most financial assets and certain other instruments that are not measured at fair value through net income. For public entities, ASU 2016-13 is required to be adopted for annual periods beginning after December 15, 2019, including interim periods within those fiscal years. Management is currently evaluating the impact of the adoption of this ASU on the Company’s consolidated financial position, results of operations, comprehensive loss, stockholders' deficit or cash flows.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Leases	The Company determines if an arrangement is a lease at inception. Operating leases are included in " Operating lease right-of-use assets ," " Other current liabilities ," and " Operating lease liabilities " on our consolidated and condensed balance sheets. As of June 30, 2019 , the Company has not entered into material financing lease arrangements. Lease expense for minimum lease payments is recognized on a straight-line basis over the lease term. The Company's lease terms may include options to extend or terminate the lease when it is reasonably certain that management will exercise that option. The Company has lease arrangements with lease and non-lease components. For the Company's warehouse and fulfillment center lease arrangements, the Company accounts for lease and non-lease components separately. For all other lease arrangements, the Company accounts for lease and non-lease components as a single lease component. Stock Compensation The Company adopted ASU No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting ("ASU 2018-07") on January 1, 2019. The ASU simplifies aspects of share-based compensation issued to non-employees by making the guidance consistent with the accounting for employee share-based compensation. The adoption of ASU 2018-07 did not have a material impact the Company’s consolidated financial position, results of operations, comprehensive loss, stockholders' deficit or cash flows. Hosting Arrangements
Recent Accounting Pronouncement and Adoption of New Accounting Pronouncement	Leases The Company adopted ASU No. 2016-02, "Leases" ("ASU 2016-02") on January 1, 2019 using the modified retrospective approach. The Company also elected the package of practical expedients, which among other things, allowed the Company to carryforward historical lease classification. The adoption of the standard resulted in (1) the derecognition of building assets and finance lease obligations for certain leases that did not pass the sale-leaseback criteria, (2) the derecognition of construction in progress assets and other long-term liabilities for certain lease arrangements whereby the Company is no longer considered the deemed construction owner of the construction projects, and (3) the recognition of operating lease right-of-use ("ROU") assets and lease liabilities for lease arrangements with an initial term greater than twelve months. Adoption of ASU 2016-02 resulted in the following adjustments to the balance sheet (in thousands): December 31, 2018 ASU 2016-02 Adjustments January 1, 2019 Balance sheet line item Property and equipment, net $ 606,977 $ (227,709 ) $ 379,268 Operating lease right-of-use assets $ — $ 524,082 $ 524,082 Other noncurrent assets $ 18,826 $ (6,814 ) $ 12,012 Other liabilities $ 160,388 $ (155,996 ) $ 4,392 Other current liabilities $ 127,995 $ (9,624 ) $ 118,371 Lease financing obligation, net of current portion $ 183,056 $ (183,056 ) $ — Operating lease liabilities $ — $ 636,385 $ 636,385 Accumulated deficit $ (1,082,689 ) $ 1,850 $ (1,080,839 ) The adoption of the standard did not have a notable impact on the Company's liquidity or a materially adverse impact on the Company's debt covenant compliance. Credit Impairment In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments – Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments ("ASU 2016-13"). This ASU revises how entities account for credit losses for most financial assets and certain other instruments that are not measured at fair value through net income. For public entities, ASU 2016-13 is required to be adopted for annual periods beginning after December 15, 2019, including interim periods within those fiscal years. Management is currently evaluating the impact of the adoption of this ASU on the Company’s consolidated financial position, results of operations, comprehensive loss, stockholders' deficit or cash flows.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	6 Months Ended
	Jun. 30, 2019
Accounting Policies [Abstract]
Adoption of ASU 2016-02	Adoption of ASU 2016-02 resulted in the following adjustments to the balance sheet (in thousands): December 31, 2018 ASU 2016-02 Adjustments January 1, 2019 Balance sheet line item Property and equipment, net $ 606,977 $ (227,709 ) $ 379,268 Operating lease right-of-use assets $ — $ 524,082 $ 524,082 Other noncurrent assets $ 18,826 $ (6,814 ) $ 12,012 Other liabilities $ 160,388 $ (155,996 ) $ 4,392 Other current liabilities $ 127,995 $ (9,624 ) $ 118,371 Lease financing obligation, net of current portion $ 183,056 $ (183,056 ) $ — Operating lease liabilities $ — $ 636,385 $ 636,385 Accumulated deficit $ (1,082,689 ) $ 1,850 $ (1,080,839 )

