Immunogen (IMGN) 10-Q 6 Nov 20 2020 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2020	Oct. 30, 2020
Document and Entity Information
Document Type	10-Q
Document Transition Report	false
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2020
Entity File Number	0-17999
Entity Registrant Name	ImmunoGen, Inc.
Entity Incorporation, State or Country Code	MA
Entity Tax Identification Number	04-2726691
Entity Address, Address Line One	830 Winter Street
Entity Address, City or Town	Waltham
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02451
City Area Code	781
Local Phone Number	895-0600
Title of 12(b) Security	Common Stock
Trading Symbol	IMGN
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		187,527,748
Entity Central Index Key	0000855654
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q3
Amendment Flag	false

CONSOLIDATED BALANCE SHEETS (UN

CONSOLIDATED BALANCE SHEETS (UNAUDITED) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
ASSETS
Cash and cash equivalents	$ 188,217	$ 176,225
Accounts receivable	5,050	7,500
Unbilled revenue/reimbursement	5	1,001
Contract assets		3,631
Non-cash royalty receivable	18,180	15,116
Prepaid and other current assets	7,875	5,425
Total current assets	219,327	208,898
Property and equipment, net of accumulated depreciation	5,578	6,993
Operating lease right-of-use assets	14,477	15,587
Other assets	8,570	3,784
Total assets	247,952	235,262
LIABILITIES AND SHAREHOLDERS? DEFICIT
Accounts payable	8,913	9,933
Accrued compensation	3,942	8,991
Other accrued liabilities	21,114	13,932
Convertible 4.5% senior notes, net of deferred financing costs of $11	2,089
Current portion of liability related to the sale of future royalties, net of deferred financing costs of $402 and $635, respectively	56,603	41,274
Current portion of operating lease liability	3,175	2,971
Current portion of deferred revenue	3,985	309
Total current liabilities	99,821	77,410
Deferred revenue, net of current portion	126,541	127,123
Operating lease liability, net of current portion	19,403	21,798
Convertible 4.5% senior notes, net of deferred financing costs of $22		2,078
Liability related to the sale of future royalties, net of current portion and deferred financing costs of $625 and $859, respectively	42,260	82,267
Other long-term liabilities	2,818	707
Total liabilities	290,843	311,383
Shareholders deficit:
Preferred stock, $.01 par value; authorized 5,000 shares; no shares issued and outstanding as of September 30, 2020 and December 31, 2020
Common stock, $.01 par value; authorized 300,000 and 200,000 shares; issued and outstanding 174,585 and 150,136 shares as of September 30, 2020 and December 31, 2019, respectively	1,746	1,501
Additional paid-in capital	1,318,591	1,209,846
Accumulated deficit	(1,363,228)	(1,287,468)
Total shareholders deficit	(42,891)	(76,121)
Total liabilities and shareholders deficit	$ 247,952	$ 235,262

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
CONSOLIDATED BALANCE SHEETS (UNAUDITED)
Current portion of deferred financing costs	$ 11
Interest rate (as a percent)	4.50%	4.50%
Non-current deferred financing costs		$ 22
Sale of future royalties, current portion and deferred financing costs	$ 402	635
Sale of future royalties, noncurrent portion and deferred financing costs	$ 625	$ 859
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, authorized shares	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, authorized shares	300,000,000	200,000,000
Common stock, issued shares	174,585,000	150,136,000
Common stock, outstanding shares	174,585,000	150,136,000

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS (UNAUDITED) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenues:
Total revenues	$ 18,189	$ 13,281	$ 46,501	$ 37,407
Operating expenses:
Research and development	24,685	21,015	75,014	88,467
General and administrative	10,231	9,208	28,862	28,686
Restructuring charge		1,020	1,524	20,921
Total operating expenses	34,916	31,243	105,400	138,074
Loss from operations	(16,727)	(17,962)	(58,899)	(100,667)
Investment income, net	11	1,032	719	3,741
Non-cash interest expense on liability related to the sale of future royalties and convertible senior notes	(5,645)	(4,275)	(17,428)	(11,525)
Interest expense on convertible senior notes	(24)	(24)	(71)	(71)
Other income (expense), net	11	(521)	(81)	(425)
Net loss	$ (22,374)	$ (21,750)	$ (75,760)	$ (108,947)
Basic and diluted net loss per common share (in dollar per share)	$ (0.13)	$ (0.15)	$ (0.44)	$ (0.74)
Basic and diluted weighted average common shares outstanding (in shares)	174,508	148,479	172,215	148,143
Total comprehensive loss	$ (22,374)	$ (21,750)	$ (75,760)	$ (108,947)
License and milestone fees
Revenues:
Total revenues	97	79	1,325	5,237
Non-cash royalty revenue related to the sale of future royalties
Revenues:
Total revenues	18,087	$ 13,202	45,159	32,102
Research and development support
Revenues:
Total revenues	$ 5		$ 17	$ 68

CONSOLIDATED STATEMENTS OF SHAR

CONSOLIDATED STATEMENTS OF SHAREHOLDERS (DEFICIT) EQUITY (UNAUDITED) - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Total
Balance at Dec. 31, 2018	$ 1,494	$ 1,192,813	$ (1,183,335)	$ 10,972
Balance (in shares) at Dec. 31, 2018	149,400
Increase (Decrease) in Shareholders' (Deficit) Equity
Net income (loss)			(43,751)	(43,751)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan		68		68
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	25
Stock option and restricted stock compensation expense		5,007		5,007
Directors' deferred share unit compensation		100		100
Balance at Mar. 31, 2019	$ 1,494	1,197,988	(1,227,086)	(27,604)
Balance (in shares) at Mar. 31, 2019	149,425
Balance at Dec. 31, 2018	$ 1,494	1,192,813	(1,183,335)	10,972
Balance (in shares) at Dec. 31, 2018	149,400
Increase (Decrease) in Shareholders' (Deficit) Equity
Net income (loss)				(108,947)
Balance at Sep. 30, 2019	$ 1,498	1,204,559	(1,292,282)	(86,225)
Balance (in shares) at Sep. 30, 2019	149,688
Balance at Mar. 31, 2019	$ 1,494	1,197,988	(1,227,086)	(27,604)
Balance (in shares) at Mar. 31, 2019	149,425
Increase (Decrease) in Shareholders' (Deficit) Equity
Net income (loss)			(43,446)	(43,446)
Issuance of common stock	$ 3	667		670
Issuance of common stock (in shares)	354
Restricted stock award, net of forfeitures	$ 1	(1)
Restricted stock award, net of forfeitures (in shares)	106
Stock option and restricted stock compensation expense		2,106		2,106
Directors' deferred share unit compensation		100		100
Balance at Jun. 30, 2019	$ 1,498	1,200,860	(1,270,532)	(68,174)
Balance (in shares) at Jun. 30, 2019	149,885
Increase (Decrease) in Shareholders' (Deficit) Equity
Net income (loss)			(21,750)	(21,750)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan		73		73
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	30
Restricted stock award, net of forfeitures (in shares)	(227)
Stock option and restricted stock compensation expense		3,580		3,580
Directors' deferred share unit compensation		46		46
Balance at Sep. 30, 2019	$ 1,498	1,204,559	(1,292,282)	(86,225)
Balance (in shares) at Sep. 30, 2019	149,688
Increase (Decrease) in Shareholders' (Deficit) Equity
Net income (loss)			4,814	4,814
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 7	2,054		2,061
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	741
Restricted stock award, net of forfeitures	$ (4)	4
Restricted stock award, net of forfeitures (in shares)	(293)
Stock option and restricted stock compensation expense		3,138		3,138
Directors' deferred share unit compensation		91		91
Balance at Dec. 31, 2019	$ 1,501	1,209,846	(1,287,468)	$ (76,121)
Balance (in shares) at Dec. 31, 2019	150,136			150,136
Increase (Decrease) in Shareholders' (Deficit) Equity
Net income (loss)			(29,088)	$ (29,088)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 1	239		240
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	86
Issuance of common stock	$ 245	97,499		97,744
Issuance of common stock (in shares)	24,524
Restricted stock units vested (in shares)	2
Restricted stock award, net of forfeitures	$ (4)	4
Restricted stock award, net of forfeitures (in shares)	(487)
Stock option and restricted stock compensation expense		3,122		3,122
Balance at Mar. 31, 2020	$ 1,743	1,310,710	(1,316,556)	(4,103)
Balance (in shares) at Mar. 31, 2020	174,261
Balance at Dec. 31, 2019	$ 1,501	1,209,846	(1,287,468)	$ (76,121)
Balance (in shares) at Dec. 31, 2019	150,136			150,136
Increase (Decrease) in Shareholders' (Deficit) Equity
Net income (loss)				$ (75,760)
Balance at Sep. 30, 2020	$ 1,746	1,318,591	(1,363,228)	$ (42,891)
Balance (in shares) at Sep. 30, 2020	174,585			174,585
Balance at Mar. 31, 2020	$ 1,743	1,310,710	(1,316,556)	$ (4,103)
Balance (in shares) at Mar. 31, 2020	174,261
Increase (Decrease) in Shareholders' (Deficit) Equity
Net income (loss)			(24,298)	(24,298)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 1	424		425
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	122
Restricted stock units vested	$ 1	(1)
Restricted stock units vested (in shares)	157
Stock option and restricted stock compensation expense		3,409		3,409
Directors' deferred share unit compensation		45		45
Adjustment of issuance costs		(1)		(1)
Balance at Jun. 30, 2020	$ 1,745	1,314,586	(1,340,854)	(24,523)
Balance (in shares) at Jun. 30, 2020	174,540
Increase (Decrease) in Shareholders' (Deficit) Equity
Net income (loss)			(22,374)	(22,374)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 1	127		128
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	45
Stock option and restricted stock compensation expense		3,729		3,729
Directors' deferred share unit compensation		149		149
Balance at Sep. 30, 2020	$ 1,746	$ 1,318,591	$ (1,363,228)	$ (42,891)
Balance (in shares) at Sep. 30, 2020	174,585			174,585

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities:
Net loss	$ (75,760)	$ (108,947)
Adjustments to reconcile net loss to net cash used for operating activities:
Non-cash royalty revenue related to sale of future royalties	(45,159)	(32,102)
Non-cash interest expense on liability related to sale of future royalties and convertible senior notes	17,428	11,525
Depreciation and amortization	1,569	3,277
(Gain) loss on sale/disposal of fixed assets and impairment charges	(691)	2,544
Operating lease right-of-use asset impairment		694
Stock and deferred share unit compensation	10,454	10,939
Change in operating assets and liabilities:
Accounts receivable	2,450	1,607
Unbilled revenue/reimbursement	996	(2,392)
Contract asset	3,631	500
Prepaid and other current assets	(2,450)	(499)
Operating lease right-of-use assets	1,110	994
Other assets	(4,786)	296
Accounts payable	(1,028)	(3,751)
Accrued compensation	(4,651)	2,336
Other accrued liabilities	8,829	(6,058)
Deferred revenue	3,094	65,050
Operating lease liability	(2,191)	(1,823)
Net cash used for operating activities	(87,155)	(55,810)
Cash flows from investing activities:
Purchases of property and equipment	(815)	(2,762)
Proceeds from sale of equipment	1,426
Net cash provided by (used for) investing activities	611	(2,762)
Cash flows from financing activities:
Proceeds from issuance of common stock under stock plans	793	811
Proceeds from common stock issuance, net of $230 of transaction costs	97,743
Net cash provided by financing activities	98,536	811
Net change in cash and cash equivalents	11,992	(57,761)
Cash and cash equivalents, beginning of period	176,225	262,252
Cash and cash equivalents, end of period	$ 188,217	$ 204,491

CONSOLIDATED STATEMENTS OF CA_2

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) $ in Thousands	9 Months Ended
	Sep. 30, 2020USD ($)
CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED)
Transaction costs	$ 230

