Immunogen (IMGN) 10-Q 10 May 21 2021 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2021	Apr. 30, 2021
Document and Entity Information
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Mar. 31, 2021
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	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		199,942,152
Entity Central Index Key	0000855654
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Amendment Flag	false

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
ASSETS
Cash and cash equivalents	$ 283,120	$ 293,856
Accounts receivable	93	35
Unbilled receivable	5,405	11
Non-cash royalty receivable	15,533	22,451
Prepaid and other current assets	15,421	7,901
Total current assets	319,572	324,254
Property and equipment, net of accumulated depreciation	5,417	5,760
Operating lease right-of-use assets	13,648	14,072
Other assets	7,325	10,986
Total assets	345,962	355,072
LIABILITIES AND SHAREHOLDERS? EQUITY
Accounts payable	12,916	9,538
Accrued compensation	2,882	4,620
Other accrued liabilities	32,820	29,320
Convertible 4.5% senior notes, net of deferred financing costs of $4 and $7, respectively	2,096	2,093
Current portion of liability related to the sale of future royalties, net of deferred financing costs of $267 and $319, respectively	29,007	44,357
Current portion of operating lease liability	3,117	3,146
Current portion of deferred revenue	51,515	29,249
Total current liabilities	134,353	122,323
Deferred revenue, net of current portion	58,522	80,860
Operating lease liability, net of current portion	17,878	18,651
Liability related to the sale of future royalties, net of current portion and deferred financing costs of $525 and $584, respectively	38,609	41,082
Other long-term liabilities	2,480	2,586
Total liabilities	251,842	265,502
Commitments and contingencies (Note I)
Shareholders? deficit:
Preferred stock, $.01 par value; authorized 5,000 shares; no shares issued and outstanding as of each of March 31, 2021 and December 31, 2020
Common stock, $.01 par value; authorized 300,000 shares; issued and outstanding 199,941 and 194,998 shares as of March 31, 2021 and December 31, 2020, respectively	1,999	1,950
Additional paid-in capital	1,458,012	1,419,460
Accumulated deficit	(1,365,891)	(1,331,840)
Total shareholders equity	94,120	89,570
Total liabilities and shareholders? equity	$ 345,962	$ 355,072

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
CONSOLIDATED BALANCE SHEETS
Current portion of deferred financing costs	$ 4	$ 7
Interest rate (as a percent)	0.045%
Sale of future royalties, current portion and deferred financing costs	$ 267	319
Sale of future royalties, noncurrent portion and deferred financing costs	$ 525	$ 584
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	300,000,000
Common stock, issued shares	199,941,000	194,998,000
Common stock, outstanding shares	199,941,000	194,998,000

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenues:
Total revenues	$ 15,706	$ 13,287
Operating expenses:
Research and development	34,413	27,408
General and administrative	10,209	8,864
Restructuring charge		825
Total operating expenses	44,622	37,097
Loss from operations	(28,916)	(23,810)
Investment income, net	13	646
Non-cash interest expense on liability related to the sale of future royalties and convertible senior notes	(4,644)	(5,702)
Interest expense on convertible senior notes	(24)	(24)
Other expense, net	(480)	(198)
Net loss	$ (34,051)	$ (29,088)
Basic and diluted net loss per common share	$ (0.17)	$ (0.17)
Basic and diluted weighted average common shares outstanding	198,835	166,947
Total comprehensive loss	$ (34,051)	$ (29,088)
License and milestone fees
Revenues:
Total revenues	157	283
Non-cash royalty revenue related to the sale of future royalties
Revenues:
Total revenues	15,545	12,997
Research and development support
Revenues:
Total revenues	$ 4	$ 7

CONSOLIDATED STATEMENTS OF SHAR

CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY (DEFICIT) - USD ($) shares in Thousands, $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Total
Balance at Dec. 31, 2019	$ 1,501	$ 1,209,846	$ (1,287,468)	$ (76,121)
Balance (in shares) at Dec. 31, 2019	150,136
Increase (Decrease) in Shareholders' Equity (Deficit)
Net 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, net of issuance costs	$ 245	97,499		97,744
Issuance of common stock, net of issuance costs (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
Increase (Decrease) in Shareholders' Equity (Deficit)
Net 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
Adjustment of issuance costs		(1)		(1)
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
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' Equity (Deficit)
Net 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
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			31,388	31,388
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 2	676		678
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	205
Issuance of common stock, net of issuance costs	$ 200	96,328		96,528
Issuance of common stock, net of issuance costs (in shares)	19,972
Restricted stock units vested	$ 2	(2)
Restricted stock units vested (in shares)	236
Stock option and restricted stock compensation expense		3,718		3,718
Directors' deferred share unit compensation		149		149
Balance at Dec. 31, 2020	$ 1,950	1,419,460	(1,331,840)	$ 89,570
Balance (in shares) at Dec. 31, 2020	194,998			194,998
Increase (Decrease) in Shareholders' Equity (Deficit)
Net loss			(34,051)	$ (34,051)
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan	$ 4	1,282		1,286
Issuance of common stock pursuant to the exercise of stock options and employee stock purchase plan (in shares)	397
Issuance of common stock, net of issuance costs	$ 45	33,447		33,492
Issuance of common stock, net of issuance costs (in shares)	4,544
Restricted stock units vested (in shares)	2
Stock option and restricted stock compensation expense		3,674		3,674
Directors' deferred share unit compensation		149		149
Balance at Mar. 31, 2021	$ 1,999	$ 1,458,012	$ (1,365,891)	$ 94,120
Balance (in shares) at Mar. 31, 2021	199,941			199,941