Marketable Securities and Fai_2

Marketable Securities and Fair Value Measurements (Tables)	6 Months Ended
	Jun. 30, 2019
Fair Value Disclosures [Abstract]
Schedule of Marketable Securities	The following tables present details of the Company’s marketable securities as of June 30, 2019 and December 31, 2018 (in thousands): June 30, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Short-term: Investment securities $ 39,343 $ 20 $ (21 ) $ 39,342 Total $ 39,343 $ 20 $ (21 ) $ 39,342 December 31, 2018 Amortized Cost Gross Unrealized Gains 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
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 June 30, 2019 and December 31, 2018 based on the three-tier value hierarchy (in thousands): June 30, 2019 Level 1 Level 2 Level 3 Total Cash equivalents: Money market funds and other funds $ 618,520 $ — $ — $ 618,520 Short-term investments: Investment securities — 39,342 — 39,342 Other non-current assets: Certificate of deposit 5,076 — — 5,076 Total $ 623,596 $ 39,342 $ — $ 662,938 December 31, 2018 Level 1 Level 2 Level 3 Total Cash equivalents: Money market 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

Property and Equipment, net (Ta

Property and Equipment, net (Tables)	6 Months Ended
	Jun. 30, 2019
Property, Plant and Equipment [Abstract]
Schedule of Property and Equipment, net	The following table summarizes property and equipment, net as of June 30, 2019 and December 31, 2018 (in thousands): June 30, December 31, Furniture and computer equipment $ 425,499 $ 339,761 Site and software development costs 228,624 172,653 Leasehold improvements 156,405 109,929 Construction in progress 27,010 90,104 Buildings (leased - Note 6) — 184,694 837,538 897,141 Less accumulated depreciation and amortization (357,046 ) (290,164 ) Property and equipment, net $ 480,492 $ 606,977

Leases (Tables)

Leases (Tables)	6 Months Ended
	Jun. 30, 2019
Leases [Abstract]
Other Information Related to Leases	The following table presents other information related to leases (in thousands): Six months ended June 30, 2019 Supplemental cash flows information Cash payments included in operating cash flows from lease arrangements $ 50,387 Right-of-use assets obtained in exchange for lease obligations $ 138,064 June 30, 2019 Additional lease information Weighted average remaining lease term 10 years Weighted average discount rate 6.9 %
Future Minimum Lease Payments	Future minimum lease payments under non-cancellable leases as of June 30, 2019 were as follows (in thousands): Amount 2019 (excluding the six months ended June 30, 2019) $ 57,089 2020 121,836 2021 123,559 2022 117,310 2023 113,546 Thereafter 507,024 Total future minimum lease payments 1,040,364 Less: Imputed interest (285,661 ) Total $ 754,703
Operating Lease Balance Sheet Items	The following table presents total operating leases (in thousands): June 30, 2019 Balance sheet line item Other current liabilities $ 69,214 Operating lease liabilities 685,489 Total operating leases $ 754,703

Equity-Based Compensation (Tabl

Equity-Based Compensation (Tables)	6 Months Ended
	Jun. 30, 2019
Share-based Payment Arrangement [Abstract]
Summary of Activity Relating to Stock Options	The following table presents activity relating to stock options for the six months ended June 30, 2019 : Options Weighted-Average Exercise Price Weighted-Average Remaining Contractual Term (Years) Outstanding at December 31, 2018 79,802 $ 3.05 2.5 Options exercised (25,179 ) $ 3.17 Outstanding and exercisable at June 30, 2019 54,623 $ 3.00 2.0
Summary of Activity Relating to Restricted Stock	The following table presents activity relating to restricted common stock for the six months ended June 30, 2019 : Shares Weighted- Average Grant Date Fair Value Unvested at December 31, 2018 20,000 $ 44.34 Unvested as of June 30, 2019 20,000 $ 44.34 The following table presents activity relating to RSUs for the six months ended June 30, 2019 : Shares Weighted- Average Grant Date Fair Value Outstanding at December 31, 2018 7,970,959 $ 72.43 RSUs granted 1,939,275 $ 151.00 RSUs vested (1,330,455 ) $ 65.87 RSUs forfeited/canceled (581,240 ) $ 83.66 Outstanding as of June 30, 2019 7,998,539 $ 91.75

