Ultragenyx Pharmaceutical (RARE) 10-Q 3 Nov 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Oct. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3
Trading Symbol	RARE
Entity Registrant Name	Ultragenyx Pharmaceutical Inc.
Entity Central Index Key	1,515,673
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		42,589,056

CONDENSED CONSOLIDATED BALANCE

CONDENSED CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 60,407	$ 161,120
Short-term investments	310,659	219,028
Restricted cash	461	1,411
Prepaid expenses and other current assets	19,376	20,136
Total current assets	390,903	401,695
Property and equipment, net	16,030	17,055
Restricted cash	1,805	2,076
Long-term investments	24,964	117,963
Other assets	1,345	1,837
Total assets	435,047	540,626
Current liabilities:
Accounts payable	9,288	5,364
Accrued liabilities	54,278	54,554
Deferred rent—current portion	695	341
Total current liabilities	64,261	60,259
Other liabilities	5,228	6,393
Total liabilities	69,489	66,652
Stockholders’ equity:
Preferred stock — 25,000,000 shares authorized; nil outstanding as of September 30, 2017 and December 31, 2016
Common stock — 250,000,000 shares authorized; 42,488,312 and 41,240,230 shares issued and outstanding as of September 30, 2017 and December 31, 2016, respectively	42	41
Additional paid-in capital	1,123,861	1,003,561
Accumulated other comprehensive income (loss)	(7,404)	905
Accumulated deficit	(750,941)	(530,533)
Total stockholders’ equity	365,558	473,974
Total liabilities and stockholders’ equity	$ 435,047	$ 540,626

CONDENSED CONSOLIDATED BALANCE3

CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - shares	Sep. 30, 2017	Dec. 31, 2016
Statement Of Financial Position [Abstract]
Preferred stock, shares authorized	25,000,000	25,000,000
Preferred stock, shares outstanding	0	0
Common stock, shares authorized	250,000,000	250,000,000
Common stock, shares issued	42,488,312	41,240,230
Common stock, shares outstanding	42,488,312	41,240,230

CONDENSED CONSOLIDATED STATEMEN

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Revenue	$ 198	$ 111	$ 198	$ 128
Operating expenses:
Research and development	60,412	48,711	170,117	132,458
General and administrative	23,499	17,183	62,189	45,128
Total operating expenses	83,911	65,894	232,306	177,586
Loss from operations	(83,713)	(65,783)	(232,108)	(177,458)
Other income (expense), net:
Interest income	1,117	922	3,350	2,877
Other income (expense), net	3,373	(46)	8,368	(6)
Total other income (expense), net	4,490	876	11,718	2,871
Loss before income taxes	(79,223)	(64,907)	(220,390)	(174,587)
Income tax provision	(4)		(18)
Net loss	$ (79,227)	$ (64,907)	$ (220,408)	$ (174,587)
Net loss per share, basic and diluted	$ (1.87)	$ (1.64)	$ (5.22)	$ (4.46)
Shares used in computing net loss per share, basic and diluted	42,471,606	39,551,923	42,222,413	39,184,994

CONDENSED CONSOLIDATED STATEME5

CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (79,227)	$ (64,907)	$ (220,408)	$ (174,587)
Other comprehensive income (loss):
Foreign currency translation adjustments	(3,326)	(28)	(8,377)	(17)
Unrealized gain (loss) on available-for-sale securities	145	(212)	68	897
Other comprehensive income (loss):	(3,181)	(240)	(8,309)	880
Total comprehensive loss	$ (82,408)	$ (65,147)	$ (228,717)	$ (173,707)

CONDENSED CONSOLIDATED STATEME6

CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating activities:
Net loss	$ (220,408)	$ (174,587)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	48,473	34,771
Amortization of premium on investment securities, net	1,451	4,230
Depreciation and amortization	3,311	2,267
Non-cash license fee from collaboration arrangement		700
Foreign currency remeasurement gain	(8,431)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	760	(5,833)
Other assets	492	(1,273)
Accounts payable	3,879	3,842
Accrued liabilities and other liabilities	(1,547)	22,588
Net cash used in operating activities	(172,020)	(113,295)
Investing activities:
Purchase of property and equipment	(1,894)	(9,522)
Purchase of investments	(230,490)	(311,523)
Proceeds from the sale of investments	27,642	94,289
Proceeds from maturities of investments	202,833	326,228
(Increase) decrease in restricted cash	1,221	(1,202)
Net cash provided by (used in) investing activities	(688)	98,270
Financing activities:
Proceeds from issuance of common stock in connection with at-the-market offering, net	67,616	33,700
Proceeds from issuance of common stock in connection with collaboration agreement, net		26,362
Proceeds from issuance of common stock from equity awards, net	4,212	3,674
Net cash provided by financing activities	71,828	63,736
Effect of exchange rate changes on cash	167
Net increase (decrease) in cash and cash equivalents	(100,713)	48,711
Cash and cash equivalents at beginning of period	161,120	93,569
Cash and cash equivalents at end of period	$ 60,407	$ 142,280