Nature of Business and Plan of

Nature of Business and Plan of Operations	9 Months Ended
	Sep. 30, 2020
Nature of Business and Plan of Operations
Nature of Business and Plan of Operations	A. Nature of Business and Plan of Operations ​ ImmunoGen, Inc. (the Company) was incorporated in Massachusetts in 1981 and is focused on the development of antibody-drug conjugates, or ADCs. The Company has generally incurred operating losses and negative cash flows from operations since inception, incurred a net loss of $75.8 million during the nine months ended September 30, 2020, and has an accumulated deficit of approximately $1.4 billion as of September 30, 2020. The Company has primarily funded these losses through payments received from its collaborations and equity, convertible debt, and other financings. To date, the Company has no product revenue and management expects operating losses to continue for the foreseeable future. At September 30, 2020, the Company had $188.2 million of cash and cash equivalents on hand. The Company anticipates that its current capital resources, inclusive of approximately $54.0 million of net proceeds generated from an Open Market Sale Agreement SM The Company is subject to risks common to companies in the biotechnology industry including, but not limited to, the development by its competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, manufacturing and marketing limitations, complexities associated with managing collaboration arrangements, third-party reimbursements, and compliance with governmental regulations.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2020
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	B. Summary of Significant Accounting Policies Basis of Presentation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, ImmunoGen Securities Corp., ImmunoGen Europe Limited, ImmunoGen BioPharma (Ireland) Limited, and Hurricane, LLC. All intercompany transactions and balances have been eliminated. The consolidated financial statements include all of the adjustments, consisting only of normal recurring adjustments, which management considers necessary for a fair presentation of the Company’s financial position in accordance with accounting principles generally accepted in the U.S. for interim financial information. The December 31, 2019 consolidated balance sheet presented for comparative purposes was derived from the Company’s audited financial statements, and certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. The preparation of interim financial statements requires the use of management’s estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the interim financial statements and the reported amounts of revenues and expenditures during the reported periods. The results of the interim periods are not necessarily indicative of the results for the entire year. Accordingly, the interim financial statements should be read in conjunction with the audited financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019 filed with the SEC on March 11, 2020. Subsequent Events On September 25, 2020, the Company entered into an Open Market Sale Agreement SM million through Jefferies. Subsequent to September 30, 2020 and through the date the Company issued these financial statements, the Company has sold approximately 12.9 million shares of its common stock under the Sale Agreement for net proceeds of approximately $54.0 million after deducting offering commissions and expenses. On October 12, 2020, the Company and Viridian Therapeutics, Inc. (Viridian) entered into a license agreement pursuant to which the Company granted Viridian the exclusive right to develop and commercialize an insulin-like growth factor-1 receptor (IGF-1R) antibody for all non-oncology indications that do not use radiopharmaceuticals in exchange for an upfront payment, with the potential to receive additional developmental milestone payments of approximately $50.0 million and up to $95.0 million in sales milestone payments plus mid-single-digit royalties on the commercial sales of any resulting product. Viridian was subsequently acquired by miRagen Therapeutics, Inc. (miRagen). miRagen is responsible for the manufacturing, development, and marketing of any products resulting from the license agreement. On October 19, 2020, the Company and Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd. (Huadong), a subsidiary of Huadong Medicine Co., Ltd., entered into a Collaboration and License Agreement (the License Agreement), pursuant to which the Company granted Huadong the exclusive right to develop and commercialize mirvetuximab soravtansine (the Licensed Product) in the People’s Republic of China, Hong Kong, Macau, and Taiwan (collectively, Greater China). The Company retains exclusive rights to the Licensed Product outside of Greater China. Under the terms of the License Agreement, the Company is entitled to receive a non-refundable $40.0 million upfront payment and up to $265.0 million in milestone payments upon achievement of certain development and regulatory objectives in the United States and in Greater China and for the achievement of certain annual net sales levels of the Licensed Product in Greater China. In addition, Huadong is obligated to pay the Company tiered percentage royalties ranging from low double digits to high teens as a percentage of commercial sales of the Licensed Product, if approved, by Huadong in Greater China, subject to adjustment in specified circumstances. Huadong is also responsible for costs related to the development of the Licensed Product in Greater China. The Company has evaluated all other events or transactions that occurred after September 30, 2020, up through the date the Company issued these financial statements. The Company did not have any other material recognized or unrecognized subsequent events during this period. Revenue Recognition The Company enters into licensing and development agreements with collaborators for the development of ADCs. The terms of these agreements contain multiple promised goods and services which may include (i) licenses, or options to obtain licenses, to the Company’s ADC technology, (ii) rights to future technological improvements, and (iii) miscellaneous other activities to be performed on behalf of the collaborative partner. Payments to the Company under these agreements may include upfront fees, option fees, exercise fees, payments for miscellaneous other activities, payments based upon the achievement of certain milestones, and royalties on product sales. The Company follows the provisions of Accounting Standards Codification, or ASC, Topic 606 - Revenue from Contracts with Customers Revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements, the Company performs the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations, including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when or as the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the Company will collect the consideration to which it is entitled in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, based on whether each promised good or service is distinct from other promised goods and services. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when or as the performance obligation is satisfied. As part of the accounting for these arrangements, the Company must develop assumptions that require judgment to determine the selling price for each performance obligation that was identified in the contract, which is discussed in further detail below. At September 30, 2020, the Company had the following types of material agreements with the parties identified below: ● Development and commercialization licenses, which provide the counterparty with the right to use the Company’s ADC technology and/or certain other intellectual property to develop and commercialize anticancer compounds to a specified antigen target: Bayer (one exclusive single-target license) CytomX (two exclusive single-target licenses) Debiopharm (one exclusive single-compound license) Fusion Pharmaceuticals (one exclusive single-target license) Novartis (five exclusive single-target licenses) Oxford BioTherapeutics/Menarini (one exclusive single target license sublicensed from Amgen) Roche, through its Genentech unit (five exclusive single-target licenses) Sanofi (five fully-paid, exclusive single-target licenses) ● Collaboration and option agreement for a defined period of time to secure a license to co-develop and co-commercialize a specified anticancer compound on established terms: Jazz Pharmaceuticals ● Collaboration and license agreement to co-develop and co-commercialize a specified anticancer compound on established terms: MacroGenics There are no performance, cancellation, termination, or refund provisions in any of the arrangements that contain material financial consequences to the Company. Development and Commercialization Licenses The obligations under a development and commercialization license agreement generally include the license to the Company’s ADC technology with respect to a specified antigen target, and may also include obligations related to rights to future technological improvements and miscellaneous other activities to be performed on behalf of the collaborative partner. In determining the performance obligations, management evaluates whether the license is distinct, and has significant standalone functionality, from the other goods and services promised to the collaborative partner based on the consideration of the relevant facts and circumstances for each arrangement. Factors considered in this determination include the research capabilities of the partner and the availability of ADC technology research expertise in the general marketplace and whether technological improvements are required for the continued functionality of the license. If the license to the Company’s intellectual property is determined to be distinct, the Company recognizes revenues from non- refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. The Company estimates the selling prices of the license and all other performance obligations based on market conditions, similar arrangements entered into by third parties, and entity-specific factors such as the terms of the Company’s previous collaborative agreements, recent preclinical and clinical testing results of therapeutic products that use the Company’s ADC technology, the Company’s pricing practices and pricing objectives, the likelihood that technological improvements will be made, and, if made, will be used by the Company’s collaborators, and the nature of the other services to be performed on behalf of its collaborators and market rates for similar services. The Company recognizes revenue related to other services as they are performed. The Company is compensated at negotiated rates that are consistent with what other third parties would charge. The Company records amounts received for services performed as a component of research and development support revenue. The Company recognizes revenue related to the rights to future technological improvements over the estimated term of the applicable license. The Company’s development and commercialization license agreements have milestone payments which for reporting purposes are aggregated into three categories: (i) development milestones, (ii) regulatory milestones, and (iii) sales milestones. Development milestones are typically payable when a product candidate initiates or advances into different clinical trial phases. Regulatory milestones are typically payable upon submission for marketing approval with the U.S. Food and Drug Administration (FDA) or other countries’ regulatory authorities or on receipt of actual marketing approvals for the compound or for additional indications. Sales milestones are typically payable when annual sales reach certain levels. At the inception of each arrangement that includes development and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service. If the milestone payment is not specifically related to the Company’s effort to satisfy a performance obligation or transfer a distinct good or service, the amount is allocated to all performance obligations using the relative standalone selling price method. In addition, the Company evaluates the milestone to determine whether the milestone is considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price to be allocated; otherwise, such amounts are considered constrained and excluded from the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development or regulatory milestones and any related constraint and, if necessary, adjusts its estimate of the transaction price. Any such adjustments to the transaction price are allocated to the performance obligations on the same basis as at contract inception. Amounts allocated to a satisfied performance obligation shall be recognized as revenue, or as a reduction of revenue, in the period in which the transaction price changes. For development and commercialization license agreements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied) in accordance with the royalty recognition constraint. Under the Company’s development and commercialization license agreements, except for the Sanofi and Debiopharm licenses, the Company receives royalty payments based upon its licensees’ net sales of covered products. Generally, under the development and commercialization agreements, the Company receives royalty reports and payments from its licensees approximately one quarter in arrears. The Company estimates the amount of royalty revenue to be recognized based on historical and forecasted sales and/or sales information from its licensees if available. Collaboration and Option Agreements/Right-to-Test Agreements The Company’s right-to-test agreements provide collaborators the right to test the Company’s ADC technology for a defined period of time through a research, or right-to-test, license. Under both right-to-test agreements and collaboration and option agreements, collaborators may (a) take options, for a defined period of time, to specified targets and (b) upon exercise of those options, secure or “take” licenses to develop and commercialize products for the specified targets on established terms. Under these agreements, fees may be due to the Company (i) at the inception of the arrangement (referred to as “upfront” fees or payments), (ii) upon the opt-in to acquire a development and commercialization license(s) (referred to as exercise fees or payments earned, if any, when the development and commercialization license is “taken”), (iii) after providing services at the collaborator’s request at negotiated prices, which are generally consistent with what other third parties would charge, or (iv) upon some combination of all of these fees. The accounting for collaboration and option agreements and right-to-test agreements is dependent on the nature of the options granted to the collaborative partner. Options are considered distinct performance obligations if they provide a collaborator with a material right. Factors that are considered in evaluating whether options convey a material right include the overall objective of the arrangement, the benefit the collaborator might obtain from the agreement without exercising the options, the cost to exercise the options relative to the fair value of the licenses, and the additional financial commitments or economic penalties imposed on the collaborator as a result of exercising the options. As of September 30, 2020, all material right-to-test agreements have expired. If the Company concludes that an option provides the customer a material right, and therefore is a separate performance obligation, the Company then determines the estimated selling prices of the option and all other units of accounting using the following inputs: (a) estimated fair value of each program, (b) the amount the partner would pay to exercise the option to obtain the license, and (c) probability of exercise. The Company does not control when or if any collaborator will exercise its options for development and commercialization licenses. As a result, the Company cannot predict when or if it will recognize revenues in connection with any of the foregoing. Upfront payments on development and commercialization licenses may be recognized upon delivery of the license if facts and circumstances dictate that the license has stand-alone functionality and is distinct from the undelivered elements. In determining whether a collaboration and option agreement is within the scope of ASC 808, Collaborative Arrangements Transaction Price Allocated to Future Performance Obligations Deferred revenue represents the transaction price of contracts for which work has not been performed (or has been partially performed) and includes unexercised contract options that are considered material rights. As of September 30, 2020, the aggregate amount of the transaction price allocated to remaining performance obligations comprising deferred revenue was $130.5 million. The Company expects to recognize revenue on approximately 3%, 86%, and 11% of the remaining performance obligations over the next 12 months , 13 to 60 months , and 61 to 120 months , respectively; however, it does not control when or if any collaborator will exercise its options for, or terminate existing development and commercialization licenses. Contract Balances from Contracts with Customers The following table presents changes in the Company’s contract assets and contract liabilities during the nine months ended September 30, 2020 and 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Nine months ended September 30, 2020 ​ December 31, 2019 Additions ​ Deductions ​ Impact of Netting ​ September 30, 2020 Contract asset ​ $ 3,631 ​ $ — ​ $ (8,000) ​ $ 4,369 ​ $ — Contract liabilities ​ $ 127,432 ​ $ 50 ​ $ (1,325) ​ $ 4,369 ​ $ 130,526 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ Balance at Nine months ended September 30, 2019 ​ December 31, 2018 ​ Additions ​ Deductions ​ September 30, 2019 Contract asset ​ $ 500 ​ $ — ​ $ (500) ​ $ — Contract liabilities ​ $ 80,802 ​ $ 65,287 ​ $ (237) ​ $ 145,852 ​ The Company recognized the following revenues as a result of changes in contract asset and contract liability balances in the respective periods (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended ​ ​ September 30, ​ September 30, ​ ​ 2020 ​ 2019 ​ 2020 ​ 2019 Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 72 ​ $ 79 ​ $ 1,300 ​ $ 237 Performance obligations satisfied in previous periods ​ $ 25 ​ $ — ​ $ 25 ​ $ 5,000 ​ During the nine months ended September 30, 2020, the Company recorded $0.2 million as license and milestone fee revenue for delivery of certain materials to CytomX that had been previously deferred and $1.1 million of amortization of deferred revenue related to numerous collaborators’ rights to technological improvements, which includes $0.9 million related to termination of the license agreement with Takeda. Additionally, a contract asset of $3.6 million, net of $4.4 million in related contract liabilities, was recorded for two probable milestones in 2019 pursuant to license agreements with CytomX and Novartis, which were subsequently achieved during the nine months ended September 30, 2020. A contract asset of $0.5 million was recorded for a probable milestone in 2018 pursuant to a license agreement with Fusion Pharmaceuticals, which was subsequently paid during the nine months ended September 30, 2019. During the nine months ended September 30, 2019, the Company received a $5.0 million regulatory milestone payment earned under its license agreement with Genentech, a member of the Roche Group. The full amount of the milestone was recognized as revenue in the period as the amount allocated to future rights to technological improvements was not material. Also during the nine months ended September 30, 2019, $65.2 million was recorded as deferred revenue as a result of a sale of the Company’s residual rights to receive royalty payments on commercial sales of Kadcyla ® The timing of revenue recognition, billings, and cash collections results in billed receivables, contract assets, and contract liabilities on the consolidated balance sheets. When consideration is received, or such consideration is unconditionally due, from a customer prior to transferring goods or services to the customer under the terms of a contract, a contract liability is recorded. Contract liabilities are recognized as revenue after control of the products or services is transferred to the customer and all revenue recognition criteria have been met. Financial Instruments and Concentration of Credit Risk Cash and cash equivalents are primarily maintained with three financial institutions in the U.S. Deposits with banks may exceed the amount of insurance provided on such deposits. Generally, these deposits may be redeemed upon demand and, therefore, bear minimal risk. The Company’s cash equivalents consist of money market funds with underlying investments primarily being U.S. Government-issued securities and high quality, short term commercial paper. Financial instruments that potentially subject the Company to concentrations of credit risk consist principally of cash, cash equivalents, and marketable securities. The Company held no marketable securities as of September 30, 2020 and December 31, 2019. The Company’s investment policy, approved by the Board of Directors, limits the amount it may invest in any one type of investment, thereby reducing credit risk concentrations. Cash and Cash Equivalents All highly liquid financial instruments with maturities of three months or less when purchased are considered cash equivalents. As of September 30, 2020 and December 31, 2019, the Company held $188.2 million and $176.2 million, respectively, in cash and money market funds, which were classified as cash and cash equivalents. Non-cash Investing and Financing Activities The Company had $0.1 million of accrued capital expenditures as of September 30, 2020, which have been treated as a non-cash investing activity and, accordingly, are not reflected in the consolidated statement of cash flows. The Company had no accrued capital expenditures as of December 31, 2019. Fair Value of Financial Instruments Fair value is defined under ASC Topic 820, Fair Value Measurements and Disclosures ● Level 1 - Quoted prices in active markets for identical assets or liabilities. ● Level 2 - Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. ● Level 3 – Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. As of September 30, 2020, the Company held certain assets that are required to be measured at fair value on a recurring basis. The following table represents the fair value hierarchy for the Company’s financial assets measured at fair value on a recurring basis as of September 30, 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at September 30, 2020 Using ​ ​ ​ ​ ​ Quoted Prices in ​ ​ ​ ​ Significant ​ ​ ​ ​ ​ Active Markets for ​ Significant Other ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Observable Inputs ​ Inputs ​ Total (Level 1) (Level 2) (Level 3) Cash equivalents ​ $ 168,967 ​ $ 168,967 ​ $ — ​ $ — As of December 31, 2019, the Company held certain assets that are required to be measured at fair value on a recurring basis. The following table represents the fair value hierarchy for the Company’s financial assets measured at fair value on a recurring basis as of December 31, 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2019 Using ​ ​ ​ ​ ​ Quoted Prices in ​ ​ ​ ​ Significant ​ ​ ​ ​ ​ Active Markets for ​ Significant Other ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Observable Inputs ​ Inputs ​ Total (Level 1) (Level 2) (Level 3) Cash equivalents ​ $ 163,674 ​ $ 163,674 $ — $ — ​ The fair value of the Company’s cash equivalents is based on quoted prices from active markets. The carrying amounts reflected in the consolidated balance sheets for accounts receivable, unbilled revenue, prepaid and other current assets, accounts payable, accrued compensation, and other accrued liabilities approximate fair value due to their short-term nature. The estimated fair value of the convertible 4.5% senior notes (the Convertible Notes) and gross carrying value were both $2.1 million as of September 30, 2020 and $3.0 million and $2.1 million, respectively, as of December 31, 2019. The fair value of the Convertible Notes is influenced by interest rates, the Company’s stock price and stock price volatility, and by prices observed in trading activity for the Convertible Notes. However, because there have been no trades involving the Convertible Notes since September 2019, the fair value as of September 30, 2020 and December 31, 2019 uses Level 3 inputs. Unbilled Revenue/Reimbursement Unbilled revenue/reimbursement substantially represents research funding earned based on actual resources utilized and external expenses incurred under certain of the Company’s collaboration agreements. Clinical Trial Accruals Clinical trial expenses are a significant component of research and development expenses, and the Company outsources a significant portion of these costs to third parties. Third party clinical trial expenses include investigator fees, site costs (patient costs), clinical research organization costs, and costs for central laboratory testing and data management. The accrual for site and patient costs includes inputs such as estimates of patient enrollment, patient cycles incurred, clinical site activations, and other pass-through costs. These inputs are required to be estimated due to a lag in receiving the actual clinical information from third parties. . Non-refundable advance payments for goods or services that will be used or rendered for future R&D activities are recorded as a prepaid asset and recognized as expense as the related goods are delivered or the related services are performed. Leases Effective January 1, 2019, the Company adopted Accounting Standards Update, or ASU, 2016-2, Leases (Topic 842) related liability recorded. ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. The Company uses the implicit rate when readily determinable. As a number of the Company’s leases do not provide an implicit rate, the Company uses an incremental borrowing rate applicable to the Company based on the information available at the commencement date in determining the present value of lease payments. As the Company has no existing or proposed collateralized borrowing arrangements, to determine a reasonable incremental borrowing rate, the Company considers collateral assumptions, the lease term, the Company’s current credit risk profile, and rates for existing borrowing arrangements for comparable peer companies. The Company accounts for the lease and fixed non-lease components as a single lease component. The Company’s lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense for operating lease payments is recognized on a straight-line basis over the lease term. Computation of Net Loss per Common Share Basic and diluted net loss per share is calculated based upon the weighted average number of common shares outstanding during the period. During periods of income, participating securities are allocated a proportional share of income determined by dividing total weighted average participating securities by the sum of the total weighted average common shares and participating securities (the two-class method). Shares of the Company’s restricted stock participate in any dividends that may be declared by the Company and are therefore considered to be participating securities. Participating securities have the effect of diluting both basic and diluted earnings per share during periods of income. During periods of loss, no loss is allocated to participating securities since they have no contractual obligation to share in the losses of the Company. Diluted (loss) income per share is computed after giving consideration to the dilutive effect of stock options, convertible notes, and restricted stock that are outstanding during the period, except where such non-participating securities would be anti-dilutive. The Company’s common stock equivalents, as calculated in accordance with the treasury-stock method for the options and unvested restricted stock and the if-converted method for the Convertible Notes, are shown in the following table (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended ​ ​ ​ September 30, ​ September 30, ​ ​ 2020 2019 2020 2019 Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock/units at end of period ​ 19,181 ​ 18,754 ​ 19,181 ​ 18,754 ​ Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock ​ 929 731 ​ 1,112 ​ 926 Shares issuable upon conversion of convertible notes at end of period ​ 501 ​ 501 ​ 501 ​ 501 ​ Common stock equivalents under if-converted method for convertible notes ​ 501 ​ 501 ​ 501 ​ 501 ​ ​ The Company’s common stock equivalents have not been included in the net loss per share calculation because their effect is anti-dilutive due to the Company’s net loss position. Stock-Based Compensation As of September 30, 2020, the Company was authorized to grant future awards under three employee share-based compensation plans, which are the ImmunoGen, Inc. 2018 Employee, Director and Consultant Equity Incentive Plan, as amended (the 2018 Plan), the Employee Stock Purchase Plan (ESPP), and the ImmunoGen Inducement Equity Incentive Plan, as amended (the Inducement Plan). At the annual meeting of shareholders on June 20, 2018, the 2018 Plan was approved and provides for the issuance of Stock Grants, the grant of Options, and the grant of Stock-Based Awards for up to 7,500,000 shares of the Company’s common stock, as well as up to 19,500,000 shares of common stock which represent awards granted under the previous stock option plans, the ImmunoGen, Inc. 2016 and 2006 Employee, Director and Consultant Equity Incentive Plans, that forfeit, expire, or cancel without delivery of shares of common stock or which resulted in the forfeiture of shares of common stock back to the Company subsequent to June 19, 2018. The Inducement Plan was approved the by Board of Directors in December 2019, and pursuant to subsequent amendments, provides for the issuance of non-qualified option grants for up to 1,500,000 shares of the Company’s common stock. Options awarded under the two plans are granted with an exercise price equal to the market price of the Company’s stock at the date of grant. Options vest at various periods of up to four years and may be exercis