Segment and Geographic Inform_2

Segment and Geographic Information (Tables)	6 Months Ended
	Jun. 30, 2019
Segment Reporting [Abstract]
Activity Related to Net Revenue, 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): Three months ended June 30, Six months ended June 30, 2019 2018 2019 2018 U.S. Direct Retail $ 1,989,026 $ 1,397,009 $ 3,633,076 $ 2,583,214 U.S. Other 11,492 14,335 25,140 29,714 U.S. segment net revenue 2,000,518 1,411,344 3,658,216 2,612,928 International Direct Retail 342,733 243,912 629,864 446,597 International segment net revenue 342,733 243,912 629,864 446,597 Total net revenue $ 2,343,251 $ 1,655,256 $ 4,288,080 $ 3,059,525 Three months ended June 30, Six months ended June 30, 2019 2018 2019 2018 Adjusted EBITDA: U.S. $ (342 ) $ 7,200 $ (28,124 ) $ (738 ) International (69,641 ) (42,009 ) (144,077 ) (84,031 ) Total reportable segments Adjusted EBITDA (69,983 ) (34,809 ) (172,201 ) (84,769 ) Less: reconciling items (1) (111,955 ) (65,925 ) (210,126 ) (123,740 ) Net loss $ (181,938 ) $ (100,734 ) $ (382,327 ) $ (208,509 ) (1) Adjustments are made to reconcile total reportable segments Adjusted EBITDA to consolidated net loss including the following (in thousands): Three months ended June 30, Six months ended June 30, 2019 2018 2019 2018 Depreciation and amortization $ 44,339 $ 28,920 $ 83,922 $ 54,882 Equity based compensation and related taxes 56,855 31,610 108,688 58,757 Interest expense, net 10,252 5,796 19,490 11,203 Other (income), net (322 ) (666 ) (3,400 ) (1,607 ) Provision for income taxes 831 265 1,426 505 Total reconciling items $ 111,955 $ 65,925 $ 210,126 $ 123,740 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 our media solutions business (in thousands): Three months ended June 30, Six months ended June 30, 2019 2018 2019 2018 Net revenue Direct Retail $ 2,331,759 $ 1,640,921 $ 4,262,940 $ 3,029,811 Other 11,492 14,335 25,140 29,714 Net revenue $ 2,343,251 $ 1,655,256 $ 4,288,080 $ 3,059,525
Schedule of Segment Assets	The following table presents total assets attributable to the Company's reportable segments reconciled to consolidated amounts (in thousands): June 30, 2019 December 31, 2018 Assets by segment U.S. $ 822,954 $ 670,344 International 179,303 56,177 Total reportable segments assets 1,002,257 726,521 Plus: reconciling corporate assets 1,179,843 1,164,329 Total assets $ 2,182,100 $ 1,890,850 U.S. and International segment assets consist primarily of "Accounts receivable," " Inventories ," " Prepaid expenses and other current assets ," " Property and equipment, net " and " Operating lease right-of-use assets ." Corporate assets include " Cash and cash equivalents ," marketable securities, " Long-term investments ," corporate facilities, " Goodwill and intangible assets, net ," and capitalized internal-use software and website development costs.

Convertible Debt (Tables)

Convertible Debt (Tables)	6 Months Ended
	Jun. 30, 2019
Debt Disclosure [Abstract]
Schedule of Outstanding Principal Amount and Carrying Value of Notes	The following table presents the outstanding principal amount and carrying value of the 2017 Notes and 2018 Notes as of the date presented (in thousands): June 30, 2019 December 31, 2018 2017 Notes 2018 Notes 2017 Notes 2018 Notes Principal amounts: Principal $ 431,250 $ 575,000 $ 431,250 $ 575,000 Unamortized debt discount (70,021 ) (174,625 ) (79,911 ) (187,435 ) Net carrying amount $ 361,229 $ 400,375 $ 351,339 $ 387,565
Schedule Of Interest Expense Related to Notes	The following table presents total interest expense recognized related to the 2017 Notes and 2018 Notes (in thousands): Three Months Ended June 30, 2019 2018 2017 Notes 2018 Notes 2017 Notes Contractual interest expense $ 404 $ 1,617 $ 404 Interest cost related to amortization of the debt discount $ 4,981 $ 6,467 $ 4,691 Six Months Ended June 30, 2019 2018 2017 Notes 2018 Notes 2017 Notes Contractual interest expense $ 809 $ 3,234 $ 809 Interest cost related to amortization of the debt discount $ 9,890 $ 12,811 $ 9,313

Net Loss per Share (Tables)