Organization

Organization	9 Months Ended
	Sep. 30, 2017
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization	1. Organization Ultragenyx Pharmaceutical Inc. (the Company) is a biopharmaceutical company and was incorporated in California on April 22, 2010. The Company subsequently reincorporated in the state of Delaware in June 2011. The Company is focused on the identification, acquisition, development, and commercialization of novel products for the treatment of rare and ultra-rare diseases, with a focus on serious, debilitating genetic diseases. The Company has completed a Phase 3 study of vestronidase alfa (recombinant human beta-glucuronidase or rhGUS) in patients with mucopolysaccharidosis 7 (MPS 7), a rare lysosomal storage disease, and is conducting Phase 2 and Phase 3 studies of burosumab (KRN23 or UX023), an antibody targeting fibroblast growth factor 23 (FGF23), in pediatric and adult patients with X-linked hypophosphatemia (XLH) and a Phase 2 study in tumor induced osteomalacia (TIO), both rare diseases that impair bone mineralization; a Phase 3 study for UX007 in patients with glucose transporter type-1 deficiency syndrome (Glut1 DS), a brain energy deficiency, who are experiencing movement disorders; and a Phase 2 clinical study of UX007 in patients severely affected by long-chain fatty acid oxidation disorders (LC-FAOD), a genetic disorder in which the body is unable to convert long chain fatty acids into energy. The Company operates as one reportable segment.

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying unaudited condensed consolidated financial statements include the amounts of the Company and our wholly-owned subsidiaries and have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information and in accordance with the instructions to Form 10-Q and Rule 10-01 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. The unaudited interim consolidated financial statements have been prepared on the same basis as the annual financial statements. In the opinion of management, the accompanying unaudited condensed consolidated financial statements reflect all adjustments (consisting only of normal recurring adjustments) considered necessary for a fair presentation. These financial statements should be read in conjunction with the audited financial statements and notes thereto for the preceding fiscal year contained in the Company’s Annual Report on Form 10-K filed on February 17, 2017 with the United States Securities and Exchange Commission (SEC). The results of operations for the three and nine months ended September 30, 2017 are not necessarily indicative of the results to be expected for the year ending December 31, 2017. The condensed consolidated balance sheet as of December 31, 2016 has been derived from audited financial statements at that date, but does not include all of the information required by GAAP for complete financial statements. Use of Estimates The accompanying consolidated financial statements have been prepared in accordance with GAAP. The preparation of the consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent liabilities and the reported amounts of expenses in the consolidated financial statements and the accompanying notes. On an ongoing basis, management evaluates its estimates, including those related to clinical trial accruals, fair value of assets and liabilities, income taxes, and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could differ from those estimates. Revenue Recognition In May 2014, the Financial Accounting Standards Board (FASB), issued Accounting Standards Update (ASU) 2014-09, Revenue from Contracts with Customers (ASC 606) Revenue from Contracts with Customers: Principal versus Agent Considerations Revenue from Contracts with Customers: Identifying Performance Obligations and Licensing Revenue from Contracts with Customers: Narrow-Scope Improvements and Practical Expedients to provide supplemental adoption guidance and clarification to ASU 2014-09 Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers Product sales revenue The Company recognizes revenue from sales of vestronidase alfa (UX003) on a “named patient” basis, which is allowed in certain countries prior to the commercial approval of the product in the territory. Under ASC 606, revenue from product sales is recognized at the point in time when the product is shipped to the customer. Prior to recognizing revenue, the Company makes estimates of the transaction price, including variable consideration that is subject to a constraint. Amounts of variable consideration are included in the transaction price to the extent that it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur when the uncertainty associated with the variable consideration is subsequently resolved. License and collaboration agreements The Company has license and collaboration agreements with Kyowa Hakko Kirin Co., Ltd. (KHK) and Takeda Pharmaceutical Company Limited (Takeda). These license and collaboration agreements are within the scope of ASC 808, Collaborative Agreements Stock-Based Compensation In March 2016, the FASB issued ASU 2016-09, Compensation — Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting , which simplifies several aspects of the Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases In October 2016, the FASB issued ASU 2016-16, Income Taxes - Intra-Entity Transfers of Assets Other Than Inventory