Agreements

Agreements	9 Months Ended
	Sep. 30, 2020
Agreements
Agreements	C. Agreements Significant Collaborative Agreements Roche In May 2000, the Company granted Genentech, now a member of the Roche Group, an exclusive license to use the Company’s maytansinoid ADC technology. Pursuant to this agreement, Roche developed and received marketing approval for its HER2-targeting ADC compound, Kadcyla, in the U.S., Europe, Japan, and numerous other countries. The Company receives royalty reports and royalty payments related to sales of Kadcyla from Roche one sales of Kadcyla were recorded and included in non-cash royalty revenue for the nine months ended September 30, 2020 and 2019, respectively. Kadcyla sales occurring after January 1, 2015 were covered by a royalty purchase agreement whereby the associated cash, except for a residual tail, was initially remitted to Immunity Royalty Holdings, L.P, or IRH. Then in January 2019, the Company sold its residual tail to OMERS, the defined benefit pension plan for municipal employees in the Province of Ontario, Canada, for a net payment of $65.2 million, as discussed further in Note E. Simultaneously, OMERS purchased IRH’s right to the royalties the Company previously sold as described above, therefore obtaining the rights to 100% of the royalties received from that date on. On May 3, 2019, Roche notified the Company that the FDA approved Kadcyla for adjuvant (after surgery) treatment of people with HER2-positive early breast cancer who have residual invasive disease after neoadjuvant (before surgery) taxane and Herceptin® (trastuzumab)-based treatment, resulting in a $5.0 million regulatory milestone payment to the Company for a first extended indication, which is included in license and milestone fees for the nine months ended September 30, 2019. The Company is entitled to receive up to a total of $44.0 million in milestone payments pursuant to the license agreement, of which the Company has received $39.0 million to date. CytomX In 2016, the Company granted CytomX an exclusive development and commercialization license to the Company’s maytansinoid ADC technology for use with Probodies™ that target CD166 under a now expired reciprocal right-to-test agreement. Pursuant to the license agreement, the Company is entitled to receive up to a total of $160.0 million in milestone payments plus royalties on the commercial sales of any resulting product. The total milestones are categorized as follows: development milestones—$10.0 million; regulatory milestones—$50.0 million; and sales milestones—$100.0 million. In December 2019, a development milestone related to dosing of a first patient in a Phase 2 clinical trial became probable of being attained, which resulted in $3.0 million of license and milestone fee revenue being recorded in 2019. In February 2020, CytomX enrolled its first patient in the aforementioned Phase 2 clinical trial, and subsequently remitted the $3.0 million milestone payment to the Company in March 2020. CytomX is responsible for the manufacturing, development, and marketing of any products resulting from the development and commercialization license taken by CytomX under this collaboration. Novartis The Company granted Novartis exclusive development and commercialization licenses to the Company’s maytansinoid and IGN ADC technology for use with antibodies to six specified targets under a now-expired right-to-test agreement established in 2010. The Company received a $45.0 million upfront payment in connection with the execution of the right-to-test agreement in 2010, and for each development and commercialization license taken for a specific target, the Company received an exercise fee of $1.0 million and is entitled to receive up to a total of $199.5 million in milestone payments, plus royalties on the commercial sales of any resulting products. The total milestones are categorized as follows: development milestones—$22.5 million; regulatory milestones—$77.0 million; and sales milestones—$100.0 million. In December 2019, a development milestone of $5.0 million related to dosing a first patient in a Phase 1 clinical trial for one of the licensed products became probable of being attained. Accordingly, $4.7 million of the $5.0 million milestone that was allocated to the delivered license was recorded as license and milestone fee revenue in 2019, and $0.3 million that was allocated to future technological improvements was deferred and is being recognized as revenue ratably over the estimated term of the license. In September 2020, Novartis enrolled its first patient in the aforementioned Phase 1 clinical trial, and consequently owes the $5.0 million milestone payment to the Company. Novartis is responsible for the manufacturing, development, and marketing of any products resulting from the development and commercialization license taken by Novartis under this collaboration. Terminated Agreements During the quarter ended September 30, 2020, each of the exclusive development and commercialization licenses granted by the Company to each of Biotest and Takeda terminated pursuant to notices of termination received during the quarter ended June 30, 2020. For additional information related to these agreements, as well as the Company’s other significant collaborative agreements, please read Note C, Agreements - Significant Collaborative Agreements, ​

Convertible 4.5% Senior Notes

Convertible 4.5% Senior Notes	9 Months Ended
	Sep. 30, 2020
Convertible 4.5% Senior Notes
Convertible 4.5% Senior Notes	D. Convertible 4.5% Senior Notes In 2016, the Company issued Convertible Notes with an aggregate principal amount of $100 million, of which $2.1 million remains outstanding as of September 30, 2020. The Convertible Notes are governed by the terms of an indenture between the Company, as issuer, and Wilmington Trust, National Association, as the trustee. The Convertible Notes are senior unsecured obligations and bear interest at a rate of 4.5% per year, payable semi-annually in arrears on January 1 and July 1 of each year, commencing on January 1, 2017. The Company recorded $0.1 million of interest expense in each of the nine months ended September 30, 2020 and 2019. The Convertible Notes will mature on July 1, 2021, unless earlier repurchased or converted. Holders may convert their notes at their option at any time prior to the close of business on the business day immediately preceding the stated maturity date. Upon conversion, the Company will deliver for each $1,000 principal amount of converted notes a number of shares equal to the conversion rate, which will initially be 238.7775 shares of common stock, equivalent to an initial conversion price of approximately $4.19. The conversion rate will be subject to adjustment in some circumstances, but will not be adjusted for any accrued and unpaid interest.

Liability Related to Sale of Fu

Liability Related to Sale of Future Royalties	9 Months Ended
	Sep. 30, 2020
Liability Related to Sale of Future Royalties
Liability Related to Sale of Future Royalties	E. Liability Related to Sale of Future Royalties In 2015, IRH purchased the right to receive 100% of the royalty payments on commercial sales of Kadcyla subsequent to December 31, 2014, arising under the Company’s development and commercialization license with Genentech, until IRH had received aggregate royalties equal to $235 million or $260 million, depending on when the aggregate royalties received by IRH reach a specified threshold. Once the applicable threshold was met, if ever, the Company would thereafter have received 85% and IRH would have received 15% of the Kadcyla royalties for the remaining royalty term. At consummation of the transaction, the Company received cash proceeds of $200 million. As part of this sale, the Company incurred $5.9 million of transaction costs, which are presented net of the liability in the accompanying consolidated balance sheet and are being amortized to interest expense over the estimated life of the royalty purchase agreement. Although the Company sold its rights to receive royalties from the sales of Kadcyla, as a result of its then ongoing involvement in the cash flows related to these royalties, the Company continues to account for these royalties as revenue and recorded the $200 million in proceeds from this transaction as a liability related to sale of future royalties (Royalty Obligation) that is being amortized using the interest method over the estimated life of the royalty purchase agreement. In January 2019, the Company sold its residual rights to receive royalty payments on commercial sales of Kadcyla to OMERS, the defined benefit pension plan for municipal employees in the Province of Ontario, Canada, for a net payment of $65.2 million (amount is net of $1.5 million in contingent broker fees). Simultaneously, OMERS purchased IRH’s right to the royalties the Company previously sold as described above, therefore obtaining the rights to 100% of the royalties received from that date on. Because the Company will not be involved with the cash flows related to the residual royalties, the $65.2 million of net proceeds received from the sale of its residual rights to receive royalty payments was recorded as long-term deferred revenue and will be amortized as the royalty revenue related to the residual rights is earned using the units of revenue approach. During the nine months ended September 30, 2020, the Company did not receive any royalties related to the residual rights, therefore, no revenue from this sale was recognized. Additionally, the purchase of IRH’s interest by OMERS did not result in an extinguishment or modification of the original instrument and, accordingly, the Company will continue to account for the remaining obligation as a liability as outlined above. The following table shows the activity within the liability account during the nine-month period ended September 30, 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended ​ ​ September 30, 2020 ​ Liability related to sale of future royalties, net — beginning balance ​ $ 123,541 ​ Kadcyla royalty payments received and paid ​ (42,095) ​ Non-cash interest expense recognized ​ ​ 17,417 ​ Liability related to sale of future royalties, net — ending balance ​ $ 98,863 ​ ​ As royalties are remitted to IRH and subsequently OMERS, the balance of the Royalty Obligation will be effectively repaid over the life of the agreement. In order to determine the amortization of the Royalty Obligation, the Company is required to estimate the total amount of future royalty payments to be received and remitted as noted above over the life of the underlying license agreement with Genentech covering Kadcyla. The sum of these amounts less the $200 million proceeds the Company received will be recorded as interest expense over the life of the Royalty Obligation. Since inception, the Company’s estimate of this total interest expense results in an imputed annual interest rate of 10.5%, and a current imputed interest rate of 20.3% as of September 30, 2020. The Company periodically assesses the estimated royalty payments to IRH/OMERS and to the extent such payments are greater or less than its initial estimates, or the timing of such payments is materially different than its original estimates, the Company will prospectively adjust the amortization of the Royalty Obligation. There are a number of factors that could materially affect the amount and timing of royalty payments from Genentech, most of which are not within the Company’s control. Such factors include, but are not limited to, changing standards of care, the introduction of competing products, manufacturing or other delays, biosimilar competition, patent protection, adverse events that result in governmental health authority imposed restrictions on the use of the drug products, significant changes in foreign exchange rates as the royalties are paid in U.S. dollars (USD) while significant portions of the underlying sales of Kadcyla are made in currencies other than USD, and other events or circumstances that could result in reduced royalty payments from Kadcyla, all of which would result in a reduction of non-cash royalty revenues and the non-cash interest expense over the life of the Royalty Obligation. Conversely, if sales of Kadcyla are more than expected, the non-cash royalty revenues and the non-cash interest expense recorded by the Company would be greater over the term of the Royalty Obligation. In addition, the royalty purchase agreement grants IRH/OMERS the right to receive certain reports and other information relating to the royalties and contains other representations and warranties, covenants, and indemnification obligations that are customary for a transaction of this nature.