Net Loss per Share (Tables)	6 Months Ended
	Jun. 30, 2019
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): Three months ended June 30, Six months ended June 30, 2019 2018 2019 2018 Net loss $ (181,938 ) $ (100,734 ) $ (382,327 ) $ (208,509 ) Weighted average common shares used for basic and diluted net loss per share computation 91,802 89,158 91,455 88,814 Net loss per common share: Basic and Diluted $ (1.98 ) $ (1.13 ) $ (4.18 ) $ (2.35 )

Basis of Presentation (Details)

Basis of Presentation (Details) supplier in Thousands, product in Millions	6 Months Ended
	Jun. 30, 2019productsupplier
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of products offered | product	14,000,000
Number of suppliers providing products offered | supplier	11

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Adoption of ASU 2016-02 (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and equipment, net	$ 480,492	$ 379,268	$ 606,977
Operating lease right-of-use assets	630,731	524,082
Other noncurrent assets	13,417	12,012	18,826
Other liabilities		4,392	160,388
Other current liabilities	179,155	118,371	127,995
Lease financing obligation, net of current portion	0	0	183,056
Operating lease liabilities	754,703	636,385
Accumulated deficit	$ (1,463,166)	(1,080,839)	$ (1,082,689)
ASU 2016-02 Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Property and equipment, net		(227,709)
Operating lease right-of-use assets		524,082
Other noncurrent assets		(6,814)
Other liabilities		(155,996)
Other current liabilities		(9,624)
Lease financing obligation, net of current portion		(183,056)
Operating lease liabilities		636,385
Accumulated deficit		$ 1,850

Marketable Securities and Fai_3

Marketable Securities and Fair Value Measurements - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Estimated fair values	$ 39,342,000		$ 39,342,000		$ 120,804,000
Other-than-temporary impairment loss	0	$ 0	0	$ 0
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities, realized gains and losses	$ 0	$ 0	$ 0	$ 0
Other long-term investments
Fair Value Disclosures [Abstract]
Estimated fair values					$ 6,526,000
Other long-term investments | Minimum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Marketable securities term			1 year
Other long-term investments | Maximum
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	Jun. 30, 2019	Dec. 31, 2018
Marketable securities and fair value measurements
Amortized Cost	$ 39,343	$ 121,005
Gross Unrealized Gains	20	0
Gross Unrealized Losses	(21)	(201)
Estimated Fair Value	39,342	120,804
Investment securities
Marketable securities and fair value measurements
Amortized Cost	39,343	114,402
Gross Unrealized Gains	20	0
Gross Unrealized Losses	(21)	(124)
Estimated Fair Value	$ 39,342	114,278
Investment securities
Marketable securities and fair value measurements
Amortized Cost		6,603
Gross Unrealized Gains		0
Gross Unrealized Losses		(77)
Estimated Fair Value		$ 6,526

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	Jun. 30, 2019	Dec. 31, 2018
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Total	$ 662,938	$ 840,890
Level 1
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Total	623,596	720,086
Level 2
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Total	39,342	120,804
Level 3
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Total	0	0
Investment securities
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Short-term investments	39,342	114,278
Long-term		6,526
Investment securities | Level 1
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Short-term investments	0	0
Long-term		0
Investment securities | Level 2
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Short-term investments	39,342	114,278
Long-term		6,526
Investment securities | Level 3
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Short-term investments	0	0
Long-term		0
Certificate of deposit
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Other non-current assets	5,076	5,000
Certificate of deposit | Level 1
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Other non-current assets	5,076	5,000
Certificate of deposit | Level 2
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Other non-current assets	0	0
Certificate of deposit | Level 3
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Other non-current assets	0	0
Money market funds and other funds
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Cash equivalents	618,520	715,086
Money market funds and other funds | Level 1
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Cash equivalents	618,520	715,086
Money market funds and other funds | Level 2
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Cash equivalents	0	0
Money market funds and other funds | Level 3
Fair Value Assets Measured on Recurring and Nonrecurring Basis [line items]
Cash equivalents	$ 0	$ 0

Intangible Assets and Goodwill

Intangible Assets and Goodwill (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jan. 31, 2019	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible assets	$ 0.2		$ 0.2			$ 0.5
Amortization expense related to intangibles	0.1	$ 0.3	0.3	$ 0.6
Indefinite-lived Intangible Assets [Line Items]
Goodwill	$ 0.4		$ 0.4			$ 2.1
Intellectual Property
Indefinite-lived Intangible Assets [Line Items]
Goodwill					$ 1.7