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements Financial assets and liabilities are recorded at fair value. The carrying amount of certain financial instruments, including cash and cash equivalents, accounts payable and accrued liabilities approximate fair value due to their relatively short maturities. Assets and liabilities recorded at fair value on a recurring basis in the balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows: Level 1 —Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date; Level 2 —Inputs are observable, unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities; and Level 3 —Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data. The following tables set forth the fair value of the Company’s financial assets remeasured on a recurring basis based on the three-tier fair value hierarchy (in thousands): September 30, 2017 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 42,229 $ — $ — $ 42,229 Corporate bonds — 127,486 — 127,486 Commercial paper — 17,306 — 17,306 U.S. Government Treasury and agency securities 4,999 185,831 — 190,830 Total financial assets $ 47,228 $ 330,623 $ — $ 377,851 December 31, 2016 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 123,536 $ — $ — $ 123,536 Corporate bonds — 207,726 — 207,726 Commercial paper — 11,970 — 11,970 Asset-backed securities — 27,713 — 27,713 U.S. Government Treasury and agency securities — 111,931 — 111,931 Total financial assets $ 123,536 $ 359,340 $ — $ 482,876

Balance Sheet Components

Balance Sheet Components	9 Months Ended
	Sep. 30, 2017
Investments Debt And Equity Securities [Abstract]
Balance Sheet Components	4. Balance Sheet Components Cash Equivalents and Investments The fair values of cash equivalents, short-term investments, and long-term investments classified as available-for-sale securities, consisted of the following (in thousands): September 30, 2017 Gross Unrealized Amortized Cost Gains Losses Estimated Fair Value Money market funds $ 42,229 $ — $ — $ 42,229 Corporate bonds 127,564 8 (86 ) 127,486 Commercial paper 17,306 — — 17,306 U.S. Government Treasury and agency securities 191,101 1 (272 ) 190,830 Total $ 378,200 $ 9 $ (358 ) $ 377,851 December 31, 2016 Gross Unrealized Amortized Cost Gains Losses Estimated Fair Value Money market funds $ 123,536 $ — $ — $ 123,536 Corporate bonds 207,909 14 (197 ) 207,726 Commercial paper 11,970 — — 11,970 Asset-backed securities 27,712 3 (2 ) 27,713 U.S. Government Treasury and agency securities 112,166 10 (245 ) 111,931 Total $ 483,293 $ 27 $ (444 ) $ 482,876 At September 30, 2017, the remaining contractual maturities of available-for-sale securities were less than two years. There have been no significant realized gains or losses on available-for-sale securities for the periods presented. All marketable securities with unrealized losses at , 2017 have been in a loss position for less than twelve months and the loss was considered to be temporary in nature. The Company does not intend to sell the investments that are in an unrealized loss position before recovery of their amortized cost basis. Accrued Liabilities Accrued liabilities consist of the following (in thousands): September 30, December 31, 2017 2016 Research and clinical study expenses $ 20,568 $ 18,593 Payroll and related expenses 19,102 17,226 Repayment and contract liability under collaboration agreement 7,962 13,650 Other 6,646 5,085 Total accrued liabilities $ 54,278 $ 54,554