Capital Stock

Capital Stock	9 Months Ended
	Sep. 30, 2020
Capital Stock
Capital Stock	F. Capital Stock Compensation Policy for Non-Employee Directors Pursuant to the Compensation Policy for Non-Employee Directors, Non-Employee Directors are granted deferred share units for their annual retainers that vest quarterly over approximately one year from the date of grant, contingent upon the individual remaining a director of ImmunoGen as of each vesting date. The number of deferred share units awarded is fixed per the plan on the date of the award. All unvested deferred share units will automatically vest immediately prior to the occurrence of a change of control. The redemption amount of deferred share units issued will be paid in shares of common stock of the Company on the date a director ceases to be a member of the Board. In addition to the deferred share units, the Non-Employee Directors are also entitled to receive a fixed number of stock options on the date of the annual meeting of shareholders. These options vest quarterly over approximately one year from the date of grant. Any new directors will receive a pro-rated award, depending on their date of election to the Board. In June 2020, the Compensation Policy for Non-Employee Directors was amended, resulting in annual deferred share unit grants increasing from 4,000 to 17,000 units, and annual stock option grants increasing from 18,000 to 50,000 options. There were no substantial changes to the terms of the awards. The directors received a total of 300,000 and 108,000 options in June 2020 and 2019, respectively, and the related compensation expense for the three and nine months ended September 30, 2020 and 2019 is included in the amounts discussed in the “Stock-Based Compensation” section of Note B above.

Restructuring Charges

Restructuring Charges	9 Months Ended
	Sep. 30, 2020
Restructuring Charge.
Restructuring Charges	G. Restructuring Charge 2019 Corporate Restructuring On June 26, 2019, the Board of Directors approved a plan to restructure the business to focus resources on continued development of mirvetuximab soravtansine and a select portfolio of three earlier-stage product candidates, resulting in a significant reduction of the Company’s workforce, with a majority of these employees separating from the business by mid-July 2019 and most of the remaining affected employees transitioning over varying periods of time of up to 12 months. Communication of the plan to the affected employees was substantially completed on June 27, 2019. As a result of the workforce reduction, during the Compensation-Nonretirement Postemployment Benefits, period, of which $2.4 million was recorded during the year ended December 31, 2019 and $1.6 million was recorded during the nine months ended September 30, 2020. A summary of activity against the corporate restructuring charge related to the employee terminations in 2020 is as follows: ​ ​ ​ ​ ​ ​ Employee ​ ​ Termination ​ Benefits Costs Balance at December 31, 2019 ​ $ 4,087 Additional charges/adjustments during the period ​ ​ (116) Payments during the period ​ ​ (2,859) Balance at September 30, 2020 ​ $ 1,112 ​ In addition to the termination benefits and other related charges, the Company has sub-leased laboratory and office space at 830 Winter Street in Waltham, Massachusetts no longer used in the business. The decision to vacate part of its corporate office resulted in a change in asset groupings and also represented an impairment indicator. The Company determined and continues to believe that the right-of-use asset and leasehold improvements are recoverable based on expected sublease income, and therefore, no impairment has been recorded. Charge Related to Unoccupied Office Space The Company has sought to sub-lease 10,281 square feet of unoccupied office space at 930 Winter Street in Waltham, Massachusetts that was leased in 2016. During the nine months ended September 30, 2019, the Company recorded a $0.6 million impairment charge related to this lease, which represented the remaining balance of the right to use asset as the likelihood of finding a sub-lessor had diminished significantly as the lease approaches termination.

Leases

Leases	9 Months Ended
	Sep. 30, 2020
Leases
Leases	H. Leases The Company currently has two real estate leases. The first is an agreement with CRP/King 830 Winter L.L.C. for the rental of approximately 120,000 square feet of laboratory and office space at 830 Winter Street, Waltham, Massachusetts through March 2026. The Company uses this space for its corporate headquarters and other operations. The Company may extend the lease for two additional terms of five years and is required to pay certain operating expenses for the leased premises subject to escalation charges for certain expense increases over a base amount. During the nine months ended September 30, 2020, the Company executed four subleases for approximately 65,000 square feet through the remaining initial term of the lease. The balance of the space will be used by the Company. The second real estate lease is an agreement with PDM 930 Unit, LLC for the rental of 10,281 square feet of additional office space at 930 Winter Street, Waltham, Massachusetts through August 31, 2021. The Company is required to pay certain operating expenses for the leased premises based on its pro-rata share of such expenses for the entire rentable space of the building. Upon adoption of ASC 842 in January 2019, a ROU asset of $17.6 million and a lease liability of $27.3 million were recorded and are identified separately in the Company’s consolidated balance sheets for the existing operating leases. There was no impact to the consolidated statements of operations. Upon adoption, the amount of the ROU assets recorded was offset by the applicable unamortized lease incentive and straight-line lease liability balances of $9.7 million and, therefore, there was no impact to accumulated deficit. There were no initial direct costs related to the leases to consider. The Company’s operating lease liabilities related to its real estate lease agreements were calculated using a collateralized incremental borrowing rate. The weighted average discount rate for the operating lease liability is approximately 11%. A 100 basis point change in the incremental borrowing rate would result in less than a $1 million impact to the ROU assets and liabilities recorded. Lease expense for operating lease payments is recognized on a straight-line basis over the lease term, which for the nine months ended September 30, 2020 and 2019 was $3.0 million and $3.3 million, respectively, and is included in operating expenses in the consolidated statement of operations. During the nine months ended September 30, 2019, the Company recorded $0.6 million of impairment charges related to its 930 Winter Street lease, which represented the remaining balance of the ROU asset as the likelihood of finding a sub-lessor had diminished significantly as the lease approaches termination. Cash paid against operating lease liabilities during the nine months ended September 30, 2020 and 2019 was $4.1 million and $4.0 million, respectively. As of September 30, 2020, the Company’s ROU asset and lease liability for operating leases totaled $14.5 million and $22.6 million, respectively, and the weighted average remaining term of the operating leases is 5.4 years. ​ The maturities of operating lease liabilities discussed above are as follows (in thousands): ​ ​ ​ ​ 2020 (three months remaining) $ 1,379 2021 ​ 5,323 2022 ​ 5,389 2023 ​ 5,510 2024 ​ 5,470 Thereafter ​ 6,866 Total lease payments ​ ​ 29,937 Less imputed interest ​ ​ (7,359) Total lease liabilities ​ $ 22,578 ​ In addition to the amounts in the table above, the Company is also responsible for variable operating expenses and real estate taxes that are expected to approximate $3.4 million per year through March 2026. Sublease Income In 2020, the Company executed four agreements to sublease a total of approximately 65,000 square feet of the Company’s leased space at 830 Winter Street, Waltham, Massachusetts through March 2026. During the nine months ended September 30, 2020, the Company recorded $1.6 million of sublease income, inclusive of the sublessees’ proportionate share of operating expenses and real estate taxes for the period. Two of the four sublease agreements include an early termination option after certain periods of time for an agreed-upon fee. Assuming no early termination option is exercised, the Company will receive $16.6 million in minimum rental payments over the remaining term of the subleases, which is not included in the operating lease liability table above. The sublessees are also responsible for their proportionate share of variable operating expenses and real estate taxes.