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities:
Net loss	$ (34,051)	$ (29,088)
Adjustments to reconcile net loss to net cash used for operating activities:
Non-cash royalty revenue related to sale of future royalties	(15,545)	(12,997)
Non-cash interest expense on liability related to sale of future royalties and convertible senior notes	4,644	5,702
Depreciation and amortization	551	529
Gain on sale/disposal of fixed assets and impairment charges		(709)
Stock and deferred share unit compensation	3,823	3,122
Change in operating assets and liabilities:
Accounts receivable	(58)	7,446
Unbilled receivable	(5,394)	(752)
Contract asset		2,641
Prepaid and other current assets	(7,520)	(2,228)
Operating lease right-of-use assets	424	353
Other assets	3,661	(3,047)
Accounts payable	3,802	(649)
Accrued compensation	(1,571)	(3,267)
Other accrued liabilities	3,487	5,253
Deferred revenue	(72)	78
Operating lease liability	(802)	(702)
Net cash used for operating activities	(44,621)	(28,315)
Cash flows from investing activities:
Purchases of property and equipment	(893)	(21)
Proceeds from sale of equipment		1,426
Net cash (used for) provided by investing activities	(893)	1,405
Cash flows from financing activities:
Proceeds from issuance of common stock under stock plans	1,286	240
Proceeds from common stock issuance, net of $70 and $229 of transaction costs, respectively	33,492	97,744
Net cash provided by financing activities	34,778	97,984
Net change in cash and cash equivalents	(10,736)	71,074
Cash and cash equivalents, beginning of period	293,856	176,225
Cash and cash equivalents, end of period	$ 283,120	$ 247,299

CONSOLIDATED STATEMENTS OF CA_2

CONSOLIDATED STATEMENTS OF CASH FLOWS (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
CONSOLIDATED STATEMENTS OF CASH FLOWS
Transaction costs	$ 70	$ 229

Nature of Business and Plan of

Nature of Business and Plan of Operations	3 Months Ended
	Mar. 31, 2021
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 (ADCs). The Company has generally incurred operating losses and negative cash flows from operations since inception, incurred a net loss of $34.1 million during the three months ended March 31, 2021, and has an accumulated deficit of approximately $1.4 billion as of March 31, 2021. 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 to continue to incur operating expenses related to research and development and potential commercialization of its portfolio over the next several years. As of March 31, 2021, the Company had $283.1 million of cash and cash equivalents on hand. The Company anticipates that its current capital resources will enable it to meet its operational expenses and capital expenditures for more than twelve months after the date these financial statements were issued. The Company may raise additional funds through equity, debt, or other financings, or generate revenues from collaborators through a combination of upfront license payments, milestone payments, royalty payments, and research funding. There can be no assurance that the Company will be able to obtain additional equity, debt, or other financing or generate revenues from collaborators on terms acceptable to the Company or at all. The failure of the Company to obtain sufficient funds on acceptable terms when needed could have a material adverse effect on the Company’s business, results of operations, and financial condition and require the Company to defer or limit some or all of its research, development, and/or clinical projects. 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.