License and Research Agreements

License and Research Agreements	9 Months Ended
	Sep. 30, 2017
Research Grant Agreement [Abstract]
License and Research Agreements	5. License and Research Agreements Kyowa Hakko Kirin Collaboration and License Agreement In August 2013, the Company entered into a collaboration and license agreement with Kyowa Hakko Kirin Co., Ltd. (KHK). Under the terms of this collaboration and license agreement, as amended, the Company and KHK will collaborate on the development and commercialization of certain products containing burosumab burosumab burosumab burosumab KHK will manufacture and supply burosumab burosumab In May 2017, Ultragenyx signed an agreement with a wholly-owned subsidiary of KHK pursuant to which Ultragenyx was granted the right to commercialize burosumab in Turkey. KHK’s subsidiary has the option to assume responsibility for commercialization efforts from Ultragenyx, after a certain minimum period. The Company’s expenses were reduced by $7.6 million and $6.8 million for the three months ended September 30, 2017 and 2016, and $22.9 million and $17.3 million for the nine months ended September 30, 2017 and 2016, respectively, for KHK’s share of the research and development expenses and were reduced by $1.2 million and $0.4 million for the three months ended September 30, 2017 and 2016, and $2.3 million and $0.9 million for the nine months ended September 30, 2017 and 2016, respectively, for KHK’s share of the general and administrative expenses. As of September 30, 2017 and December 31, 2016, the Company had receivables in the amount of $8.8 million and $8.6 million, respectively, for this collaboration arrangement. Takeda License and Collaboration and Purchase Agreements In June 2016, the Company executed a collaboration and license agreement with Takeda Pharmaceutical Company Limited (Takeda). Pursuant to the agreement, which became effective in July 2016, the Company obtained an exclusive license for a pre-clinical compound from Takeda in a pre-determined field of use, which includes an option to an additional field of use for this product with the terms to be negotiated, and an option to a specified product candidate (identified option product). The Company is responsible for the development costs for the pre-clinical compound and the identified option product pursuant to an initial development plan. Because the license to the pre-clinical compound has no alternative future use, the estimated fair value of $0.7 million was recorded as a research and development expense upon acquisition. Under the license for the pre-clinical compound, the Company may be required to make future milestone payments to Takeda of up to $7.5 million if certain development milestones are met, $75.0 million if certain regulatory milestones are met, and $150.0 million if certain commercial milestones are met, as well as royalties with respect to net sales in the high-single digits to low-teens. Any products resulting from the pre-clinical compound or the identified option product is referred to in this report as the “licensed product.” The Company discontinued the development efforts on the pre-clinical compound in the pre-determined field of use and the identified option product. As part of the agreement, the Company and Takeda established a five-year research collaboration whereby the parties may mutually agree to add additional option products candidates to the collaboration, in which case the Company will bear the cost of the development activities, with certain exceptions. In July 2016, the Company consummated a common stock purchase agreement, executed in conjunction with the collaboration and license agreement, whereby Takeda purchased 374,590 shares of the Company’s common stock for $40.0 million in cash. The fair market value of the common stock issued to Takeda was $27.3 million, based on the closing stock price of $72.95 on the date of issuance, resulting in a $12.7 million premium paid to the Company. The Company also received a put option to require Takeda to purchase an additional $25.0 million in shares of the Company’s common stock at the then-current 30-day volume-weighted average price (VWAP). The put option was exercised in October 2016, whereby Takeda purchased 352,530 shares of the Company’s common stock for $25.0 million in cash. Takeda is subject to a five-year standstill (subject to customary exceptions or release). The Company estimated the fair value of the put options to be $0.9 million and recorded the put options in additional paid-in capital. The Company also granted Takeda an exclusive option for Asian rights, for a limited period, to any licensed products and any additional products resulting from the collaboration, as well as an option to exclusively license one of the Company’s products for development and commercialization in Japan. If Takeda exercises any of its option rights to license a product pursuant to the agreement, Takeda will pay for the development costs within the licensed territory, will share in a portion of the global development costs, and will make a milestone payment upon regulatory approval. Takeda will also owe royalties on net sales in the licensed territory for any licensed product, depending on the development stage when the product is licensed as well as sales levels. The royalties related to the option to license the Company’s product, as well as the additional product are subject to future good faith negotiations at the time that the option is exercised. The research and license agreement and the stock purchase agreement are being accounted for as one arrangement because they were entered into at the same time with interrelated financial terms. The Company analogized to Topic 606 The Company is responsible for the costs under the initial development plan. A significant portion of this work is expected to be performed by Takeda which has an estimated cost of approximately $10.0 million to $12.3 million and is subject to changes as development activities are adjusted and cost estimates are refined. The Company concluded that the payment to Takeda is not in return for a distinct service that Takeda transfers to the Company, therefore, the payment made to Takeda is accounted for as a reduction in the transaction price. As of September 30, 2017, the Company concluded that $2.4 million of the estimated transaction price should not be constrained because it is Based on the high level of uncertainty in the amount of transaction price that is refundable to Takeda, the Company concluded that the remaining transaction price should continue to be constrained as of September 30, 2017. The Company will continue to re-evaluate the application of the constraint to the transaction price at each reporting period end date. Costs incurred by the Company associated with co-development activities performed under this collaboration are included in research and development expense in the accompanying consolidated statements of operations. As of September 30, 2017 and December 31, 2016, the Company had a repayment liability in the amount of $7.3 million and $14.3 million, respectively, and a contract liability in the amount of $0.7 million as of September 30, 2017 and none as of December 31, 2016.

Stock-Based Awards

Stock-Based Awards	9 Months Ended
	Sep. 30, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Awards	6 . Stock-Based Awards The 2014 Incentive Plan (the 2014 Plan) provides for automatic annual increases in shares available for grant, beginning on January 1, 2015 through January 1, 2024. As of September 30, 2017, there were 1,881,670 shares reserved under the 2014 Plan for the future issuance of equity awards and 1,816,052 shares reserved for the 2014 Employee Stock Purchase Plan. The table below sets forth the functional classification of stock-based compensation expense, net of estimated forfeitures, for the periods presented (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Research and development $ 9,951 $ 8,294 $ 28,528 $ 21,364 General and administrative 7,229 5,400 19,945 13,407 Total stock-based compensation $ 17,180 $ 13,694 $ 48,473 $ 34,771