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2020
Commitments and Contingencies
Commitments and Contingencies	I. Commitments and Contingencies Manufacturing Commitments As of September 30, 2020, the Company has noncancelable obligations under several agreements related to in-process and future manufacturing of antibody and cytotoxic agents required for supply of the Company’s product candidates totaling $5.2 million, which will be substantially paid in 2021. In 2018, the Company executed a commercial agreement with one of its manufacturers for the future production of antibody through calendar 2025. In May 2019, the agreement was amended to reduce the number of committed antibody batches for an agreed-upon exit fee, which was recorded as research and development expense in the first quarter of 2019. After further negotiations, the Company’s noncancelable commitment for future production is approximately €16 million at September 30, 2020.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2020
Summary of Significant Accounting Policies
Basis of Presentation	Basis of Presentation The consolidated financial statements include the accounts of the Company and its wholly owned subsidiaries, ImmunoGen Securities Corp., ImmunoGen Europe Limited, ImmunoGen BioPharma (Ireland) Limited, and Hurricane, LLC. All intercompany transactions and balances have been eliminated. The consolidated financial statements include all of the adjustments, consisting only of normal recurring adjustments, which management considers necessary for a fair presentation of the Company’s financial position in accordance with accounting principles generally accepted in the U.S. for interim financial information. The December 31, 2019 consolidated balance sheet presented for comparative purposes was derived from the Company’s audited financial statements, and certain information and footnote disclosures normally included in the Company’s annual financial statements have been condensed or omitted. The preparation of interim financial statements requires the use of management’s estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the interim financial statements and the reported amounts of revenues and expenditures during the reported periods. The results of the interim periods are not necessarily indicative of the results for the entire year. Accordingly, the interim financial statements should be read in conjunction with the audited financial statements and notes thereto included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2019 filed with the SEC on March 11, 2020.
Subsequent Events	Subsequent Events On September 25, 2020, the Company entered into an Open Market Sale Agreement SM million through Jefferies. Subsequent to September 30, 2020 and through the date the Company issued these financial statements, the Company has sold approximately 12.9 million shares of its common stock under the Sale Agreement for net proceeds of approximately $54.0 million after deducting offering commissions and expenses. On October 12, 2020, the Company and Viridian Therapeutics, Inc. (Viridian) entered into a license agreement pursuant to which the Company granted Viridian the exclusive right to develop and commercialize an insulin-like growth factor-1 receptor (IGF-1R) antibody for all non-oncology indications that do not use radiopharmaceuticals in exchange for an upfront payment, with the potential to receive additional developmental milestone payments of approximately $50.0 million and up to $95.0 million in sales milestone payments plus mid-single-digit royalties on the commercial sales of any resulting product. Viridian was subsequently acquired by miRagen Therapeutics, Inc. (miRagen). miRagen is responsible for the manufacturing, development, and marketing of any products resulting from the license agreement. On October 19, 2020, the Company and Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd. (Huadong), a subsidiary of Huadong Medicine Co., Ltd., entered into a Collaboration and License Agreement (the License Agreement), pursuant to which the Company granted Huadong the exclusive right to develop and commercialize mirvetuximab soravtansine (the Licensed Product) in the People’s Republic of China, Hong Kong, Macau, and Taiwan (collectively, Greater China). The Company retains exclusive rights to the Licensed Product outside of Greater China. Under the terms of the License Agreement, the Company is entitled to receive a non-refundable $40.0 million upfront payment and up to $265.0 million in milestone payments upon achievement of certain development and regulatory objectives in the United States and in Greater China and for the achievement of certain annual net sales levels of the Licensed Product in Greater China. In addition, Huadong is obligated to pay the Company tiered percentage royalties ranging from low double digits to high teens as a percentage of commercial sales of the Licensed Product, if approved, by Huadong in Greater China, subject to adjustment in specified circumstances. Huadong is also responsible for costs related to the development of the Licensed Product in Greater China. The Company has evaluated all other events or transactions that occurred after September 30, 2020, up through the date the Company issued these financial statements. The Company did not have any other material recognized or unrecognized subsequent events during this period.
Revenue Recognition	Revenue Recognition The Company enters into licensing and development agreements with collaborators for the development of ADCs. The terms of these agreements contain multiple promised goods and services which may include (i) licenses, or options to obtain licenses, to the Company’s ADC technology, (ii) rights to future technological improvements, and (iii) miscellaneous other activities to be performed on behalf of the collaborative partner. Payments to the Company under these agreements may include upfront fees, option fees, exercise fees, payments for miscellaneous other activities, payments based upon the achievement of certain milestones, and royalties on product sales. The Company follows the provisions of Accounting Standards Codification, or ASC, Topic 606 - Revenue from Contracts with Customers Revenue is recognized when a customer obtains control of promised goods or services, in an amount that reflects the consideration which the entity expects to receive in exchange for those goods or services. In determining the appropriate amount of revenue to be recognized as it fulfills its obligations under the agreements, the Company performs the following five steps: (i) identification of the promised goods or services in the contract; (ii) determination of whether the promised goods or services are performance obligations, including whether they are distinct in the context of the contract; (iii) measurement of the transaction price, including the constraint on variable consideration; (iv) allocation of the transaction price to the performance obligations; and (v) recognition of revenue when or as the Company satisfies each performance obligation. The Company only applies the five-step model to contracts when it is probable that the Company will collect the consideration to which it is entitled in exchange for the goods or services it transfers to the customer. At contract inception, once the contract is determined to be within the scope of ASC 606, the Company assesses the goods or services promised within each contract and determines those that are performance obligations, based on whether each promised good or service is distinct from other promised goods and services. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when or as the performance obligation is satisfied. As part of the accounting for these arrangements, the Company must develop assumptions that require judgment to determine the selling price for each performance obligation that was identified in the contract, which is discussed in further detail below. At September 30, 2020, the Company had the following types of material agreements with the parties identified below: ● Development and commercialization licenses, which provide the counterparty with the right to use the Company’s ADC technology and/or certain other intellectual property to develop and commercialize anticancer compounds to a specified antigen target: Bayer (one exclusive single-target license) CytomX (two exclusive single-target licenses) Debiopharm (one exclusive single-compound license) Fusion Pharmaceuticals (one exclusive single-target license) Novartis (five exclusive single-target licenses) Oxford BioTherapeutics/Menarini (one exclusive single target license sublicensed from Amgen) Roche, through its Genentech unit (five exclusive single-target licenses) Sanofi (five fully-paid, exclusive single-target licenses) ● Collaboration and option agreement for a defined period of time to secure a license to co-develop and co-commercialize a specified anticancer compound on established terms: Jazz Pharmaceuticals ● Collaboration and license agreement to co-develop and co-commercialize a specified anticancer compound on established terms: MacroGenics There are no performance, cancellation, termination, or refund provisions in any of the arrangements that contain material financial consequences to the Company. Development and Commercialization Licenses The obligations under a development and commercialization license agreement generally include the license to the Company’s ADC technology with respect to a specified antigen target, and may also include obligations related to rights to future technological improvements and miscellaneous other activities to be performed on behalf of the collaborative partner. In determining the performance obligations, management evaluates whether the license is distinct, and has significant standalone functionality, from the other goods and services promised to the collaborative partner based on the consideration of the relevant facts and circumstances for each arrangement. Factors considered in this determination include the research capabilities of the partner and the availability of ADC technology research expertise in the general marketplace and whether technological improvements are required for the continued functionality of the license. If the license to the Company’s intellectual property is determined to be distinct, the Company recognizes revenues from non- refundable, upfront fees allocated to the license when the license is transferred to the customer and the customer is able to use and benefit from the license. The Company estimates the selling prices of the license and all other performance obligations based on market conditions, similar arrangements entered into by third parties, and entity-specific factors such as the terms of the Company’s previous collaborative agreements, recent preclinical and clinical testing results of therapeutic products that use the Company’s ADC technology, the Company’s pricing practices and pricing objectives, the likelihood that technological improvements will be made, and, if made, will be used by the Company’s collaborators, and the nature of the other services to be performed on behalf of its collaborators and market rates for similar services. The Company recognizes revenue related to other services as they are performed. The Company is compensated at negotiated rates that are consistent with what other third parties would charge. The Company records amounts received for services performed as a component of research and development support revenue. The Company recognizes revenue related to the rights to future technological improvements over the estimated term of the applicable license. The Company’s development and commercialization license agreements have milestone payments which for reporting purposes are aggregated into three categories: (i) development milestones, (ii) regulatory milestones, and (iii) sales milestones. Development milestones are typically payable when a product candidate initiates or advances into different clinical trial phases. Regulatory milestones are typically payable upon submission for marketing approval with the U.S. Food and Drug Administration (FDA) or other countries’ regulatory authorities or on receipt of actual marketing approvals for the compound or for additional indications. Sales milestones are typically payable when annual sales reach certain levels. At the inception of each arrangement that includes development and regulatory milestone payments, the Company evaluates whether the achievement of each milestone specifically relates to the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service within a performance obligation. If the achievement of a milestone is considered a direct result of the Company’s efforts to satisfy a performance obligation or transfer a distinct good or service and the receipt of the payment is based upon the achievement of the milestone, the associated milestone value is allocated to that distinct good or service. If the milestone payment is not specifically related to the Company’s effort to satisfy a performance obligation or transfer a distinct good or service, the amount is allocated to all performance obligations using the relative standalone selling price method. In addition, the Company evaluates the milestone to determine whether the milestone is considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal would not occur, the associated milestone value is included in the transaction price to be allocated; otherwise, such amounts are considered constrained and excluded from the transaction price. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development or regulatory milestones and any related constraint and, if necessary, adjusts its estimate of the transaction price. Any such adjustments to the transaction price are allocated to the performance obligations on the same basis as at contract inception. Amounts allocated to a satisfied performance obligation shall be recognized as revenue, or as a reduction of revenue, in the period in which the transaction price changes. For development and commercialization license agreements that include sales-based royalties, including milestone payments based on the level of sales, and the license is deemed to be the predominant item to which the royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur, or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied) in accordance with the royalty recognition constraint. Under the Company’s development and commercialization license agreements, except for the Sanofi and Debiopharm licenses, the Company receives royalty payments based upon its licensees’ net sales of covered products. Generally, under the development and commercialization agreements, the Company receives royalty reports and payments from its licensees approximately one quarter in arrears. The Company estimates the amount of royalty revenue to be recognized based on historical and forecasted sales and/or sales information from its licensees if available. Collaboration and Option Agreements/Right-to-Test Agreements The Company’s right-to-test agreements provide collaborators the right to test the Company’s ADC technology for a defined period of time through a research, or right-to-test, license. Under both right-to-test agreements and collaboration and option agreements, collaborators may (a) take options, for a defined period of time, to specified targets and (b) upon exercise of those options, secure or “take” licenses to develop and commercialize products for the specified targets on established terms. Under these agreements, fees may be due to the Company (i) at the inception of the arrangement (referred to as “upfront” fees or payments), (ii) upon the opt-in to acquire a development and commercialization license(s) (referred to as exercise fees or payments earned, if any, when the development and commercialization license is “taken”), (iii) after providing services at the collaborator’s request at negotiated prices, which are generally consistent with what other third parties would charge, or (iv) upon some combination of all of these fees. The accounting for collaboration and option agreements and right-to-test agreements is dependent on the nature of the options granted to the collaborative partner. Options are considered distinct performance obligations if they provide a collaborator with a material right. Factors that are considered in evaluating whether options convey a material right include the overall objective of the arrangement, the benefit the collaborator might obtain from the agreement without exercising the options, the cost to exercise the options relative to the fair value of the licenses, and the additional financial commitments or economic penalties imposed on the collaborator as a result of exercising the options. As of September 30, 2020, all material right-to-test agreements have expired. If the Company concludes that an option provides the customer a material right, and therefore is a separate performance obligation, the Company then determines the estimated selling prices of the option and all other units of accounting using the following inputs: (a) estimated fair value of each program, (b) the amount the partner would pay to exercise the option to obtain the license, and (c) probability of exercise. The Company does not control when or if any collaborator will exercise its options for development and commercialization licenses. As a result, the Company cannot predict when or if it will recognize revenues in connection with any of the foregoing. Upfront payments on development and commercialization licenses may be recognized upon delivery of the license if facts and circumstances dictate that the license has stand-alone functionality and is distinct from the undelivered elements. In determining whether a collaboration and option agreement is within the scope of ASC 808, Collaborative Arrangements Transaction Price Allocated to Future Performance Obligations Deferred revenue represents the transaction price of contracts for which work has not been performed (or has been partially performed) and includes unexercised contract options that are considered material rights. As of September 30, 2020, the aggregate amount of the transaction price allocated to remaining performance obligations comprising deferred revenue was $130.5 million. The Company expects to recognize revenue on approximately 3%, 86%, and 11% of the remaining performance obligations over the next 12 months , 13 to 60 months , and 61 to 120 months , respectively; however, it does not control when or if any collaborator will exercise its options for, or terminate existing development and commercialization licenses. Contract Balances from Contracts with Customers The following table presents changes in the Company’s contract assets and contract liabilities during the nine months ended September 30, 2020 and 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Nine months ended September 30, 2020 ​ December 31, 2019 Additions ​ Deductions ​ Impact of Netting ​ September 30, 2020 Contract asset ​ $ 3,631 ​ $ — ​ $ (8,000) ​ $ 4,369 ​ $ — Contract liabilities ​ $ 127,432 ​ $ 50 ​ $ (1,325) ​ $ 4,369 ​ $ 130,526 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ Balance at Nine months ended September 30, 2019 ​ December 31, 2018 ​ Additions ​ Deductions ​ September 30, 2019 Contract asset ​ $ 500 ​ $ — ​ $ (500) ​ $ — Contract liabilities ​ $ 80,802 ​ $ 65,287 ​ $ (237) ​ $ 145,852 ​ The Company recognized the following revenues as a result of changes in contract asset and contract liability balances in the respective periods (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended ​ ​ September 30, ​ September 30, ​ ​ 2020 ​ 2019 ​ 2020 ​ 2019 Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 72 ​ $ 79 ​ $ 1,300 ​ $ 237 Performance obligations satisfied in previous periods ​ $ 25 ​ $ — ​ $ 25 ​ $ 5,000 ​ During the nine months ended September 30, 2020, the Company recorded $0.2 million as license and milestone fee revenue for delivery of certain materials to CytomX that had been previously deferred and $1.1 million of amortization of deferred revenue related to numerous collaborators’ rights to technological improvements, which includes $0.9 million related to termination of the license agreement with Takeda. Additionally, a contract asset of $3.6 million, net of $4.4 million in related contract liabilities, was recorded for two probable milestones in 2019 pursuant to license agreements with CytomX and Novartis, which were subsequently achieved during the nine months ended September 30, 2020. A contract asset of $0.5 million was recorded for a probable milestone in 2018 pursuant to a license agreement with Fusion Pharmaceuticals, which was subsequently paid during the nine months ended September 30, 2019. During the nine months ended September 30, 2019, the Company received a $5.0 million regulatory milestone payment earned under its license agreement with Genentech, a member of the Roche Group. The full amount of the milestone was recognized as revenue in the period as the amount allocated to future rights to technological improvements was not material. Also during the nine months ended September 30, 2019, $65.2 million was recorded as deferred revenue as a result of a sale of the Company’s residual rights to receive royalty payments on commercial sales of Kadcyla ® The timing of revenue recognition, billings, and cash collections results in billed receivables, contract assets, and contract liabilities on the consolidated balance sheets. When consideration is received, or such consideration is unconditionally due, from a customer prior to transferring goods or services to the customer under the terms of a contract, a contract liability is recorded. Contract liabilities are recognized as revenue after control of the products or services is transferred to the customer and all revenue recognition criteria have been met.