Basis of Presentation and Signi

Basis of Presentation and Significant Accounting Policies	3 Months Ended
	Mar. 31, 2021
Basis of Presentation and Significant Accounting Policies
Basis of Presentation and Significant Accounting Policies	B. Basis of Presentation and 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, 2020 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, 2020 filed with the SEC on March 1, 2021. Significant Accounting Policies The significant accounting policies used in preparation of these condensed consolidated financial statements for the three months ended March 31, 2021 are consistent with those discussed in Note B. to the consolidated financial statements included in the Company’s 2020 Annual Report on Form 10-K, except as described under Recently Adopted Accounting Pronouncements ​ Subsequent Events The Company has evaluated all events or transactions that occurred after March 31, 2021, up through the date the Company issued these financial statements. The Company did not have any material recognized or unrecognized subsequent events during this period. Revenue Recognition Transaction Price Allocated to Future Performance Obligations Deferred revenue under ASC 606, Revenue from Contracts with Customers 13 61 Contract Balances from Contracts with Customers The following tables present changes in the Company’s contract assets and contract liabilities during the three months ended March 31, 2021 and 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Three months ended March 31, 2021 ​ December 31, 2020 Additions ​ Deductions ​ Impact of Netting ​ March 31, 2021 Contract asset ​ $ — ​ $ — ​ $ — ​ $ — ​ $ — Contract liabilities (deferred revenue) ​ $ 110,109 ​ $ — ​ $ (72) ​ $ — ​ $ 110,037 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Three months ended March 31, 2020 ​ December 31, 2019 ​ Additions ​ Deductions ​ Impact of Netting ​ March 31, 2020 Contract asset ​ $ 3,631 ​ $ — ​ $ (3,000) ​ $ 359 ​ $ 990 Contract liabilities (deferred revenue) ​ $ 127,432 ​ $ — ​ $ (283) ​ $ 361 ​ $ 127,510 ​ 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 ​ ​ ​ March 31, ​ ​ ​ 2021 ​ 2020 ​ ​ Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 72 ​ $ 283 ​ ​ ​ During the three months ended March 31, 2021, the Company recorded $0.1 million of amortization of deferred revenue related to numerous collaborators’ rights to technological improvements. During the three months ended March 31, 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 $0.1 million of amortization related to numerous collaborators’ rights to technological improvements. Additionally, a contract asset of $2.7 million, net of a $0.3 million related contract liability, was recorded for a probable milestone in 2019 pursuant to a license agreement with CytomX, which was subsequently achieved and paid during the three months ended March 31, 2020. The timing of revenue recognition, billings, and cash collections results in billed receivables, unbilled 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 (under the caption deferred revenue). 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 March 31, 2021 and December 31, 2020. 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 March 31, 2021 and December 31, 2020, the Company held $283.1 million and $293.9 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 $0.7 million of accrued capital expenditures as of December 31, 2020, which were subsequently paid during the three months ended March 31, 2021. The Company had no accrued capital expenditures as of March 31, 2021. Fair Value of Financial Instruments Fair value is defined under ASC 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 March 31, 2021, 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 March 31, 2021 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at March 31, 2021 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 ​ $ 253,092 ​ $ 253,092 ​ $ — ​ $ — As of December 31, 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 December 31, 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 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 ​ $ 194,525 ​ $ 194,525 $ — $ — ​ 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 gross carrying amount and estimated fair value of the convertible 4.5% senior notes was $2.1 million and $5.4 million, respectively, as of March 31, 2021 compared to $2.1 million and $4.3 million, respectively, as of December 31, 2020. 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 March 31, 2021 and December 31, 2020 uses Level 3 inputs. 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 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 ​ ​ March 31, ​ 2021 2020 Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock/units at end of period ​ 21,320 ​ 19,021 Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock ​ 3,553 ​ 1,428 Shares issuable upon conversion of convertible notes at end of period ​ 501 ​ 501 Common stock equivalents under if-converted method for convertible notes ​ 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 March 31, 2021, 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 (the 2018 Plan), the Employee Stock Purchase Plan (the ESPP), and the ImmunoGen Inducement Equity Incentive Plan (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-based plans, including each of 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 by the Board of Directors in December 2019, and pursuant to subsequent amendments, provides for the issuance of non-qualified option grants for up to 3,500,000 shares of the Company’s common stock as of March 31, 2021. 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 under each of these plans. The stock-based awards are accounted for under ASC 718, Compensation—Stock Compensation ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, ​ 2021 ​ 2020 Dividend ​ None ​ None Volatility ​ 85.4% ​ 84.20% Risk-free interest rate ​ 0.62% ​ 1.45% Expected life (years) ​ 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 March 31, 2021 and 2020 were $5.47 and $3.23 per share, respectively. A summary of option activity under the Company’s equity plans for the three months ended March 31, 2021 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ Number ​ Average ​ ​ ​ of Stock ​ Exercise ​ ​ ​ Options ​ Price ​ Outstanding at December 31, 2020 ​ 18,398 ​ $ 6.10 ​ Granted ​ 3,659 ​ ​ 7.70 ​ Exercised ​ (397) ​ ​ 3.24 ​ Forfeited/Canceled ​ (399) ​ ​ 7.29 ​ Outstanding at March 31, 2021 ​ 21,261 ​ ​ 6.41 ​ ​ 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 March 31, 2021, 128,700 of these options were still outstanding. In 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 is $9.4 million. A summary of restricted stock and restricted stock unit activity under the Company’s equity plans for the three months ended March 31, 2021 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ Weighted- ​ ​ Restricted ​ Average Grant ​ ​ Stock Shares ​ Date Fair Value Unvested at December 31, 2020 ​ 61 ​ $ 2.47 Vested (2) ​ ​ 2.53 Unvested at March 31, 2021 ​ 59 ​ $ 2.47 ​ 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 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 is $0.1 million. During the three months ended March 31, 2021, holders of options issued under the Company’s equity plans exercised their rights to acquire an aggregate of approximately 397,000 shares of common stock at prices ranging from $1.84 to $6.34 per share. The total proceeds to the Company from these option exercises were $1.3 million. In June 2018, the Company's Board of Directors, with shareholder approval, adopted the ESPP. Following the automatic share increase on January 1, 2021, under the ESPP’s “evergreen” provision, an aggregate of 2,000,000 shares of common stock have been reserved for issuance under the ESPP. No shares were issued to participating employees during the three months ended March 31, 2021 or 2020. 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 Company recognizes share-based compensation expense equal to the fair value of the ESPP awards on a straight-line basis over the offering period. Stock compensation expense related to stock options and restricted stock awards granted under the stock plans and the ESPP was $3.7 million and $3.1 million during the three months ended March 31, 2021 and 2020, respectively. As of March 31, 2021, the estimated fair value of unvested employee awards, exclusive of performance awards, was $33.9 million. The weighted-average remaining vesting period for these awards is approximately 3.3 years. Segment Information During all periods presented, the Company continued to operate in one reportable business segment under the management approach of ASC 280, Segment Reporting During the three months ended March 31, 2021 and 2020, 99% and 98%, respectively, of revenues were from Roche, consisting primarily of non-cash royalty revenue. There were no other customers of the Company that generated significant revenues in the three months ended March 31, 2021 and 2020. Recently Adopted Accounting Pronouncements In December 2019, the Financial Accounting Standards Board issued Accounting Standards Update (ASU) 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes 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.

Agreements

Agreements	3 Months Ended
	Mar. 31, 2021
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., Japan, the European Union, and numerous other countries. The Company receives royalty reports and royalty payments related to sales of Kadcyla from Roche one For additional information related to this agreement, as well as the Company’s other significant collaborative agreements, please read Note C, “Agreements - Significant Collaborative Agreements,” to the audited financial statements included within the Company’s Annual Report on Form 10-K for the year ended December 31, 2020 filed with the SEC on March 1, 2021. ​

Convertible 4.5% Senior Notes

Convertible 4.5% Senior Notes	3 Months Ended
	Mar. 31, 2021
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 March 31, 2021. 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 $24,000 of interest expense in each of the three months ended March 31, 2021 and 2020. 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 is currently 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. The Company analyzed the terms of the convertible notes and determined that under current accounting guidance the notes would be entirely accounted for as debt and none of the terms of the notes require separate accounting.