Net Loss Per Share

Net Loss Per Share	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Net Loss Per Share	7 . Net Loss Per Share Basic net loss per share has been computed by dividing the net loss by the weighted-average number of shares of common stock outstanding during the period. Diluted net loss per share is calculated by dividing net loss by the weighted-average number of shares of common stock and potential dilutive securities outstanding during the period. The following weighted-average outstanding common stock equivalents were excluded from the computation of diluted net loss per share for the periods presented because including them would have been antidilutive: Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Options to purchase common stock and restricted stock units 5,835,271 5,196,197 5,736,646 4,575,124 Employee stock purchase plan 32,014 — 17,942 — Common stock warrants 149,700 149,700 149,700 149,700 6,016,985 5,345,897 5,904,288 4,724,824

Equity Transactions

Equity Transactions	9 Months Ended
	Sep. 30, 2017
Equity [Abstract]
Equity Transactions	8 . Equity Transactions In July 2016, the Company entered into an At-The-Market, or ATM, sales agreement, with Cowen and Company, LLC (Cowen), whereby the Company could sell up to $150.0 million in aggregate proceeds of common stock from time to time, through Cowen as its sales agent. During the three months ended March 31, 2017, the Company sold 912,351 shares of common stock, resulting in net proceeds of approximately $67.6 million, after commissions and other offering costs, which completed the sales of the total amount authorized in the agreement. In July 27, 2017, the Company entered into an additional ATM sales agreement with Cowen whereby the Company can sell up to $150.0 million in aggregate proceeds of common stock from time to time, through Cowen as its sales agent. No shares were sold during the three months ended September 30, 2017. During the period of October 1, 2017 through October 30, 2017, the Company sold an additional 64,468 shares of common stock, resulting in net proceeds of approximately $3.6 million, after commissions and other offering costs.

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Loss)	9 Months Ended
	Sep. 30, 2017
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease [Abstract]
Accumulated Other Comprehensive Income (Loss)	9. Accumulated Other Comprehensive Income (Loss) Total accumulated other comprehensive income (loss) consisted of the following (in thousands): September 30, December 31, 2017 2016 Unrealized loss on securities available-for-sale $ (349 ) $ (417 ) Foreign currency translation adjustments (7,055 ) 1,322 Total accumulated other comprehensive income (loss) $ (7,404 ) $ 905

Merger Agreement

Merger Agreement	9 Months Ended
	Sep. 30, 2017
Business Combinations [Abstract]
Merger Agreement	10 . Merger Agreement On October 2, 2017, the Company entered into an Agreement and Plan of Merger (the “Merger Agreement”) with Dimension Therapeutics, Inc. (“Dimension”) and Mystic River Merger Sub Inc., a wholly owned subsidiary of the Company (“Purchaser”), pursuant to which the Company will acquire Dimension. In connection with the Merger Agreement, the Company, on behalf of Dimension, paid a $2.9 million termination fee to REGENXBIO, as a result of a previously existing merger agreement between REGENXBIO Inc. and Dimension. Pursuant to the terms of the Merger Agreement, Purchaser commenced a cash tender offer (the “Offer”) for all outstanding shares of common stock, par value $0.0001 per share, of Dimension (the “Dimension Common Stock”) at a purchase price of $6.00 per share, net to each holder in cash, without interest, or approximately $151.0 million, subject to Dimension total common stock outstanding at close, plus acquisition costs. As a condition to consummating the Offer, Dimension stockholders must validly tender (and not withdraw) shares constituting at least a majority of the outstanding shares of Dimension Common Stock calculated on a fully diluted basis in accordance with the Merger Agreement, and the satisfaction of other customary closing conditions. The transaction is expected to close in the fourth quarter of 2017.