Property and Equipment, net (De

Property and Equipment, net (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jan. 01, 2019	Dec. 31, 2018
Property and equipment, net
Property and equipment, gross	$ 837,538		$ 837,538			$ 897,141
Less accumulated depreciation and amortization	(357,046)		(357,046)			(290,164)
Property and equipment, net	480,492		480,492		$ 379,268	606,977
Depreciation and amortization	44,200	$ 28,600	83,600	$ 54,300
Furniture and computer equipment
Property and equipment, net
Property and equipment, gross	425,499		425,499			339,761
Site and software development costs
Property and equipment, net
Property and equipment, gross	228,624		228,624			172,653
Leasehold improvements
Property and equipment, net
Property and equipment, gross	156,405		156,405			109,929
Construction in progress
Property and equipment, net
Property and equipment, gross	27,010		27,010			90,104
Buildings
Property and equipment, net
Property and equipment, gross	$ 0		$ 0			$ 184,694

Leases - Narrative (Details)

Leases - Narrative (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2019USD ($)	Jun. 30, 2018USD ($)	Dec. 31, 2018USD ($)lease	Jan. 01, 2019USD ($)
Lessee, Lease, Description [Line Items]
Operating lease expense	$ 29,400	$ 16,100	$ 55,000	$ 30,600
Operating leases not yet commenced			498,700
Letter of credit issued as security for lease agreements	45,700		45,700		$ 26,800
Lease financing obligation, net of current portion	$ 0		$ 0		$ 183,056	$ 0
Property Lease Guarantee
Lessee, Lease, Description [Line Items]
Number of warehouse lease arrangements | lease					3
Lease financing obligation, net of current portion					$ 101,000
Property Lease Guarantee | Warehouse Lease Arrangement, One 2018
Lessee, Lease, Description [Line Items]
Letter of credit issued as security for lease agreements					$ 2,500
Minimum
Lessee, Lease, Description [Line Items]
Operating leases not yet commenced, term of contract	2 years		2 years
Maximum
Lessee, Lease, Description [Line Items]
Operating leases not yet commenced, term of contract	15 years		15 years

Leases - Other Information Rela

Leases - Other Information Related to Leases (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019USD ($)
Supplemental cash flows information
Cash payments included in operating cash flows from lease arrangements	$ 50,387
Right-of-use assets obtained in exchange for lease obligations	$ 138,064
Additional lease information
Weighted average remaining lease term	10 years
Weighted average discount rate	6.90%

Leases - Future Minimum Lease P

Leases - Future Minimum Lease Payments (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019
Leases [Abstract]
2019 (excluding the six months ended June 30, 2019)	$ 57,089
2020	121,836
2021	123,559
2022	117,310
2023	113,546
Thereafter	507,024
Total future minimum lease payments	1,040,364
Less: Imputed interest	(285,661)
Operating lease liabilities	$ 754,703	$ 636,385

Leases - Operating Leases Balan

Leases - Operating Leases Balance Sheet Locations (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019
Leases [Abstract]
Other current liabilities	$ 69,214
Operating lease liabilities	685,489
Operating lease liabilities	$ 754,703	$ 636,385

Commitments and Contingencies (

Commitments and Contingencies (Details) $ in Millions	Jan. 16, 2019plaintiffofficer	Jun. 30, 2019USD ($)	Dec. 31, 2018USD ($)
Commitments and Contingencies Disclosure [Abstract]
Letter of credit issued as security for lease agreements | $		$ 45.7	$ 26.8
Number of Company's officers | officer	3
Number of plaintiffs | plaintiff	2

Equity-Based Compensation - Nar

Equity-Based Compensation - Narrative (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jan. 01, 2019
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Intrinsic value of stock options exercised	$ 3.7
Aggregate intrinsic value of stock options outstanding	7.8
Aggregate intrinsic value of stock options exercisable	7.8
Restricted common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate Intrinsic value of stock unvested	2.9
Unrecognized equity based compensation	$ 0.9
Weighted average remaining vesting term	6 months
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Aggregate Intrinsic value of stock unvested	$ 1,200
Unrecognized equity based compensation	$ 671.8
Weighted average remaining vesting term	1 year 6 months
Intrinsic value of stock vested	$ 187.3
2014 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)	8,603,066
Number of shares available for future grant (in shares)		6,202,378