Liability Related to Sale of Fu

Liability Related to Sale of Future Royalties	3 Months Ended
	Mar. 31, 2021
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 milestone. 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 the 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 broker fees). Simultaneously, OMERS purchased IRH’s right to the royalties the Company previously sold to IRH 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 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 three months ended March 31, 2021, no revenue related to the residual rights 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 three-month period ended March 31, 2021 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ ​ March 31, 2021 ​ Liability related to sale of future royalties, net — beginning balance ​ $ 85,439 ​ Proceeds from sale of future royalties, net ​ — ​ Kadcyla royalty payments received and paid ​ (22,463) ​ Non-cash interest expense recognized ​ ​ 4,640 ​ Liability related to sale of future royalties, net — ending balance ​ $ 67,616 ​ ​ As royalties are remitted to 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 agreement. The sum of these amounts less the $200 million proceeds the Company received from IRH 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 21.0% as of March 31, 2021. 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	3 Months Ended
	Mar. 31, 2021
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 as part of 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 policy 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 of Directors. Pursuant to the Compensation Policy for Non-Employee Directors, as amended, non-employee directors also receive stock option awards upon initial election to the Board of Directors and annually thereafter. 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 months ended March 31, 2021 and 2020 is included in the amounts discussed in the “Stock-Based Compensation” section of Note B above.

Restructuring Charge

Restructuring Charge	3 Months Ended
	Mar. 31, 2021
Restructuring Charge
Restructuring Charge	G. Restructuring Charge 2019 Corporate Restructuring In June 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. As a result of the workforce reduction, during the Compensation-Nonretirement Postemployment Benefits, A summary of activity against the corporate restructuring charge related to the employee terminations in 2021 is as follows: ​ ​ ​ ​ ​ ​ Employee ​ ​ Termination ​ Benefits Costs Balance at December 31, 2020 ​ $ 784 Payments during the period ​ ​ (128) Balance at March 31, 2021 ​ $ 656 ​ 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.

Leases

Leases	3 Months Ended
	Mar. 31, 2021
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 2020, the Company executed four subleases for approximately 65,000 square feet of this space 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. 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 was $1.0 million in each of the three month periods ended March 31, 2021 and 2020, and is included in operating expenses in the consolidated statement of operations. Cash paid against operating lease liabilities was $1.4 million in each of the three month periods ended March 31, 2021 and 2020. As of March 31, 2021, the Company’s ROU asset and lease liability for operating leases totaled $13.6 million and $21.0 million, respectively, and the weighted average remaining term of the operating leases is five years. ​ The maturities of operating lease liabilities discussed above are as follows (in thousands): ​ ​ ​ ​ 2021 (nine months remaining) $ 3,945 2022 ​ 5,389 2023 ​ 5,510 2024 ​ 5,470 2025 ​ 5,490 Thereafter ​ 1,376 Total lease payments ​ ​ 27,180 Less imputed interest ​ ​ (6,185) Total lease liabilities ​ $ 20,995 ​ 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 $3.1 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 three months ended March 31, 2021 and 2020, the Company recorded $1.2 million and $0.3 million of sublease income, respectively, 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 $15.3 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	3 Months Ended
	Mar. 31, 2021
Commitments and Contingencies
Commitments and Contingencies	I. Commitments and Contingencies Manufacturing Commitments As of March 31, 2021, 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 $6.4 million, which will be paid in 2021. Additionally, pursuant to commercial agreements for future production of antibody, our noncancelable commitments total approximately $30.0 million at March 31, 2021.