Financial Instruments and Concentration of Credit Risk	Financial Instruments and Concentration of Credit Risk Cash and cash equivalents are primarily maintained with three financial institutions in the U.S. Deposits with banks may exceed the amount of insurance provided on such deposits. Generally, these deposits may be redeemed upon demand and, therefore, bear minimal risk. The Company’s cash equivalents consist of money market funds with underlying investments primarily being U.S. Government-issued securities and high quality, short term commercial paper. Financial instruments that potentially subject the Company to concentrations of credit risk consist principally of cash, cash equivalents, and marketable securities. The Company held no marketable securities as of September 30, 2020 and December 31, 2019. The Company’s investment policy, approved by the Board of Directors, limits the amount it may invest in any one type of investment, thereby reducing credit risk concentrations.
Cash and Cash Equivalents	Cash and Cash Equivalents All highly liquid financial instruments with maturities of three months or less when purchased are considered cash equivalents. As of September 30, 2020 and December 31, 2019, the Company held $188.2 million and $176.2 million, respectively, in cash and money market funds, which were classified as cash and cash equivalents.
Non-cash Investing and Financing Activities	Non-cash Investing and Financing Activities The Company had $0.1 million of accrued capital expenditures as of September 30, 2020, which have been treated as a non-cash investing activity and, accordingly, are not reflected in the consolidated statement of cash flows. The Company had no accrued capital expenditures as of December 31, 2019.
Fair Value of Financial Instruments	Fair Value of Financial Instruments Fair value is defined under ASC Topic 820, Fair Value Measurements and Disclosures ● Level 1 - Quoted prices in active markets for identical assets or liabilities. ● Level 2 - Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities. ● Level 3 – Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities. As of September 30, 2020, the Company held certain assets that are required to be measured at fair value on a recurring basis. The following table represents the fair value hierarchy for the Company’s financial assets measured at fair value on a recurring basis as of September 30, 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at September 30, 2020 Using ​ ​ ​ ​ ​ Quoted Prices in ​ ​ ​ ​ Significant ​ ​ ​ ​ ​ Active Markets for ​ Significant Other ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Observable Inputs ​ Inputs ​ Total (Level 1) (Level 2) (Level 3) Cash equivalents ​ $ 168,967 ​ $ 168,967 ​ $ — ​ $ — As of December 31, 2019, the Company held certain assets that are required to be measured at fair value on a recurring basis. The following table represents the fair value hierarchy for the Company’s financial assets measured at fair value on a recurring basis as of December 31, 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2019 Using ​ ​ ​ ​ ​ Quoted Prices in ​ ​ ​ ​ Significant ​ ​ ​ ​ ​ Active Markets for ​ Significant Other ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Observable Inputs ​ Inputs ​ Total (Level 1) (Level 2) (Level 3) Cash equivalents ​ $ 163,674 ​ $ 163,674 $ — $ — ​ The fair value of the Company’s cash equivalents is based on quoted prices from active markets. The carrying amounts reflected in the consolidated balance sheets for accounts receivable, unbilled revenue, prepaid and other current assets, accounts payable, accrued compensation, and other accrued liabilities approximate fair value due to their short-term nature. The estimated fair value of the convertible 4.5% senior notes (the Convertible Notes) and gross carrying value were both $2.1 million as of September 30, 2020 and $3.0 million and $2.1 million, respectively, as of December 31, 2019. The fair value of the Convertible Notes is influenced by interest rates, the Company’s stock price and stock price volatility, and by prices observed in trading activity for the Convertible Notes. However, because there have been no trades involving the Convertible Notes since September 2019, the fair value as of September 30, 2020 and December 31, 2019 uses Level 3 inputs.
Unbilled Revenue/Reimbursement	Unbilled Revenue/Reimbursement Unbilled revenue/reimbursement substantially represents research funding earned based on actual resources utilized and external expenses incurred under certain of the Company’s collaboration agreements.
Clinical Trial Accruals	Clinical Trial Accruals Clinical trial expenses are a significant component of research and development expenses, and the Company outsources a significant portion of these costs to third parties. Third party clinical trial expenses include investigator fees, site costs (patient costs), clinical research organization costs, and costs for central laboratory testing and data management. The accrual for site and patient costs includes inputs such as estimates of patient enrollment, patient cycles incurred, clinical site activations, and other pass-through costs. These inputs are required to be estimated due to a lag in receiving the actual clinical information from third parties. . Non-refundable advance payments for goods or services that will be used or rendered for future R&D activities are recorded as a prepaid asset and recognized as expense as the related goods are delivered or the related services are performed.
Leases	Leases Effective January 1, 2019, the Company adopted Accounting Standards Update, or ASU, 2016-2, Leases (Topic 842) related liability recorded. ROU assets represent the Company’s right to use an underlying asset for the lease term and lease liabilities represent the Company’s obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. The Company uses the implicit rate when readily determinable. As a number of the Company’s leases do not provide an implicit rate, the Company uses an incremental borrowing rate applicable to the Company based on the information available at the commencement date in determining the present value of lease payments. As the Company has no existing or proposed collateralized borrowing arrangements, to determine a reasonable incremental borrowing rate, the Company considers collateral assumptions, the lease term, the Company’s current credit risk profile, and rates for existing borrowing arrangements for comparable peer companies. The Company accounts for the lease and fixed non-lease components as a single lease component. The Company’s lease terms may include options to extend or terminate the lease when it is reasonably certain that the Company will exercise that option. Lease expense for operating lease payments is recognized on a straight-line basis over the lease term.
Computation of Net Loss per Common Share	Computation of Net Loss per Common Share Basic and diluted net loss per share is calculated based upon the weighted average number of common shares outstanding during the period. During periods of income, participating securities are allocated a proportional share of income determined by dividing total weighted average participating securities by the sum of the total weighted average common shares and participating securities (the two-class method). Shares of the Company’s restricted stock participate in any dividends that may be declared by the Company and are therefore considered to be participating securities. Participating securities have the effect of diluting both basic and diluted earnings per share during periods of income. During periods of loss, no loss is allocated to participating securities since they have no contractual obligation to share in the losses of the Company. Diluted (loss) income per share is computed after giving consideration to the dilutive effect of stock options, convertible notes, and restricted stock that are outstanding during the period, except where such non-participating securities would be anti-dilutive. The Company’s common stock equivalents, as calculated in accordance with the treasury-stock method for the options and unvested restricted stock and the if-converted method for the Convertible Notes, are shown in the following table (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended ​ ​ ​ September 30, ​ September 30, ​ ​ 2020 2019 2020 2019 Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock/units at end of period ​ 19,181 ​ 18,754 ​ 19,181 ​ 18,754 ​ Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock ​ 929 731 ​ 1,112 ​ 926 Shares issuable upon conversion of convertible notes at end of period ​ 501 ​ 501 ​ 501 ​ 501 ​ Common stock equivalents under if-converted method for convertible notes ​ 501 ​ 501 ​ 501 ​ 501 ​ ​ The Company’s common stock equivalents have not been included in the net loss per share calculation because their effect is anti-dilutive due to the Company’s net loss position.
Stock-Based Compensation	Stock-Based Compensation As of September 30, 2020, the Company was authorized to grant future awards under three employee share-based compensation plans, which are the ImmunoGen, Inc. 2018 Employee, Director and Consultant Equity Incentive Plan, as amended (the 2018 Plan), the Employee Stock Purchase Plan (ESPP), and the ImmunoGen Inducement Equity Incentive Plan, as amended (the Inducement Plan). At the annual meeting of shareholders on June 20, 2018, the 2018 Plan was approved and provides for the issuance of Stock Grants, the grant of Options, and the grant of Stock-Based Awards for up to 7,500,000 shares of the Company’s common stock, as well as up to 19,500,000 shares of common stock which represent awards granted under the previous stock option plans, the ImmunoGen, Inc. 2016 and 2006 Employee, Director and Consultant Equity Incentive Plans, that forfeit, expire, or cancel without delivery of shares of common stock or which resulted in the forfeiture of shares of common stock back to the Company subsequent to June 19, 2018. The Inducement Plan was approved the by Board of Directors in December 2019, and pursuant to subsequent amendments, provides for the issuance of non-qualified option grants for up to 1,500,000 shares of the Company’s common stock. Options awarded under the two plans are granted with an exercise price equal to the market price of the Company’s stock at the date of grant. Options vest at various periods of up to four years and may be exercised within ten years of the date of grant. The stock-based awards are accounted for under ASC Topic 718, Compensation-Stock Compensation ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ 2020 ​ 2019 ​ 2020 ​ 2019 Dividend ​ None ​ None ​ None ​ None Volatility ​ 88.3% ​ 81.6% ​ 85.0% ​ 76.3% Risk-free interest rate ​ .36% ​ 1.78% ​ 1.23% ​ 2.24% Expected life (years) ​ 6.0 ​ 6.0 ​ 6.0 ​ 6.0 ​ Using the Black-Scholes option-pricing model, the weighted average grant date fair values of options granted during the three months ended September 30, 2020 and 2019 were $4.56 and $1.68 per share, respectively, and $4.57 and $2.85 for options granted during the nine months ended September 30, 2020 and 2019, respectively. A summary of option activity under the Company’s equity plans as of September 30, 2020, and changes during the nine-month period then ended is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ Number ​ Average ​ ​ ​ of Stock ​ Exercise ​ ​ ​ Options ​ Price ​ Outstanding at December 31, 2019 ​ 13,518 ​ $ 7.53 ​ Granted ​ 7,256 ​ ​ 4.57 ​ Exercised ​ (175) ​ ​ 2.81 ​ Forfeited/Canceled ​ (1,715) ​ ​ 9.52 ​ Outstanding at September 30, 2020 ​ 18,884 ​ $ 6.26 ​ ​ In September 2018, the Company granted 295,200 performance-based stock options to certain employees that will vest in two equal installments upon the achievement of specified performance goals. At September 30, 2020, 128,700 of these options are still outstanding. In the nine months ended September 30, 2020, the Company issued 2.6 million additional performance stock options that will vest in four installments upon the achievement of specified performance goals. The Company determined it is not currently probable that any of these performance goals will be achieved and, therefore, no expense has been recorded to date. The fair value of the performance-based options that could be expensed in future periods, net of estimated forfeitures, is $9.4 million. A summary of restricted stock and restricted stock unit activity, inclusive of performance-based restricted stock awards, under the Company’s equity plans as of September 30, 2020, and changes during the nine-month period then ended is presented below (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ Weighted- ​ ​ Restricted ​ Average Grant ​ ​ Stock Shares ​ Date Fair Value Unvested at December 31, 2019 1,297 ​ $ 2.97 Vested (513) ​ ​ 2.62 Forfeited ​ (487) ​ ​ 3.62 Unvested at September 30, 2020 ​ 297 ​ ​ 2.55 ​ In 2016, 2017, and 2019, the Company granted shares of performance-based restricted common stock to certain employees of the Company. All but 57,400 of these granted shares have since been forfeited. The restrictions on these shares will lapse in three equal installments upon the achievement of specified performance goals. The Company determined it is not currently probable that these performance goals will be achieved and, therefore, no expense has been recorded to date. The fair value of the performance-based shares that could be expensed in future periods, net of estimated forfeitures, is $142,000. During the nine months ended September 30, 2020, holders of options issued under the Company’s equity plans exercised their rights to acquire an aggregate of approximately 175,000 shares of common stock at prices ranging from $2.31 to $3.05 per share. The total proceeds to the Company from these option exercises were $0.5 million. In June 2018, the Company's Board of Directors, with shareholder approval, adopted the ESPP. An aggregate of 1,000,000 shares of common stock have been reserved for issuance under the ESPP. On June 30, 2020 and June 30, 2019, 78,000, and 323,000 shares, respectively, were issued to participating employees at a fair value of $1.86 and $1.63 per share, respectively. The fair value of each ESPP award is estimated on the first day of the offering period using the Black- Scholes option-pricing model. The assumptions used in the calculations for each offering period are noted in the table below. The Company recognizes share-based compensation expense equal to the fair value of the ESPP awards on a straight-line basis over the offering period. ​ ​ ​ ​ ​ ​ ​ ​ June 30, 2020 ​ June 30, 2019 ​ Dividend ​ None ​ None ​ Volatility ​ 85.7% ​ 67.3% ​ Risk-free interest rate ​ 1.57% ​ 2.51% ​ Expected life (years) ​ 0.5 ​ 0.5 ​ ​ Stock compensation expense related to stock options and restricted stock awards granted under the stock plans and related to the ESPP was $3.7 million and $10.3 million during the three and nine months ended September 30, 2020, respectively, compared to stock compensation expense of $3.6 million and $10.7 million for the three and nine months ended September 30, 2019, respectively. Stock compensation expense related to the ESPP was $188,000 and $345,000 for the nine months ended September 30, 2020 and 2019, respectively. As of September 30, 2020, the estimated fair value of unvested employee awards, exclusive of performance awards, was $20.7 million, net of estimated forfeitures. The weighted-average remaining vesting period for these awards is approximately three years .
Segment Information	Segment Information During the nine months ended September 30, 2020, the Company continued to operate in one operating segment, which is the business of development of monoclonal antibody-based anticancer therapeutics. During the three and nine months ended September 30, 2020, 99% and 97%, respectively, of revenues were from Roche, consisting primarily of non-cash royalty revenue, compared to 99% of revenue from Roche in each of the three and nine month periods ended September 30, 2019. There were no other customers of the Company that generated significant revenues in the three or nine months ended September 30, 2020 and 2019.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606 Collaborative Arrangements In June 2016, the FASB issued ASU 2016-13, Measurement of Credit Losses on Financial Instruments No other recently issued or effective ASUs had, or are expected to have, a material effect on the Company's results of operations, financial condition, or liquidity.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2020
Summary of Significant Accounting Policies
Contract assets and contract liabilities	The following table presents changes in the Company’s contract assets and contract liabilities during the nine months ended September 30, 2020 and 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Nine months ended September 30, 2020 ​ December 31, 2019 Additions ​ Deductions ​ Impact of Netting ​ September 30, 2020 Contract asset ​ $ 3,631 ​ $ — ​ $ (8,000) ​ $ 4,369 ​ $ — Contract liabilities ​ $ 127,432 ​ $ 50 ​ $ (1,325) ​ $ 4,369 ​ $ 130,526 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ Balance at Nine months ended September 30, 2019 ​ December 31, 2018 ​ Additions ​ Deductions ​ September 30, 2019 Contract asset ​ $ 500 ​ $ — ​ $ (500) ​ $ — Contract liabilities ​ $ 80,802 ​ $ 65,287 ​ $ (237) ​ $ 145,852 ​ The Company recognized the following revenues as a result of changes in contract asset and contract liability balances in the respective periods (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended ​ ​ September 30, ​ September 30, ​ ​ 2020 ​ 2019 ​ 2020 ​ 2019 Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 72 ​ $ 79 ​ $ 1,300 ​ $ 237 Performance obligations satisfied in previous periods ​ $ 25 ​ $ — ​ $ 25 ​ $ 5,000
Schedule of assets that are required to be measured at fair value on a recurring basis	As of September 30, 2020, the Company held certain assets that are required to be measured at fair value on a recurring basis. The following table represents the fair value hierarchy for the Company’s financial assets measured at fair value on a recurring basis as of September 30, 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at September 30, 2020 Using ​ ​ ​ ​ ​ Quoted Prices in ​ ​ ​ ​ Significant ​ ​ ​ ​ ​ Active Markets for ​ Significant Other ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Observable Inputs ​ Inputs ​ Total (Level 1) (Level 2) (Level 3) Cash equivalents ​ $ 168,967 ​ $ 168,967 ​ $ — ​ $ — As of December 31, 2019, the Company held certain assets that are required to be measured at fair value on a recurring basis. The following table represents the fair value hierarchy for the Company’s financial assets measured at fair value on a recurring basis as of December 31, 2019 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2019 Using ​ ​ ​ ​ ​ Quoted Prices in ​ ​ ​ ​ Significant ​ ​ ​ ​ ​ Active Markets for ​ Significant Other ​ Unobservable ​ ​ ​ ​ ​ Identical Assets ​ Observable Inputs ​ Inputs ​ Total (Level 1) (Level 2) (Level 3) Cash equivalents ​ $ 163,674 ​ $ 163,674 $ — $ —
Schedule of common stock equivalents, as calculated in accordance with the treasury-stock method	The Company’s common stock equivalents, as calculated in accordance with the treasury-stock method for the options and unvested restricted stock and the if-converted method for the Convertible Notes, are shown in the following table (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended ​ ​ ​ September 30, ​ September 30, ​ ​ 2020 2019 2020 2019 Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock/units at end of period ​ 19,181 ​ 18,754 ​ 19,181 ​ 18,754 ​ Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock ​ 929 731 ​ 1,112 ​ 926 Shares issuable upon conversion of convertible notes at end of period ​ 501 ​ 501 ​ 501 ​ 501 ​ Common stock equivalents under if-converted method for convertible notes ​ 501 ​ 501 ​ 501 ​ 501 ​
Schedule of assumptions	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ 2020 ​ 2019 ​ 2020 ​ 2019 Dividend ​ None ​ None ​ None ​ None Volatility ​ 88.3% ​ 81.6% ​ 85.0% ​ 76.3% Risk-free interest rate ​ .36% ​ 1.78% ​ 1.23% ​ 2.24% Expected life (years) ​ 6.0 ​ 6.0 ​ 6.0 ​ 6.0 ​ ​ ​ ​ ​ ​ ​ ​ June 30, 2020 ​ June 30, 2019 ​ Dividend ​ None ​ None ​ Volatility ​ 85.7% ​ 67.3% ​ Risk-free interest rate ​ 1.57% ​ 2.51% ​ Expected life (years) ​ 0.5 ​ 0.5 ​
Summary of stock option activity	A summary of option activity under the Company’s equity plans as of September 30, 2020, and changes during the nine-month period then ended is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ Number ​ Average ​ ​ ​ of Stock ​ Exercise ​ ​ ​ Options ​ Price ​ Outstanding at December 31, 2019 ​ 13,518 ​ $ 7.53 ​ Granted ​ 7,256 ​ ​ 4.57 ​ Exercised ​ (175) ​ ​ 2.81 ​ Forfeited/Canceled ​ (1,715) ​ ​ 9.52 ​ Outstanding at September 30, 2020 ​ 18,884 ​ $ 6.26 ​
Summary of restricted stock activity	A summary of restricted stock and restricted stock unit activity, inclusive of performance-based restricted stock awards, under the Company’s equity plans as of September 30, 2020, and changes during the nine-month period then ended is presented below (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ Weighted- ​ ​ Restricted ​ Average Grant ​ ​ Stock Shares ​ Date Fair Value Unvested at December 31, 2019 1,297 ​ $ 2.97 Vested (513) ​ ​ 2.62 Forfeited ​ (487) ​ ​ 3.62 Unvested at September 30, 2020 ​ 297 ​ ​ 2.55