Summary of Significant Accoun17

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited condensed consolidated financial statements include the amounts of the Company and our wholly-owned subsidiaries and have been prepared in accordance with U.S. generally accepted accounting principles (GAAP) for interim financial information and in accordance with the instructions to Form 10-Q and Rule 10-01 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by GAAP for complete financial statements. The unaudited interim consolidated financial statements have been prepared on the same basis as the annual financial statements. In the opinion of management, the accompanying unaudited condensed consolidated financial statements reflect all adjustments (consisting only of normal recurring adjustments) considered necessary for a fair presentation. These financial statements should be read in conjunction with the audited financial statements and notes thereto for the preceding fiscal year contained in the Company’s Annual Report on Form 10-K filed on February 17, 2017 with the United States Securities and Exchange Commission (SEC). The results of operations for the three and nine months ended September 30, 2017 are not necessarily indicative of the results to be expected for the year ending December 31, 2017. The condensed consolidated balance sheet as of December 31, 2016 has been derived from audited financial statements at that date, but does not include all of the information required by GAAP for complete financial statements.
Use of Estimates	Use of Estimates The accompanying consolidated financial statements have been prepared in accordance with GAAP. The preparation of the consolidated financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, disclosure of contingent liabilities and the reported amounts of expenses in the consolidated financial statements and the accompanying notes. On an ongoing basis, management evaluates its estimates, including those related to clinical trial accruals, fair value of assets and liabilities, income taxes, and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could differ from those estimates.
Revenue Recognition	Revenue Recognition In May 2014, the Financial Accounting Standards Board (FASB), issued Accounting Standards Update (ASU) 2014-09, Revenue from Contracts with Customers (ASC 606) Revenue from Contracts with Customers: Principal versus Agent Considerations Revenue from Contracts with Customers: Identifying Performance Obligations and Licensing Revenue from Contracts with Customers: Narrow-Scope Improvements and Practical Expedients to provide supplemental adoption guidance and clarification to ASU 2014-09 Technical Corrections and Improvements to Topic 606, Revenue from Contracts with Customers Product sales revenue The Company recognizes revenue from sales of vestronidase alfa (UX003) on a “named patient” basis, which is allowed in certain countries prior to the commercial approval of the product in the territory. Under ASC 606, revenue from product sales is recognized at the point in time when the product is shipped to the customer. Prior to recognizing revenue, the Company makes estimates of the transaction price, including variable consideration that is subject to a constraint. Amounts of variable consideration are included in the transaction price to the extent that it is probable that a significant reversal in the amount of cumulative revenue recognized will not occur when the uncertainty associated with the variable consideration is subsequently resolved. License and collaboration agreements The Company has license and collaboration agreements with Kyowa Hakko Kirin Co., Ltd. (KHK) and Takeda Pharmaceutical Company Limited (Takeda). These license and collaboration agreements are within the scope of ASC 808, Collaborative Agreements
Stock-Based Compensation	Stock-Based Compensation In March 2016, the FASB issued ASU 2016-09, Compensation — Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting , which simplifies several aspects of the
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements In February 2016, the FASB issued ASU No. 2016-02, Leases In October 2016, the FASB issued ASU 2016-16, Income Taxes - Intra-Entity Transfers of Assets Other Than Inventory

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Summary of Financial Assets Measured on Recurring Basis	The following tables set forth the fair value of the Company’s financial assets remeasured on a recurring basis based on the three-tier fair value hierarchy (in thousands): September 30, 2017 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 42,229 $ — $ — $ 42,229 Corporate bonds — 127,486 — 127,486 Commercial paper — 17,306 — 17,306 U.S. Government Treasury and agency securities 4,999 185,831 — 190,830 Total financial assets $ 47,228 $ 330,623 $ — $ 377,851 December 31, 2016 Level 1 Level 2 Level 3 Total Financial Assets: Money market funds $ 123,536 $ — $ — $ 123,536 Corporate bonds — 207,726 — 207,726 Commercial paper — 11,970 — 11,970 Asset-backed securities — 27,713 — 27,713 U.S. Government Treasury and agency securities — 111,931 — 111,931 Total financial assets $ 123,536 $ 359,340 $ — $ 482,876

Balance Sheet Components (Table

Balance Sheet Components (Tables)	9 Months Ended
	Sep. 30, 2017
Investments Debt And Equity Securities [Abstract]
Summary of Fair Values of Cash Equivalents, Short-Term Investments and Long-Term Investments Classified as Available-for-Sale Securities	The fair values of cash equivalents, short-term investments, and long-term investments classified as available-for-sale securities, consisted of the following (in thousands): September 30, 2017 Gross Unrealized Amortized Cost Gains Losses Estimated Fair Value Money market funds $ 42,229 $ — $ — $ 42,229 Corporate bonds 127,564 8 (86 ) 127,486 Commercial paper 17,306 — — 17,306 U.S. Government Treasury and agency securities 191,101 1 (272 ) 190,830 Total $ 378,200 $ 9 $ (358 ) $ 377,851 December 31, 2016 Gross Unrealized Amortized Cost Gains Losses Estimated Fair Value Money market funds $ 123,536 $ — $ — $ 123,536 Corporate bonds 207,909 14 (197 ) 207,726 Commercial paper 11,970 — — 11,970 Asset-backed securities 27,712 3 (2 ) 27,713 U.S. Government Treasury and agency securities 112,166 10 (245 ) 111,931 Total $ 483,293 $ 27 $ (444 ) $ 482,876
Accrued Liabilities	Accrued liabilities consist of the following (in thousands): September 30, December 31, 2017 2016 Research and clinical study expenses $ 20,568 $ 18,593 Payroll and related expenses 19,102 17,226 Repayment and contract liability under collaboration agreement 7,962 13,650 Other 6,646 5,085 Total accrued liabilities $ 54,278 $ 54,554