Equity-Based Compensation - Sum

Equity-Based Compensation - Summary of Activity Relating to Stock Options (Details) - Stock options - $ / shares	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Options
Outstanding at the beginning of the period (in shares)	79,802
Options exercised (in shares)	(25,179)
Outstanding and exercisable at the end of the period (in shares)	54,623	79,802
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)	$ 3.05
Options exercised (in dollars per share)	3.17
Outstanding and exercisable at the end of the period (in dollars per share)	$ 3	$ 3.05
Weighted-Average Remaining Contractual Term (Years)
Outstanding	2 years	2 years 6 months

Equity-Based Compensation - S_2

Equity-Based Compensation - Summary of Activity Relating to Restricted Stock (Details)	6 Months Ended
	Jun. 30, 2019$ / sharesshares
Restricted stock
Shares
Outstanding at the beginning of the period (in shares) | shares	20,000
Outstanding at the end of the period (in shares) | shares	20,000
Weighted- Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 44.34
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 44.34
RSUs
Shares
Outstanding at the beginning of the period (in shares) | shares	7,970,959
RSUs granted (in shares) | shares	1,939,275
RSUs vested (in shares) | shares	(1,330,455)
RSUs forfeited/canceled (in shares) | shares	(581,240)
Outstanding at the end of the period (in shares) | shares	7,998,539
Weighted- Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 72.43
RSUs granted (in dollars per share) | $ / shares	151
RSUs vested (in dollars per share) | $ / shares	65.87
RSUs forfeited/canceled (in dollars per share) | $ / shares	83.66
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 91.75

Unearned Revenue (Details)

Unearned Revenue (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Unearned revenue
Disaggregation of Revenue [Line Items]
Contract liabilities	$ 155.8	$ 148.1
Revenue recognized that was included in deferred revenue	121.9
Other current liabilities
Disaggregation of Revenue [Line Items]
Contract liabilities	3.1	$ 3.1
Revenue recognized that was included in deferred revenue	$ 1.5

Segment and Geographic Inform_3

Segment and Geographic Information Narrative (Details)	6 Months Ended
	Jun. 30, 2019site
U.S.
Segment Reporting Information [Line Items]
Distinct sites	5

Segment and Geographic Inform_4

Segment and Geographic Information - Activity Related to Net Revenue and Adjusted EBITDA (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Total net revenue	$ 2,343,251	$ 1,655,256	$ 4,288,080	$ 3,059,525
Adjusted EBITDA:	(69,983)	(34,809)	(172,201)	(84,769)
Less: reconciling items	(111,955)	(65,925)	(210,126)	(123,740)
Net loss	(181,938)	(100,734)	(382,327)	(208,509)
Depreciation and amortization	44,339	28,920	83,922	54,882
Equity based compensation and related taxes	56,855	31,610	108,688	58,757
Interest expense, net	10,252	5,796	19,490	11,203
Other (income), net	(322)	(666)	(3,400)	(1,607)
Provision for income taxes	831	265	1,426	505
Less: reconciling items	111,955	65,925	210,126	123,740
U.S.
Segment Reporting Information [Line Items]
Total net revenue	2,000,518	1,411,344	3,658,216	2,612,928
Adjusted EBITDA:	(342)	7,200	(28,124)	(738)
International
Segment Reporting Information [Line Items]
Total net revenue	342,733	243,912	629,864	446,597
Adjusted EBITDA:	(69,641)	(42,009)	(144,077)	(84,031)
Direct Retail
Segment Reporting Information [Line Items]
Total net revenue	2,331,759	1,640,921	4,262,940	3,029,811
Direct Retail | U.S.
Segment Reporting Information [Line Items]
Total net revenue	1,989,026	1,397,009	3,633,076	2,583,214
Direct Retail | International
Segment Reporting Information [Line Items]
Total net revenue	342,733	243,912	629,864	446,597
Other
Segment Reporting Information [Line Items]
Total net revenue	11,492	14,335	25,140	29,714
Other | U.S.
Segment Reporting Information [Line Items]
Total net revenue	$ 11,492	$ 14,335	$ 25,140	$ 29,714

Segment and Geographic Inform_5

Segment and Geographic Information - Schedule of Segment Reconciled to Consolidated Amounts (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Assets by segment	$ 2,182,100	$ 1,890,850
U.S.
Segment Reporting Information [Line Items]
Assets by segment	822,954	670,344
International
Segment Reporting Information [Line Items]
Assets by segment	179,303	56,177
Total reportable segments assets
Segment Reporting Information [Line Items]
Assets by segment	1,002,257	726,521
Plus: reconciling corporate assets
Segment Reporting Information [Line Items]
Assets by segment	$ 1,179,843	$ 1,164,329