Liability Related to Sale of _2

Liability Related to Sale of Future Royalties (Tables)	9 Months Ended
	Sep. 30, 2020
Liability Related to Sale of Future Royalties
Schedule of Liability account during the period from the inception of the royalty transaction	The following table shows the activity within the liability account during the nine-month period ended September 30, 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended ​ ​ September 30, 2020 ​ Liability related to sale of future royalties, net — beginning balance ​ $ 123,541 ​ Kadcyla royalty payments received and paid ​ (42,095) ​ Non-cash interest expense recognized ​ ​ 17,417 ​ Liability related to sale of future royalties, net — ending balance ​ $ 98,863 ​

Restructuring Charges (Tables)

Restructuring Charges (Tables)	9 Months Ended
	Sep. 30, 2020
Restructuring Charge.
Schedule activity against the restructuring charge related to the employee terminations	A summary of activity against the corporate restructuring charge related to the employee terminations in 2020 is as follows: ​ ​ ​ ​ ​ ​ Employee ​ ​ Termination ​ Benefits Costs Balance at December 31, 2019 ​ $ 4,087 Additional charges/adjustments during the period ​ ​ (116) Payments during the period ​ ​ (2,859) Balance at September 30, 2020 ​ $ 1,112

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2020
Leases
Schedule of maturities of operating lease liabilities	The maturities of operating lease liabilities discussed above are as follows (in thousands): ​ ​ ​ ​ 2020 (three months remaining) $ 1,379 2021 ​ 5,323 2022 ​ 5,389 2023 ​ 5,510 2024 ​ 5,470 Thereafter ​ 6,866 Total lease payments ​ ​ 29,937 Less imputed interest ​ ​ (7,359) Total lease liabilities ​ $ 22,578

Nature of Business and Plan o_2

Nature of Business and Plan of Operations (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Net loss				$ (75,760)	$ (108,947)
Accumulated deficit		$ (1,363,228)		(1,363,228)		$ (1,287,468)
Total revenues		18,189	$ 13,281	46,501	37,407
Cash and cash equivalents		$ 188,217	$ 204,491	$ 188,217	$ 204,491	$ 176,225	$ 262,252
Number of months Capital resources meets capital expenditures				12 months
Product
Total revenues				$ 0
Subsequent event | Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd. | Upfront payment
Upfront payments received under collaboration agreement	$ 40,000
Open Market Sale Offer | Subsequent event
Consideration received from sale of common stock	$ 54,000

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Subsequent Events (Details) - USD ($) shares in Millions	Oct. 19, 2020	Oct. 12, 2020	Oct. 31, 2020	Sep. 30, 2020	Sep. 25, 2020
Open Market Sale Offer
Subsequent Event [Line Items]
Aggregate sales price					$ 100
Subsequent event | Open Market Sale Offer
Subsequent Event [Line Items]
Number of shares sold			12.9
Consideration received from sale of common stock			$ 54,000,000
Development milestones | CytomX
Subsequent Event [Line Items]
Maximum entitled to receive				$ 10,000,000
Development milestones | Subsequent event | Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd.
Subsequent Event [Line Items]
Maximum entitled to receive	$ 265,000,000
Milestone payments plus royalties on the commercial sales | Subsequent event | Viridian Therapeutics, Inc.
Subsequent Event [Line Items]
Maximum entitled to receive		$ 95,000,000
Upfront payment | Subsequent event | Viridian Therapeutics, Inc.
Subsequent Event [Line Items]
Upfront payments received under collaboration agreement		$ 50,000,000
Upfront payment | Subsequent event | Hangzhou Zhongmei Huadong Pharmaceutical Co., Ltd.
Subsequent Event [Line Items]
Upfront payments received under collaboration agreement			$ 40,000,000
Maximum entitled to receive	$ 40,000,000

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Revenue Recognition (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2020USD ($)item	Dec. 31, 2010item
Bayer
Summary of Significant Accounting Policies
Number of single-target licenses	1
CytomX
Summary of Significant Accounting Policies
Number of single-target licenses	2
Debiopharm
Summary of Significant Accounting Policies
Number of single-compound licenses	1
Future milestones or royalties to be received | $	$ 0
Fusion Pharmaceuticals
Summary of Significant Accounting Policies
Number of single-compound licenses	1
Novartis
Summary of Significant Accounting Policies
Number of single-target licenses	5	6
Oxford BioTherapeutics Ltd Member
Summary of Significant Accounting Policies
Number of single-target licenses	1
Roche
Summary of Significant Accounting Policies
Number of single-target licenses	5
Sanofi
Summary of Significant Accounting Policies
Number of single-target licenses	5
Future milestones or royalties to be received | $	$ 0

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Performance Obligations (Details) $ in Millions	Sep. 30, 2020USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 130.5
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, percent	3.00%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, percent	86.00%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2030-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, percent	11.00%
Minimum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction	12 months
Minimum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction	13 months
Minimum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2030-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction	61 months
Maximum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2025-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction	60 months
Maximum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2030-10-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction	120 months

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Contract Balances (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Changes in the Company's contract assets and contract liabilities
Contract asset, Beginning balance	$ 3,631	$ 500
Contract asset, Additions		0
Contract asset, Deductions	8,000	500
Contract Asset, Impact Of Netting.	4,369
Contract liabilities:
Contract liabilities, Beginning balance	127,432	80,802
Contract liabilities, Additions	50	65,287
Contract liabilities, Deductions	(1,325)	(237)
Contract Liabilities, Impact Of Netting	4,369
Contract liabilities, Ending balance	$ 130,526	$ 145,852

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Revenues Recognized as a Result of Changes in Contract Asset and Liability Balances (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue recognized in the period from:
Amounts included in contract liabilities at the beginning of the period	$ 72	$ 79	$ 1,300	$ 237
Performance obligations satisfied in previous periods	$ 25		$ 25	$ 5,000

Summary of Significant Accoun_9

Summary of Significant Accounting Policies - Contract Balances from Contracts with Customers - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Revenue recognized in the period from:
Revenue from contract with customer	$ 18,189	$ 13,281	$ 46,501	$ 37,407
Deferred revenue	130,526	145,852	130,526	145,852	$ 127,432	$ 80,802
Contract assets					3,631	$ 500
Contract liability	3,985		3,985		309
Contract Liabilities, Impact Of Netting	4,369		4,369
Revenue previously deferred	72	79	1,300	237
Milestone earned, included in accounts receivable			8,000	500
Probable milestone earned and paid				0
License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer	$ 97	79	1,325	5,237
Genentech | Regulatory milestones
Revenue recognized in the period from:
Revenue previously deferred				5,000
CytomX | License and milestone fees
Revenue recognized in the period from:
Revenue previously deferred			200
CytomX | Probable Milestone
Revenue recognized in the period from:
Contract assets					3,600
Contract liability					$ 4,400
Kadcyla | Royalty revenue
Revenue recognized in the period from:
Deferred revenue		65,200		65,200
Takeda | Technological Improvements
Revenue recognized in the period from:
Amortization of deferred revenue			900
Fusion Pharmaceuticals | Probable Milestone
Revenue recognized in the period from:
Contract assets		$ 500		500
Other Collaborators | Technological Improvements
Revenue recognized in the period from:
Amortization of deferred revenue			$ 1,100	$ 200

Summary of Significant Accou_10

Summary of Significant Accounting Policies - Financial Instruments and Concentration of Credit Risk (Details)	9 Months Ended
	Sep. 30, 2020USD ($)item	Dec. 31, 2019USD ($)
Financial Instruments and Concentration of Credit Risk
Number of financial institutions in the U.S. in which cash and cash equivalents are primarily maintained | item	3
Marketable securities held by entity | $	$ 0	$ 0

Summary of Significant Accou_11

Summary of Significant Accounting Policies - Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Summary of Significant Accounting Policies
Cash and cash equivalents	$ 188,217	$ 176,225	$ 204,491	$ 262,252

Summary of Significant Accou_12

Summary of Significant Accounting Policies - Non-cash Investing and Financing Activities (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Summary of Significant Accounting Policies
Accrued capital expenditures	$ 100,000	$ 0

Summary of Significant Accou_13

Summary of Significant Accounting Policies - Fair Value of Financial Instruments (Details)	9 Months Ended
	Sep. 30, 2020USD ($)item	Dec. 31, 2019USD ($)	Dec. 31, 2017	Dec. 31, 2016USD ($)
Fair value hierarchy for the Company's financial assets measured at fair value
Interest rate (as a percent)	4.50%	4.50%
Number of trades | item	0
Convertible Notes
Fair value hierarchy for the Company's financial assets measured at fair value
Interest rate (as a percent)	4.50%		4.50%
Principal amount of debt for conversion calculations				$ 1,000
Significant Other Observable Inputs (Level 2) | Convertible Notes | Face Value
Fair value hierarchy for the Company's financial assets measured at fair value
Convertible debt fair value	$ 2,100,000	$ 3,000,000
Significant Other Observable Inputs (Level 2) | Convertible Notes | Estimated fair value
Fair value hierarchy for the Company's financial assets measured at fair value
Convertible 4.5% senior notes gross carrying value	2,100,000	2,100,000
Recurring basis
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	168,967,000	163,674,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	168,967,000	163,674,000
Recurring basis | Significant Other Observable Inputs (Level 2)
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	0	0
Recurring basis | Significant Unobservable Inputs (Level 3)
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	$ 0	$ 0

Summary of Significant Accou_14

Summary of Significant Accounting Policies - Leases (Details) $ in Thousands	Sep. 30, 2020USD ($)
Commitments and Contingencies
Obligations under finance leases	$ 0

Summary of Significant Accou_15

Summary of Significant Accounting Policies - Computation of Net Loss per Common Share (Details) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020USD ($)shares	Sep. 30, 2019USD ($)shares	Sep. 30, 2020USD ($)shares	Sep. 30, 2019USD ($)shares
Computation of Net Loss per Common Share
Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock at end of period	19,181	18,754	19,181	18,754
Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock	929	731	1,112	926
Shares issuable upon conversion of convertible notes at end of period (in shares) | $	501	501	501	501
Common stock equivalents under if-converted method for convertible notes (in shares)	501	501	501	501