Basis of Presentation and Sig_2

Basis of Presentation and Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2021
Basis of Presentation and 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, 2020 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, 2020 filed with the SEC on March 1, 2021. Significant Accounting Policies The significant accounting policies used in preparation of these condensed consolidated financial statements for the three months ended March 31, 2021 are consistent with those discussed in Note B. to the consolidated financial statements included in the Company’s 2020 Annual Report on Form 10-K, except as described under Recently Adopted Accounting Pronouncements
Subsequent Events	Subsequent Events The Company has evaluated all events or transactions that occurred after March 31, 2021, up through the date the Company issued these financial statements. The Company did not have any material recognized or unrecognized subsequent events during this period.
Revenue Recognition	Revenue Recognition Transaction Price Allocated to Future Performance Obligations Deferred revenue under ASC 606, Revenue from Contracts with Customers 13 61 Contract Balances from Contracts with Customers The following tables present changes in the Company’s contract assets and contract liabilities during the three months ended March 31, 2021 and 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Three months ended March 31, 2021 ​ December 31, 2020 Additions ​ Deductions ​ Impact of Netting ​ March 31, 2021 Contract asset ​ $ — ​ $ — ​ $ — ​ $ — ​ $ — Contract liabilities (deferred revenue) ​ $ 110,109 ​ $ — ​ $ (72) ​ $ — ​ $ 110,037 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Three months ended March 31, 2020 ​ December 31, 2019 ​ Additions ​ Deductions ​ Impact of Netting ​ March 31, 2020 Contract asset ​ $ 3,631 ​ $ — ​ $ (3,000) ​ $ 359 ​ $ 990 Contract liabilities (deferred revenue) ​ $ 127,432 ​ $ — ​ $ (283) ​ $ 361 ​ $ 127,510 ​ 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 ​ ​ ​ March 31, ​ ​ ​ 2021 ​ 2020 ​ ​ Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 72 ​ $ 283 ​ ​ ​ During the three months ended March 31, 2021, the Company recorded $0.1 million of amortization of deferred revenue related to numerous collaborators’ rights to technological improvements. During the three months ended March 31, 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 $0.1 million of amortization related to numerous collaborators’ rights to technological improvements. Additionally, a contract asset of $2.7 million, net of a $0.3 million related contract liability, was recorded for a probable milestone in 2019 pursuant to a license agreement with CytomX, which was subsequently achieved and paid during the three months ended March 31, 2020. The timing of revenue recognition, billings, and cash collections results in billed receivables, unbilled 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 (under the caption deferred revenue). 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 March 31, 2021 and December 31, 2020. 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 March 31, 2021 and December 31, 2020, the Company held $283.1 million and $293.9 million, respectively, in cash and money market funds, which were classified as cash and cash equivalents.
Non-cash Investing	Non-cash Investing and Financing Activities The Company had $0.7 million of accrued capital expenditures as of December 31, 2020, which were subsequently paid during the three months ended March 31, 2021. The Company had no accrued capital expenditures as of March 31, 2021.
Fair Value of Financial Instruments	Fair Value of Financial Instruments Fair value is defined under ASC 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 March 31, 2021, 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 March 31, 2021 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at March 31, 2021 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 ​ $ 253,092 ​ $ 253,092 ​ $ — ​ $ — As of December 31, 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 December 31, 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 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 ​ $ 194,525 ​ $ 194,525 $ — $ — ​ 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 gross carrying amount and estimated fair value of the convertible 4.5% senior notes was $2.1 million and $5.4 million, respectively, as of March 31, 2021 compared to $2.1 million and $4.3 million, respectively, as of December 31, 2020. 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 March 31, 2021 and December 31, 2020 uses Level 3 inputs.
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 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 ​ ​ March 31, ​ 2021 2020 Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock/units at end of period ​ 21,320 ​ 19,021 Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock ​ 3,553 ​ 1,428 Shares issuable upon conversion of convertible notes at end of period ​ 501 ​ 501 Common stock equivalents under if-converted method for convertible notes ​ 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 March 31, 2021, 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 (the 2018 Plan), the Employee Stock Purchase Plan (the ESPP), and the ImmunoGen Inducement Equity Incentive Plan (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-based plans, including each of 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 by the Board of Directors in December 2019, and pursuant to subsequent amendments, provides for the issuance of non-qualified option grants for up to 3,500,000 shares of the Company’s common stock as of March 31, 2021. 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 under each of these plans. The stock-based awards are accounted for under ASC 718, Compensation—Stock Compensation ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, ​ 2021 ​ 2020 Dividend ​ None ​ None Volatility ​ 85.4% ​ 84.20% Risk-free interest rate ​ 0.62% ​ 1.45% Expected life (years) ​ 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 March 31, 2021 and 2020 were $5.47 and $3.23 per share, respectively. A summary of option activity under the Company’s equity plans for the three months ended March 31, 2021 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ Number ​ Average ​ ​ ​ of Stock ​ Exercise ​ ​ ​ Options ​ Price ​ Outstanding at December 31, 2020 ​ 18,398 ​ $ 6.10 ​ Granted ​ 3,659 ​ ​ 7.70 ​ Exercised ​ (397) ​ ​ 3.24 ​ Forfeited/Canceled ​ (399) ​ ​ 7.29 ​ Outstanding at March 31, 2021 ​ 21,261 ​ ​ 6.41 ​ ​ 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 March 31, 2021, 128,700 of these options were still outstanding. In 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 is $9.4 million. A summary of restricted stock and restricted stock unit activity under the Company’s equity plans for the three months ended March 31, 2021 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ Weighted- ​ ​ Restricted ​ Average Grant ​ ​ Stock Shares ​ Date Fair Value Unvested at December 31, 2020 ​ 61 ​ $ 2.47 Vested (2) ​ ​ 2.53 Unvested at March 31, 2021 ​ 59 ​ $ 2.47 ​ 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 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 is $0.1 million. During the three months ended March 31, 2021, holders of options issued under the Company’s equity plans exercised their rights to acquire an aggregate of approximately 397,000 shares of common stock at prices ranging from $1.84 to $6.34 per share. The total proceeds to the Company from these option exercises were $1.3 million. In June 2018, the Company's Board of Directors, with shareholder approval, adopted the ESPP. Following the automatic share increase on January 1, 2021, under the ESPP’s “evergreen” provision, an aggregate of 2,000,000 shares of common stock have been reserved for issuance under the ESPP. No shares were issued to participating employees during the three months ended March 31, 2021 or 2020. 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 Company recognizes share-based compensation expense equal to the fair value of the ESPP awards on a straight-line basis over the offering period. Stock compensation expense related to stock options and restricted stock awards granted under the stock plans and the ESPP was $3.7 million and $3.1 million during the three months ended March 31, 2021 and 2020, respectively. As of March 31, 2021, the estimated fair value of unvested employee awards, exclusive of performance awards, was $33.9 million. The weighted-average remaining vesting period for these awards is approximately 3.3 years.
Segment Information	Segment Information During all periods presented, the Company continued to operate in one reportable business segment under the management approach of ASC 280, Segment Reporting During the three months ended March 31, 2021 and 2020, 99% and 98%, respectively, of revenues were from Roche, consisting primarily of non-cash royalty revenue. There were no other customers of the Company that generated significant revenues in the three months ended March 31, 2021 and 2020.
Recently Adopted Accounting Pronouncements	Recently Adopted Accounting Pronouncements In December 2019, the Financial Accounting Standards Board issued Accounting Standards Update (ASU) 2019-12, Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes 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.