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income tax expense	$ 831	$ 265	$ 1,426	$ 505
Unrecognized tax benefits	$ 0		$ 0		$ 0

Stockholders' Deficit (Details)

Stockholders' Deficit (Details)	6 Months Ended	54 Months Ended
	Jun. 30, 2019vote$ / sharesshares	Mar. 31, 2019shares	Dec. 31, 2018$ / sharesshares
Stockholders' Equity
Preferred stock, shares authorized (in shares)	10,000,000
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.001
Preferred stock, shares issued (in shares)	0
Preferred stock, shares outstanding (in shares)	0
Class A common stock
Stockholders' Equity
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)	64,591,843		62,329,701
Number of votes each holder is entitled | vote	1
Class B common stock
Stockholders' Equity
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)	27,506,309		28,417,882
Number of votes each holder is entitled | vote	10
Conversion ratio following the completion of the offering	1
Conversion ratio upon transfer	1
Number of shares converted into Class A common stock (in shares)		54,532,689
Class B common stock | Maximum
Stockholders' Equity
Aggregate number of shares outstanding Class A common stock and Class B common stock that shall convert automatically (less than)	10.00%
Class B common stock | Minimum
Stockholders' Equity
Percentage of outstanding shares of Class B common stock that shall convert automatically in the event of the affirmative vote or written consent of holders	66.67%

Credit Agreement (Details)

Credit Agreement (Details) - USD ($)	Feb. 19, 2019	Jun. 30, 2019	Feb. 21, 2019	Dec. 31, 2018
Line of Credit Facility [Line Items]
Borrowings under Revolver credit agreement		$ 0		$ 0
Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity			$ 165,000,000
Right to increase			50,000,000
Borrower and guarantor obligations under debt agreement, capital stock of first-tier foreign subsidiaries, percent	65.00%
Revolving Credit Facility | Federal Funds Effective Swap Rate
Line of Credit Facility [Line Items]
Basis spread	0.50%
Revolving Credit Facility | Eurodollar
Line of Credit Facility [Line Items]
Basis spread	1.00%
Applicable margin	0.0175
Revolving Credit Facility | Base Rate
Line of Credit Facility [Line Items]
Applicable margin	0.0075
Revolving Credit Facility | Letter of Credit Sublimit
Line of Credit Facility [Line Items]
Maximum borrowing capacity			100,000,000
Revolving Credit Facility | Swing Line Sublimit
Line of Credit Facility [Line Items]
Maximum borrowing capacity			$ 15,000,000

Convertible Debt - Narrative (D

Convertible Debt - Narrative (Details)	Nov. 19, 2018USD ($)day$ / shares	Nov. 14, 2018$ / shares	Sep. 15, 2017USD ($)day$ / shares	Sep. 11, 2017institution$ / shares	Nov. 30, 2018USD ($)	Mar. 31, 2019day	Jun. 30, 2019USD ($)day	Dec. 31, 2018USD ($)
Debt Instrument [Line Items]
Number of initial purchasers | institution				2
Convertible Debt | 0.375% Senior Note due 2022
Debt Instrument [Line Items]
Debt, aggregate principal			$ 431,250,000				$ 431,250,000	$ 431,250,000
Interest rate, stated percentage			0.375%
Proceeds from sale of Notes, net of discounts and offering expenses			$ 420,400,000
Payments for Capped Call Transactions			44,200,000
Equity component of Notes			$ 95,800,000
Debt, fair value							650,700,000
If-converted value in excess of principal							$ 173,800,000
Initial conversion rate			0.00961
Conversion price (in usd per share) | $ / shares			$ 104.06
Number of trading days (whether or not consecutive) | day			20				20
Number of trading days (consecutive) | day			30			30
Percentage of conversion stock price			130.00%			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 trading days immediately preceding date notice of redemption provided | day			1
Trading days immediately preceding date 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 | 0.375% Senior Note due 2022 | Class A common stock
Debt Instrument [Line Items]
Cap price, per share (in usd per share) | $ / shares				$ 154.16
Premium over last reported sale price, percentage				100.00%
Convertible Debt | 0.375% 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			6.00%
Convertible Debt | 1.125% 2018 Senior Note Due 2024
Debt Instrument [Line Items]
Debt, aggregate principal	$ 500,000,000				$ 575,000,000		$ 575,000,000	$ 575,000,000
Interest rate, stated percentage					1.125%
Over-allotment option included in issuance of notes	$ 75,000,000				$ 75,000,000
Proceeds from sale of Notes, net of discounts and offering expenses					562,000,000
Payments for Capped Call Transactions					$ 93,400,000
Effective interest rate					8.10%
Equity component of Notes					$ 181,500,000
Debt, fair value							842,900,000
If-converted value in excess of principal							$ 146,200,000
Initial conversion rate	0.008591
Number of trading days (whether or not consecutive) | day	20					20	20
Number of trading days (consecutive) | day	30					30	30
Percentage of conversion stock price	130.00%					130.00%	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 usd per share) | $ / shares	$ 116.40
Cap price, per share (in usd per share) | $ / shares	$ 219.63	$ 219.63
Premium over last reported sale price, percentage		150.00%