Summary of Significant Accou_16

Summary of Significant Accounting Policies - Stock-Based Compensation (Details)	1 Months Ended	3 Months Ended						6 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018installmentshares	Sep. 30, 2020USD ($)$ / sharesshares	Jun. 30, 2020USD ($)shares	Dec. 31, 2019USD ($)$ / sharesshares	Sep. 30, 2019USD ($)$ / shares	Jun. 30, 2019USD ($)shares	Mar. 31, 2019USD ($)	Jun. 30, 2020$ / sharesshares	Jun. 30, 2019$ / sharesshares	Sep. 30, 2020USD ($)planinstallment$ / sharesshares	Sep. 30, 2019USD ($)$ / shares	Dec. 31, 2019item$ / sharesshares	Dec. 31, 2017itemshares	Dec. 31, 2016itemshares	Jun. 30, 2018shares
Stock-Based Compensation
Number of employee share-based compensation plans | plan										3
Number of Stock Options
Exercised (in shares)										(175,000)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Proceeds from Stock Options Exercised | $										$ 793,000	$ 811,000
Directors' deferred share unit compensation | $		$ 149,000	$ 45,000	$ 91,000	$ 46,000	$ 100,000	$ 100,000
ESPP
Weighted-average assumptions used to estimate the fair value of each stock option
Dividend (as a percent)			0.00%			0.00%
Volatility (as a percent)			85.70%			67.30%
Risk-free interest rate (as a percent)			1.57%			2.51%
Expected life			6 months			6 months
Weighted-Average Grant Date Fair Value
Awarded (in dollars per share) | $ / shares								$ 1.86	$ 1.63
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Aggregate number of common shares reserved for future issuance															1,000,000
Allocated Share-based Compensation Expense | $										$ 188,000	345,000
Shares, Issued			78,000			323,000		78,000	323,000
Stock options and restricted stock awards
Stock-Based Compensation
Vesting period										3 years
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Allocated Share-based Compensation Expense | $		3,700,000			$ 3,600,000					$ 10,300,000	$ 10,700,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $		$ 20,700,000								$ 20,700,000
Stock options
Weighted-average assumptions used to estimate the fair value of each stock option
Dividend (as a percent)		0.00%			0.00%					0.00%	0.00%
Volatility (as a percent)		88.30%			81.60%					85.00%	76.30%
Risk-free interest rate (as a percent)		0.36%			1.78%					1.23%	2.24%
Expected life		6 years			6 years					6 years	6 years
Weighted-average grant date fair value (in dollars per share) | $ / shares		$ 4.56			$ 1.68					$ 4.57	$ 2.85
Number of Stock Options
Outstanding at the beginning of the period (in shares)								13,518,000		13,518,000
Granted (in shares)										7,256,000
Exercised (in shares)										(175,000)
Forfeited/Canceled (in shares)										(1,715,000)
Outstanding at the end of the period (in shares)		18,884,000		13,518,000						18,884,000		13,518,000
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares								$ 7.53		$ 7.53
Granted (in dollars per share) | $ / shares										4.57
Exercised (in dollars per share) | $ / shares										2.81
Forfeited/Canceled (in dollars per share) | $ / shares										9.52
Outstanding at the end of the period (in dollars per share) | $ / shares		$ 6.26		$ 7.53						$ 6.26		$ 7.53
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Proceeds from Stock Options Exercised | $										$ 500,000
Stock options | Minimum
Weighted-Average Exercise Price
Exercised (in dollars per share) | $ / shares										$ 2.31
Stock options | Maximum
Weighted-Average Exercise Price
Exercised (in dollars per share) | $ / shares										$ 3.05
Performance shares
Stock-Based Compensation
Number of equal installments | installment	2									4
Number of Stock Options
Granted (in shares)										2,600,000
Outstanding at the end of the period (in shares)		128,700								128,700
Number of Restricted Stock Shares
Awarded (in shares)	295,200											57,400	57,400	57,400
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Allocated Share-based Compensation Expense | $										$ 0
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $		$ 9,400,000								$ 9,400,000
Restricted stock
Stock-Based Compensation
Number of equal installments | item												3	3	3
Number of Restricted Stock Shares
Unvested at the beginning of the period (in shares)								1,297,000		1,297,000
Vested (in shares)										(513,000)
Forfeited (in shares)										(487,000)
Unvested at the end of the period (in shares)		297,000		1,297,000						297,000		1,297,000
Weighted-Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share) | $ / shares								$ 2.97		$ 2.97
Vested (in dollars per share) | $ / shares										2.62
Forfeited (in dollars per share) | $ / shares										3.62
Unvested at the end of the period (in dollars per share) | $ / shares		$ 2.55		$ 2.97						$ 2.55		$ 2.97
Restricted stock | Officers
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Allocated Share-based Compensation Expense | $										$ 0
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $		$ 142,000								$ 142,000
Previous 2016 and 2006 stock option plans
Stock-Based Compensation
Common stock authorized for issuance (in shares)		19,500,000								19,500,000
2018 Plan
Stock-Based Compensation
Common stock authorized for issuance (in shares)		7,500,000								7,500,000
Inducement Plan
Stock-Based Compensation
Number of employee share-based compensation plans | plan										2
Common stock authorized for issuance (in shares)				1,500,000								1,500,000
Vesting period										4 years
Exercise period										10 years

Summary of Significant Accou_17

Summary of Significant Accounting Policies - Segments (Details) - item	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Information
Number of operating segments			1
Other customers
Segment Information
Percentages of revenue recognized	0.00%	0.00%	0.00%	0.00%
Royalty revenue | Roche | Revenues percentage
Segment Information
Percentages of revenue recognized	99.00%	99.00%	97.00%	99.00%

Agreements - Roche (Details)

Agreements - Roche (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Jan. 31, 2019	May 31, 2000	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Collaborative Agreements disclosures
Non-cash royalty revenue related to sale of future royalties					$ 45,159	$ 32,102
Revenue previously deferred			$ 72	$ 79	1,300	237
Milestone payments
Collaborative Agreements disclosures
Revenue previously deferred					39,000
Roche
Collaborative Agreements disclosures
Period in arrears to receive royalty reports and payments related to sales of Kadcyla		3 months
Percentage of royalty payments	100.00%
Roche | License and milestone fees | Regulatory milestones
Collaborative Agreements disclosures
Revenue previously deferred						5,000
Roche | Kadcyla
Collaborative Agreements disclosures
Non-cash royalty revenue related to sale of future royalties					45,200	$ 32,100
OMERS | Kadcyla
Collaborative Agreements disclosures
Non-cash royalty revenue related to sale of future royalties	$ 65,200
Percentage of royalty payments	100.00%
Maximum | Roche | Milestone payments
Collaborative Agreements disclosures
Potential milestone payments			$ 44,000		$ 44,000

Agreements - CytomX (Details)

Agreements - CytomX (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Collaborative Agreements disclosures
Revenue from contract with customer		$ 18,189	$ 13,281	$ 46,501	$ 37,407
CytomX | Development milestones
Collaborative Agreements disclosures
Maximum entitled to receive				10,000
Revenue from contract with customer						$ 3,000
CytomX | Phase 2 clinical trial
Collaborative Agreements disclosures
Revenue from contract with customer	$ 3,000
CytomX | Regulatory milestones
Collaborative Agreements disclosures
Maximum entitled to receive				50,000
CytomX | Milestone payments
Collaborative Agreements disclosures
Maximum entitled to receive				100,000
Milestone payments plus royalties on the commercial sales | CytomX
Collaborative Agreements disclosures
Maximum entitled to receive				$ 160,000

Agreements - Novartis (Details)

Agreements - Novartis (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2020USD ($)item	Sep. 30, 2019USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2010USD ($)item	Dec. 31, 2018USD ($)
Collaborative Agreements disclosures
Revenue from contract with customer	$ 18,189	$ 13,281	$ 46,501	$ 37,407
Contract with Customer, Liability	130,526	145,852	$ 130,526	145,852	$ 127,432		$ 80,802
Novartis
Collaborative Agreements disclosures
Number of single-target licenses | item			5			6
Novartis | Development milestones
Collaborative Agreements disclosures
Potential milestone payment						$ 22,500
Novartis | Phase 1 clinical trial
Collaborative Agreements disclosures
Potential milestone payment	5,000		$ 5,000
Development Milestone amount					5,000
Novartis | Regulatory milestones
Collaborative Agreements disclosures
Potential milestone payment						77,000
Novartis | Milestone payments
Collaborative Agreements disclosures
Potential milestone payment						100,000
Upfront payment | Novartis
Collaborative Agreements disclosures
Revenue from contract with customer						45,000
License and milestone fees
Collaborative Agreements disclosures
Revenue from contract with customer	97	$ 79	1,325	5,237
License and milestone fees | Novartis
Collaborative Agreements disclosures
Revenue from contract with customer					4,700
Potential milestone payment					5,000	199,500
License and milestone fees | Novartis | Technological Improvements
Collaborative Agreements disclosures
Development Milestone amount					$ 300
Exercise fee | Novartis
Collaborative Agreements disclosures
Revenue from contract with customer						$ 1,000
Research and development support
Collaborative Agreements disclosures
Revenue from contract with customer	$ 5		$ 17	$ 68

Agreements - Terminated Agreeme

Agreements - Terminated Agreements (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue from contract with customer	$ 18,189	$ 13,281	$ 46,501	$ 37,407
License and milestone fees
Revenue from contract with customer	$ 97	$ 79	1,325	$ 5,237
Takeda | Technological Improvements
Amortization of deferred revenue			$ 900

Convertible 4.5% Senior Notes (

Convertible 4.5% Senior Notes (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2017	Dec. 31, 2016USD ($)$ / shares
Convertible debt
Convertible 4.5% Senior Notes outstanding	$ 2,100,000		$ 2,100,000
Interest rate (as a percent)	4.50%		4.50%			4.50%
Interest expense	$ 24,000	$ 24,000	$ 71,000	$ 71,000
Convertible Notes
Convertible debt
Principal amount of debt								$ 100,000,000
Interest rate (as a percent)	4.50%		4.50%				4.50%
Interest expense			$ 100,000	$ 100,000
Principal amount of debt for conversion calculations								$ 1,000
Ratio issued upon conversion					238.7775
Initial conversion price (in dollars per share) | $ / shares								$ 4.19

Liability Related to Sale of _3

Liability Related to Sale of Future Royalties (Details) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended		12 Months Ended
	Jan. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2015
Liability Related to Sale of Future Royalties
Non-cash royalty revenue related to the sale of future royalties		$ 45,159	$ 32,102
Kadcyla
Liability Related to Sale of Future Royalties
Percentage of royalty payments if applicable threshold is met				85.00%
Change in liability related to sale of future royalties
Liability related to sale of future royalties, net - beginning balance		123,541
Royalty payments received and paid		42,095
Non-cash interest expense recognized		17,417
Liability related to sale of future royalties, net - ending balance		98,863
IRH | Kadcyla
Liability Related to Sale of Future Royalties
Percentage of royalty payments				100.00%
Percentage of royalty payments if applicable threshold is met				15.00%
Proceeds from sale of future royalties - net		$ 200,000		$ 200,000
Transaction costs for royalty agreements				5,900
Change in liability related to sale of future royalties
Effective annual interest rate		10.50%
Current effective interest rate		20.3
IRH | Kadcyla | Maximum
Liability Related to Sale of Future Royalties
Royalties threshold				260,000
IRH | Kadcyla | Minimum
Liability Related to Sale of Future Royalties
Royalties threshold				$ 235,000
OMERS | Kadcyla
Liability Related to Sale of Future Royalties
Percentage of royalty payments	100.00%
Non-cash royalty revenue related to the sale of future royalties	$ 65,200
Contingent broker fees	1,500
Net proceeds from sale of residual rights to receive royalty payments	$ 65,200	$ 0

Capital Stock (Details)

Capital Stock (Details) - shares	Dec. 09, 2016	Jun. 30, 2020	Jun. 30, 2019	May 31, 2020	Sep. 30, 2020
Stock options
Stock-based compensation disclosure
Stock options granted to directors (in shares)					7,256,000
Compensation Policy for Non-Employee Directors | Stock options
Stock-based compensation disclosure
Vesting period					1 year
Stock options granted to directors (in shares)		300,000	108,000
Compensation Policy for Non-Employee Directors | Deferred share units
Stock-based compensation disclosure
Vesting period	1 year
Non-employee directors-initial grant | Stock options
Stock-based compensation disclosure
Annual deferred share units granted		17,000		4,000
Annual stock options grants		50,000		18,000

Restructuring Charge (Details)

Restructuring Charge (Details) $ in Thousands	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2019USD ($)	Jun. 30, 2019USD ($)	Dec. 31, 2019USD ($)	Sep. 30, 2020USD ($)ft²	Sep. 30, 2019USD ($)	Dec. 31, 2019USD ($)
Restructuring charge
Restructuring charge	$ 1,020			$ 1,524	$ 20,921
Workforce reduction
Restructuring charge
Balance at beginning of the period				4,087
Additional charges/adjustments during the period				(116)
Payments during the period				(2,859)
Balance at end of the period			$ 4,087	1,112		$ 4,087
Workforce reduction | 2019 Corporate Restructuring
Restructuring
Charge for severance related to a pre-existing plan		$ 16,000
Costs incurred			$ 15,300
Incremental retention benefits | 2019 Corporate Restructuring
Restructuring
Costs incurred		$ 4,000		$ 1,600		$ 2,400
Winter Street 930 Waltham MA
Restructuring charge
Unoccupied office space for sub-lease | ft²				10,281
Leasehold impairment charge					$ 600

Leases - Operating Leases (Deta

Leases - Operating Leases (Details) $ in Thousands	1 Months Ended	9 Months Ended
	Jan. 31, 2019USD ($)	Sep. 30, 2020USD ($)ft²leaseagreementitem	Sep. 30, 2019USD ($)	Dec. 31, 2019USD ($)
Lessee, Lease, Description [Line Items]
Number of real estate leases | lease		2
Right-of-use assets		$ 14,477		$ 15,587
Lease liabilities		$ 22,578
Unamortized lease incentive and straight-line lease liability balances	$ 9,700
Weighted-average discount rate		11.00%
Lease expense for operating lease payments		$ 3,000	$ 3,300
Cash paid against operating lease liabilities		$ 4,100	4,000
Weighted average remaining term of the operating leases		5 years 4 months 24 days
Sublease income		$ 1,600
Maximum
Lessee, Lease, Description [Line Items]
100 basis point change effect on ROU asset		$ 1,000
CRP/King 830 Winter L.L.C.
Lessee, Lease, Description [Line Items]
Area of space leased | ft²		120,000
Number of additional terms for which lease agreement can be extended | item		2
Operating lease term extension period		5 years
Number of executed sub-lease spaces | lease		4
Area of executed sublease space | ft²		65,000
PDM 930 Unit, LLC
Lessee, Lease, Description [Line Items]
Area of executed sublease space | ft²		10,281
Winter Street 930 Waltham MA
Lessee, Lease, Description [Line Items]
Number of executed sub-lease spaces | agreement		4
Area of executed sublease space | ft²		65,000
Leasehold impairment charge			$ 600
Minimum rental payments over the remaining term of the sublease		$ 16,600
Adjustment | ASU 2016-2
Lessee, Lease, Description [Line Items]
Right-of-use assets	17,600
Lease liabilities	$ 27,300

Leases - Maturities of Operatin

Leases - Maturities of Operating Lease Liabilities (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2020USD ($)
Leases
2020 (three months remaining)	$ 1,379
2021	5,323
2022	5,389
2023	5,510
2024	5,470
Thereafter	6,866
Total lease payments	29,937
Less imputed interest	(7,359)
Total lease liabilities	22,578
Variable operating costs and real estate taxes	$ 3,400

Commitments and Contingencies (

Commitments and Contingencies (Details) - Sep. 30, 2020 € in Millions, $ in Millions	USD ($)	EUR (€)
In-process and future manufacturing of antibody and cytotoxic agents
Collaborations and Manufacturing Commitments
Noncancelable obligations under several agreements | $	$ 5.2
Minimum
Collaborations and Manufacturing Commitments
Manufacturing commitment | €		€ 16