Basis of Presentation and Sig_3

Basis of Presentation and Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2021
Basis of Presentation and Significant Accounting Policies
Contract assets and contract liabilities	The following tables present changes in the Company’s contract assets and contract liabilities during the three months ended March 31, 2021 and 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Three months ended March 31, 2021 ​ December 31, 2020 Additions ​ Deductions ​ Impact of Netting ​ March 31, 2021 Contract asset ​ $ — ​ $ — ​ $ — ​ $ — ​ $ — Contract liabilities (deferred revenue) ​ $ 110,109 ​ $ — ​ $ (72) ​ $ — ​ $ 110,037 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Balance at Three months ended March 31, 2020 ​ December 31, 2019 ​ Additions ​ Deductions ​ Impact of Netting ​ March 31, 2020 Contract asset ​ $ 3,631 ​ $ — ​ $ (3,000) ​ $ 359 ​ $ 990 Contract liabilities (deferred revenue) ​ $ 127,432 ​ $ — ​ $ (283) ​ $ 361 ​ $ 127,510 ​ 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 ​ ​ ​ March 31, ​ ​ ​ 2021 ​ 2020 ​ ​ Revenue recognized in the period from: ​ ​ ​ ​ ​ ​ ​ ​ Amounts included in contract liabilities at the beginning of the period ​ $ 72 ​ $ 283 ​ ​
Schedule of assets that are required to be measured at fair value on a recurring basis	As of March 31, 2021, 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 March 31, 2021 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at March 31, 2021 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 ​ $ 253,092 ​ $ 253,092 ​ $ — ​ $ — As of December 31, 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 December 31, 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 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 ​ $ 194,525 ​ $ 194,525 $ — $ —
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 ​ ​ March 31, ​ 2021 2020 Options outstanding to purchase common stock, shares issuable under the employee stock purchase plan, and unvested restricted stock/units at end of period ​ 21,320 ​ 19,021 Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock ​ 3,553 ​ 1,428 Shares issuable upon conversion of convertible notes at end of period ​ 501 ​ 501 Common stock equivalents under if-converted method for convertible notes ​ 501 ​ 501
Schedule of assumptions	​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended March 31, ​ 2021 ​ 2020 Dividend ​ None ​ None Volatility ​ 85.4% ​ 84.20% Risk-free interest rate ​ 0.62% ​ 1.45% Expected life (years) ​ 6.0 ​ 6.0
Summary of stock option activity	A summary of option activity under the Company’s equity plans for the three months ended March 31, 2021 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ Number ​ Average ​ ​ ​ of Stock ​ Exercise ​ ​ ​ Options ​ Price ​ Outstanding at December 31, 2020 ​ 18,398 ​ $ 6.10 ​ Granted ​ 3,659 ​ ​ 7.70 ​ Exercised ​ (397) ​ ​ 3.24 ​ Forfeited/Canceled ​ (399) ​ ​ 7.29 ​ Outstanding at March 31, 2021 ​ 21,261 ​ ​ 6.41 ​
Summary of restricted stock activity	A summary of restricted stock and restricted stock unit activity under the Company’s equity plans for the three months ended March 31, 2021 is presented below (in thousands, except weighted-average data): ​ ​ ​ ​ ​ ​ ​ ​ Number of ​ Weighted- ​ ​ Restricted ​ Average Grant ​ ​ Stock Shares ​ Date Fair Value Unvested at December 31, 2020 ​ 61 ​ $ 2.47 Vested (2) ​ ​ 2.53 Unvested at March 31, 2021 ​ 59 ​ $ 2.47

Liability Related to Sale of _2

Liability Related to Sale of Future Royalties (Tables)	3 Months Ended
	Mar. 31, 2021
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 three-month period ended March 31, 2021 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ ​ March 31, 2021 ​ Liability related to sale of future royalties, net — beginning balance ​ $ 85,439 ​ Proceeds from sale of future royalties, net ​ — ​ Kadcyla royalty payments received and paid ​ (22,463) ​ Non-cash interest expense recognized ​ ​ 4,640 ​ Liability related to sale of future royalties, net — ending balance ​ $ 67,616 ​

Restructuring Charge (Tables)

Restructuring Charge (Tables)	3 Months Ended
	Mar. 31, 2021
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 2021 is as follows: ​ ​ ​ ​ ​ ​ Employee ​ ​ Termination ​ Benefits Costs Balance at December 31, 2020 ​ $ 784 Payments during the period ​ ​ (128) Balance at March 31, 2021 ​ $ 656

Leases (Tables)

Leases (Tables)	3 Months Ended
	Mar. 31, 2021
Leases
Schedule of maturities of operating lease liabilities	The maturities of operating lease liabilities discussed above are as follows (in thousands): ​ ​ ​ ​ 2021 (nine months remaining) $ 3,945 2022 ​ 5,389 2023 ​ 5,510 2024 ​ 5,470 2025 ​ 5,490 Thereafter ​ 1,376 Total lease payments ​ ​ 27,180 Less imputed interest ​ ​ (6,185) Total lease liabilities ​ $ 20,995

Nature of Business and Plan o_2

Nature of Business and Plan of Operations (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Net loss	$ (34,051)	$ (29,088)
Accumulated deficit	(1,365,891)		$ (1,331,840)
Total revenues	15,706	13,287
Cash and cash equivalents	$ 283,120	$ 247,299	$ 293,856	$ 176,225
Number of months Capital resources meets capital expenditures	12 months
Product
Total revenues	$ 0

Basis of Presentation and Sig_4

Basis of Presentation and Significant Accounting Policies - Performance Obligations (Details) $ in Millions	Mar. 31, 2021USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, Remaining Performance Obligation	$ 110
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, percent	0.47%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, percent	0.39%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2031-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligations, percent	0.14%
Minimum | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-04-01
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]: 2026-04-01
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]: 2031-04-01
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]: 2026-04-01
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]: 2031-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, expected timing of satisfaction	120 months