Convertible Debt - Schedule of

Convertible Debt - Schedule of Outstanding Principal Amount and Carrying Value of Notes (Details) - USD ($)	Jun. 30, 2019	Dec. 31, 2018	Nov. 30, 2018	Nov. 19, 2018	Sep. 15, 2017
Debt Instrument [Line Items]
Net carrying amount	$ 761,604,000	$ 738,904,000
Convertible Debt | 2017 Notes
Debt Instrument [Line Items]
Principal	431,250,000	431,250,000			$ 431,250,000
Unamortized debt discount	(70,021,000)	(79,911,000)
Net carrying amount	361,229,000	351,339,000
Convertible Debt | 2018 Notes
Debt Instrument [Line Items]
Principal	575,000,000	575,000,000	$ 575,000,000	$ 500,000,000
Unamortized debt discount	(174,625,000)	(187,435,000)
Net carrying amount	$ 400,375,000	$ 387,565,000

Convertible Debt - Schedule o_2

Convertible Debt - Schedule of Interest Expense Related to Notes (Details) - Convertible Debt - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
2017 Notes
Debt Instrument [Line Items]
Contractual interest expense	$ 404	$ 404	$ 809	$ 809
Interest cost related to amortization of the debt discount	4,981	$ 4,691	9,890	$ 9,313
2018 Notes
Debt Instrument [Line Items]
Contractual interest expense	1,617		3,234
Interest cost related to amortization of the debt discount	$ 6,467		$ 12,811

Net Loss per Share (Details)

Net Loss per Share (Details) shares in Millions	Nov. 19, 2018$ / sharesshares	Sep. 15, 2017$ / sharesshares	Sep. 11, 2017$ / sharesshares	Jun. 30, 2019shares	Jun. 30, 2018shares	Jun. 30, 2019shares	Jun. 30, 2018shares	Nov. 14, 2018$ / shares
Class of Stock [Line Items]
Allocation of undistributed earnings between stock classes, conversion ratio						1
0.375% Senior Note due 2022 | Convertible Debt
Class of Stock [Line Items]
Conversion price (in usd per share) | $ / shares		$ 104.06
0.375% Senior Note due 2022 | Convertible Debt | Class A common stock
Class of Stock [Line Items]
Conversion of stock, shares issued (in shares)		4.1
Cap price, per share (in usd per share) | $ / shares			$ 154.16
Capped call transaction, shares issued (in shares)			2.8
1.125% 2018 Senior Note Due 2024 | Convertible Debt | Class A common stock
Class of Stock [Line Items]
Conversion price (in usd per share) | $ / shares	$ 116.40
Conversion of stock, shares issued (in shares)	4.9
Cap price, per share (in usd per share) | $ / shares	$ 219.63							$ 219.63
Capped call transaction, shares issued (in shares)	2.6
Common Stock Options, Restricted Stock Options And Restricted Stock Units
Class of Stock [Line Items]
Antidilutive securities excluded from diluted earnings per share calculation (in shares)				8.1	7.7	8.1	7.7

Net Loss per Share - Calculatio

Net Loss per Share - Calculation of Basic and Diluted Net Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Net loss	$ (181,938)	$ (100,734)	$ (382,327)	$ (208,509)
Weighted average common shares used for basic and diluted net loss per share computation (in shares)	91,802	89,158	91,455	88,814
Net loss per common share:
Basic and Diluted (in usd per share)	$ (1.98)	$ (1.13)	$ (4.18)	$ (2.35)

Uncategorized Items - a2019-06x

Label	Element	Value
Accounting Standards Update 2014-09 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 4,657,000
Accounting Standards Update 2014-09 [Member] | Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	4,657,000
Accounting Standards Update 2016-02 [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	1,850,000
Accounting Standards Update 2016-02 [Member] | Retained Earnings [Member]
Cumulative Effect of New Accounting Principle in Period of Adoption	us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption	$ 1,850,000