Stock-Based Awards (Tables)

Stock-Based Awards (Tables)	9 Months Ended
	Sep. 30, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Functional Classification of Stock-Based Compensation Expense, Net of Estimated Forfeitures	The table below sets forth the functional classification of stock-based compensation expense, net of estimated forfeitures, for the periods presented (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Research and development $ 9,951 $ 8,294 $ 28,528 $ 21,364 General and administrative 7,229 5,400 19,945 13,407 Total stock-based compensation $ 17,180 $ 13,694 $ 48,473 $ 34,771

Net Loss Per Share (Tables)

Net Loss Per Share (Tables)	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Outstanding Shares of Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share	The following weighted-average outstanding common stock equivalents were excluded from the computation of diluted net loss per share for the periods presented because including them would have been antidilutive: Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Options to purchase common stock and restricted stock units 5,835,271 5,196,197 5,736,646 4,575,124 Employee stock purchase plan 32,014 — 17,942 — Common stock warrants 149,700 149,700 149,700 149,700 6,016,985 5,345,897 5,904,288 4,724,824

Accumulated Other Comprehensi22

Accumulated Other Comprehensive Income (Loss) (Tables)	9 Months Ended
	Sep. 30, 2017
Other Comprehensive Income Loss Net Of Tax Period Increase Decrease [Abstract]
Schedule of Total Accumulated Other Comprehensive Income (Loss)	Total accumulated other comprehensive income (loss) consisted of the following (in thousands): September 30, December 31, 2017 2016 Unrealized loss on securities available-for-sale $ (349 ) $ (417 ) Foreign currency translation adjustments (7,055 ) 1,322 Total accumulated other comprehensive income (loss) $ (7,404 ) $ 905

Organization - Additional Infor

Organization - Additional Information (Details)	9 Months Ended
	Sep. 30, 2017Segment
United States of America
Organization And Nature Of Business [Line Items]
Number of reportable segments	1

Summary of Significant Accoun24

Summary of Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jan. 01, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Revenue recognized	$ 198	$ 111	$ 198	$ 128
Accounting Standards Update 2016-09
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Unrecognized excess tax benefits which was fully offset by increase in valuation allowance					$ 19,700
New Accounting Pronouncement, Early Adoption, Effect | Accounting Standards Update 2014-09
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Revenue recognized			$ 100

Fair Value Measurements - Summa

Fair Value Measurements - Summary of Financial Assets Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financial Assets:
Fair value	$ 377,851	$ 482,876
Money Market Funds
Financial Assets:
Fair value	42,229	123,536
Asset-backed Securities
Financial Assets:
Fair value		27,713
Corporate Bonds
Financial Assets:
Fair value	127,486	207,726
Commercial Paper
Financial Assets:
Fair value	17,306	11,970
U.S. Government Treasury and Agency Securities
Financial Assets:
Fair value	190,830	111,931
Level 1
Financial Assets:
Fair value	47,228	123,536
Level 1 | Money Market Funds
Financial Assets:
Fair value	42,229	123,536
Level 1 | U.S. Government Treasury and Agency Securities
Financial Assets:
Fair value	4,999
Level 2
Financial Assets:
Fair value	330,623	359,340
Level 2 | Asset-backed Securities
Financial Assets:
Fair value		27,713
Level 2 | Corporate Bonds
Financial Assets:
Fair value	127,486	207,726
Level 2 | Commercial Paper
Financial Assets:
Fair value	17,306	11,970
Level 2 | U.S. Government Treasury and Agency Securities
Financial Assets:
Fair value	$ 185,831	$ 111,931

Balance Sheet Components - Summ

Balance Sheet Components - Summary of Fair Values of Cash Equivalents, Short-Term Investments and Long-Term Investments Classified as Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 378,200	$ 483,293
Gross Unrealized Gains	9	27
Gross Unrealized Losses	(358)	(444)
Estimated Fair Value	377,851	482,876
Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	42,229	123,536
Estimated Fair Value	42,229	123,536
Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	17,306	11,970
Estimated Fair Value	17,306	11,970
Corporate Bonds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	127,564	207,909
Gross Unrealized Gains	8	14
Gross Unrealized Losses	(86)	(197)
Estimated Fair Value	127,486	207,726
Asset-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost		27,712
Gross Unrealized Gains		3
Gross Unrealized Losses		(2)
Estimated Fair Value		27,713
U.S. Government Treasury and Agency Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	191,101	112,166
Gross Unrealized Gains	1	10
Gross Unrealized Losses	(272)	(245)
Estimated Fair Value	$ 190,830	$ 111,931