Basis of Presentation and Sig_5

Basis of Presentation and Significant Accounting Policies - Contract Balances (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Changes in the Company's contract assets and contract liabilities
Contract asset, Beginning balance		$ 3,631
Contract asset, Additions		0
Contract asset, Deductions		3,000
Contract Asset, Impact Of Netting.		359
Contract asset, Ending balance		990
Contract liabilities:
Contract liabilities (deferred revenue), Beginning balance	$ 110,109	127,432
Contract liabilities (deferred revenue), Deductions	(72)	(283)
Contract Liabilities (deferred revenue), Impact Of Netting		361
Contract liabilities (deferred revenue), Ending balance	$ 110,037	$ 127,510

Basis of Presentation and Sig_6

Basis of Presentation and Significant Accounting Policies - Revenues Recognized as a Result of Changes in Contract Asset and Liability Balances (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenue recognized in the period from:
Amounts included in contract liabilities at the beginning of the period	$ 72	$ 283

Basis of Presentation and Sig_7

Basis of Presentation and Significant Accounting Policies - Contract Balances from Contracts with Customers - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
Revenue recognized in the period from:
Revenue from contract with customer	$ 15,706	$ 13,287
Deferred revenue	110,037	127,510	$ 110,109	$ 127,432
Contract assets		990		$ 3,631
Contract liability	51,515		$ 29,249
License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer	157	283
CytomX and Novartis | License and milestone fees
Revenue recognized in the period from:
Revenue from contract with customer		200
CytomX and Novartis | Technological Improvements
Revenue recognized in the period from:
Deferred revenue		100
CytomX and Novartis | Probable Milestone
Revenue recognized in the period from:
Contract assets		2,700
Contract liability		$ 300
Other Collaborators | Technological Improvements
Revenue recognized in the period from:
Amortization of deferred revenue	$ 100

Basis of Presentation and Sig_8

Basis of Presentation and Significant Accounting Policies - Financial Instruments and Concentration of Credit Risk (Details)	3 Months Ended
	Mar. 31, 2021USD ($)item	Dec. 31, 2020USD ($)
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

Basis of Presentation and Sig_9

Basis of Presentation and Significant Accounting Policies - Cash and Cash Equivalents (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Basis of Presentation and Significant Accounting Policies
Cash and cash equivalents	$ 283,120	$ 293,856	$ 247,299	$ 176,225

Basis of Presentation and Si_10

Basis of Presentation and Significant Accounting Policies - Non-cash Investing (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Basis of Presentation and Significant Accounting Policies
Accrued capital expenditures	$ 0	$ 700,000

Basis of Presentation and Si_11

Basis of Presentation and Significant Accounting Policies - Fair Value of Financial Instruments (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021USD ($)item	Dec. 31, 2020USD ($)item	Jan. 01, 2017
Fair value hierarchy for the Company's financial assets measured at fair value
Interest rate (as a percent)	0.045%
Number of trades | item	0	0
Convertible Notes
Fair value hierarchy for the Company's financial assets measured at fair value
Interest rate (as a percent)	0.045%		4.50%
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	$ 2,100
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 debt fair value	5,400	4,300
Recurring basis
Fair value hierarchy for the Company's financial assets measured at fair value
Cash equivalents	253,092	194,525
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	253,092	194,525
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

Basis of Presentation and Si_12

Basis of Presentation and Significant Accounting Policies - Computation of Net Loss per Common Share (Details) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)shares	Mar. 31, 2020USD ($)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/units at end of period	21,320	19,021
Common stock equivalents under treasury stock method for options, shares issuable under the employee stock purchase plan, and unvested restricted stock	3,553	1,428
Shares issuable upon conversion of convertible notes at end of period (in shares) | $	501	501
Common stock equivalents under if-converted method for convertible notes (in shares)	501	501

Basis of Presentation and Si_13

Basis of Presentation and Significant Accounting Policiess - Stock-Based Compensation (Details)	1 Months Ended	3 Months Ended					12 Months Ended
	Sep. 30, 2018installmentshares	Mar. 31, 2021USD ($)plan$ / sharesshares	Dec. 31, 2020USD ($)$ / sharesshares	Sep. 30, 2020USD ($)	Jun. 30, 2020USD ($)	Mar. 31, 2020USD ($)$ / sharesshares	Dec. 31, 2020installment$ / sharesshares	Dec. 31, 2019itemshares	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)		(397,000)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Proceeds from Stock Options Exercised | $		$ 1,286,000				$ 240,000
Directors' deferred share unit compensation | $		$ 149,000	$ 149,000	$ 149,000	$ 45,000
ESPP
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Aggregate number of common shares reserved for future issuance											2,000,000
Shares issued to participating employees		0				0
Stock options and restricted stock awards
Stock-Based Compensation
Vesting period		3 years 3 months 18 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Allocated Share-based Compensation Expense | $		$ 3,700,000				$ 3,100,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized | $		$ 33,900,000
Stock options
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)		85.40%				84.20%
Risk-free interest rate (as a percent)		0.62%				1.45%
Expected life		6 years				6 years
Weighted-average grant date fair value (in dollars per share) | $ / shares		$ 5.47				$ 3.23
Number of Stock Options
Outstanding at the beginning of the period (in shares)		18,398,000
Granted (in shares)		3,659,000
Exercised (in shares)		(397,000)
Forfeited/Canceled (in shares)		(399,000)
Outstanding at the end of the period (in shares)		21,261,000	18,398,000				18,398,000
Weighted-Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares		$ 6.10
Granted (in dollars per share) | $ / shares		7.70
Exercised (in dollars per share) | $ / shares		3.24
Forfeited/Canceled (in dollars per share) | $ / shares		7.29
Outstanding at the end of the period (in dollars per share) | $ / shares		$ 6.41	$ 6.10				$ 6.10
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Proceeds from Stock Options Exercised | $		$ 1,300,000
Stock options | Minimum
Weighted-Average Exercise Price
Exercised (in dollars per share) | $ / shares		$ 1.84
Stock options | Maximum
Weighted-Average Exercise Price
Exercised (in dollars per share) | $ / shares		$ 6.34
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
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
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)		61,000
Vested (in shares)		(2,000)
Unvested at the end of the period (in shares)		59,000	61,000				61,000
Weighted-Average Grant Date Fair Value
Unvested at the beginning of the period (in dollars per share) | $ / shares		$ 2.47
Vested (in dollars per share) | $ / shares		2.53
Unvested at the end of the period (in dollars per share) | $ / shares		$ 2.47	$ 2.47				$ 2.47
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 | $		$ 100,000
Previous 2016 and 2006 stock option plans
Stock-Based Compensation
Common stock authorized for issuance (in shares)		19,500,000
2018 Plan
Stock-Based Compensation
Common stock authorized for issuance (in shares)		7,500,000
Inducement Plan
Stock-Based Compensation
Number of employee share-based compensation plans | plan		2
Common stock authorized for issuance (in shares)								3,500,000
Vesting period		4 years
Exercise period		10 years

Basis of Presentation and Si_14

Basis of Presentation and Significant Accounting Policies - Segment Information (Details) - segment	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Information
Number of operating segments	1
Other customers
Segment Information
Percentages of revenue recognized	0.00%	0.00%
Royalty revenue | Roche | Revenues percentage
Segment Information
Percentages of revenue recognized	99.00%	98.00%

Agreements - Roche (Details)

Agreements - Roche (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended
	Jan. 31, 2019	May 31, 2000	Mar. 31, 2021	Mar. 31, 2020
Collaborative Agreements disclosures
Non-cash royalty revenue related to sale of future royalties			$ 15,545	$ 12,997
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 | Kadcyla
Collaborative Agreements disclosures
Non-cash royalty revenue related to sale of future royalties			$ 15,500	$ 13,000
OMERS | Kadcyla
Collaborative Agreements disclosures
Non-cash royalty revenue related to sale of future royalties	$ 65,200
Percentage of royalty payments	100.00%

Convertible 4.5% Senior Notes (

Convertible 4.5% Senior Notes (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2021USD ($)	Mar. 31, 2020USD ($)	Dec. 31, 2016USD ($)$ / shares	Jan. 01, 2017
Convertible debt
Interest rate (as a percent)	0.045%
Interest expense	$ 24,000	$ 24,000
Convertible Notes
Convertible debt
Principal amount of debt			$ 100,000,000
Convertible 4.5% Senior Notes outstanding	$ 2,100,000
Interest rate (as a percent)	0.045%			4.50%
Interest expense	$ 24,000,000	$ 24,000,000
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	3 Months Ended		12 Months Ended
	Jan. 31, 2019	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2015
Liability Related to Sale of Future Royalties
Non-cash royalty revenue related to the sale of future royalties		$ 15,545	$ 12,997
Kadcyla
Liability Related to Sale of Future Royalties
Percentage of royalty payments if applicable threshold is met				85.00%
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
Liability related to sale of future royalties, net - beginning balance		85,439
Proceeds from sale of future royalties, net		200,000		200,000
Royalty payments received and paid		(22,463)
Non-cash interest expense recognized		4,640
Liability related to sale of future royalties, net - ending balance		$ 67,616
Effective annual interest rate		10.50%
Current effective interest rate		21
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	Mar. 31, 2021
Stock options
Stock-based compensation disclosure
Stock options granted to directors (in shares)				3,659,000
Compensation Policy for Non-Employee Directors | Stock options
Stock-based compensation disclosure
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

Restructuring Charge (Details)

Restructuring Charge (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended			6 Months Ended
	Jun. 30, 2020	Mar. 31, 2021	Mar. 31, 2020	Jun. 30, 2019	Dec. 31, 2019
Workforce reduction
Restructuring charge
Balance at beginning of the period		$ 784
Payments during the period		(128)
Balance at end of the period		$ 656
Workforce reduction | 2019 Corporate Restructuring
Restructuring
Charge for severance related to a pre-existing plan				$ 16,000
Severance costs, charges incurred					$ 15,300
Incremental retention benefits | 2019 Corporate Restructuring
Restructuring
Severance costs, charges incurred	$ 4,000		$ 800

Leases - Operating Leases (Deta

Leases - Operating Leases (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021USD ($)ft²leaseitem	Mar. 31, 2020USD ($)	Dec. 31, 2020USD ($)ft²leaseagreement
Lessee, Lease, Description [Line Items]
Number of real estate leases | lease	2
Right-of-use assets	$ 13,648		$ 14,072
Lease liabilities	$ 20,995
Weighted-average discount rate	11.00%
Lease expense for operating lease payments	$ 1,000
Cash paid against operating lease liabilities	$ 1,400	$ 1,400
Weighted average remaining term of the operating leases	5 years
Sublease income	$ 1,200	$ 300
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
Minimum rental payments over the remaining term of the sublease	$ 15,300

Leases - Maturities of Operatin

Leases - Maturities of Operating Lease Liabilities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021USD ($)
Leases
2021 (nine months remaining)	$ 3,945
2022	5,389
2023	5,510
2024	5,470
2025	5,490
Thereafter	1,376
Total lease payments	27,180
Less imputed interest	(6,185)
Total lease liabilities	20,995
Variable operating costs and real estate taxes	$ 3,100

Commitments and Contingencies (

Commitments and Contingencies (Details) $ in Millions	Mar. 31, 2021USD ($)
In-process and future manufacturing of antibody and cytotoxic agents
Collaborations and Manufacturing Commitments
Noncancelable obligations under several agreements	$ 6.4
Minimum
Collaborations and Manufacturing Commitments
Manufacturing commitment	$ 30