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details)	9 Months Ended
	Sep. 30, 2017USD ($)
Schedule Of Available For Sale Securities [Line Items]
Significant realized gains or losses on available-for-sale securities	$ 0
Maximum
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities remaining contractual maturities	2 years

Balance Sheet Components - Accr

Balance Sheet Components - Accrued Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Research and clinical study expenses	$ 20,568	$ 18,593
Payroll and related expenses	19,102	17,226
Repayment and contract liability under collaboration agreement	7,962	13,650
Other	6,646	5,085
Total accrued liabilities	$ 54,278	$ 54,554

License and Research Agreemen29

License and Research Agreements - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended			3 Months Ended		9 Months Ended
	Oct. 31, 2016	Jul. 31, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Common stock purchased				42,488,312		42,488,312		41,240,230
Kyowa Hakko Kirin Collaboration | License Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction in research and development expenses				$ 7.6	$ 6.8	$ 22.9	$ 17.3
Reduction in general and administrative expenses				1.2	$ 0.4	2.3	$ 0.9
License agreement receivable				8.8		$ 8.8		$ 8.6
Kyowa Hakko Kirin Collaboration | License Agreement | Profit Share Territory
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Remaining profit or loss share percentage on commercializing products						50.00%
Tiered double-digit revenue share percentage entitled to receive						20.00%
Kyowa Hakko Kirin Collaboration | Maximum | License Agreement | European Territory
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of royalty on net sales entitled to receive						10.00%
Takeda License and Collaboration and Purchase Agreements
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Reduction in research and development expenses				1.4		$ 1.7
Estimated fair value of license recorded as research and development expense			$ 0.7
Research collaboration agreement, period			5 years
Common stock purchased		374,590
Common stock value		$ 40
Common closing stock price per share		$ 72.95
Common stock shares issued premium amount		$ 12.7
Option to purchase additional common stock, value		$ 25
Volume weighted average price, period		30 days
Total arrangement consideration received		$ 14.3
Estimated transaction price						2.4
Repayment liability				7.3		7.3		14.3
Contract liability				$ 0.7		$ 0.7		$ 0
Takeda License and Collaboration and Purchase Agreements | Put Option
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated fair value of future options		0.9
Common stock issued upon option exercised in period, shares	352,530
Common stock issued upon option exercised in period, value	$ 25
Takeda License and Collaboration and Purchase Agreements | Common Stock
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Fair market value of shares issued		27.3
Takeda License and Collaboration and Purchase Agreements | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated approximate cost for expected work performed		12.3
Takeda License and Collaboration and Purchase Agreements | Maximum | Development Milestones
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future contingent milestone payments			$ 7.5
Takeda License and Collaboration and Purchase Agreements | Maximum | Regulatory Milestones
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future contingent milestone payments			75
Takeda License and Collaboration and Purchase Agreements | Maximum | Commercial Milestones
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Future contingent milestone payments			$ 150
Takeda License and Collaboration and Purchase Agreements | Minimum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Estimated approximate cost for expected work performed		$ 10

Stock-Based Awards - Additional

Stock-Based Awards - Additional Information (Details)	9 Months Ended
	Sep. 30, 2017shares
2014 Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share reserved for future issuance	1,881,670
Automatic increases in shares available for grant effective date	Jan. 1, 2015
Shares available for grant, ending date	Jan. 1, 2024
2014 Employee Stock Purchase Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share reserved for future issuance	1,816,052

Stock-Based Awards - Functional

Stock-Based Awards - Functional Classification of Stock-Based Compensation Expense, Net of Estimated Forfeitures (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 17,180	$ 13,694	$ 48,473	$ 34,771
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	9,951	8,294	28,528	21,364
General and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Stock-based compensation expense	$ 7,229	$ 5,400	$ 19,945	$ 13,407

Net Loss Per Share - Outstandin

Net Loss Per Share - Outstanding Shares of Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share (Details) - shares	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	6,016,985	5,345,897	5,904,288	4,724,824
Employee Stock Purchase Plan
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	32,014		17,942
Options to Purchase Common Stock and Restricted Stock Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	5,835,271	5,196,197	5,736,646	4,575,124
Common Stock Warrants
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	149,700	149,700	149,700	149,700

Equity Transactions - Additiona

Equity Transactions - Additional Information (Details) - USD ($)	Jul. 27, 2017	Oct. 30, 2017	Jul. 31, 2016	Sep. 30, 2017	Mar. 31, 2017
ATM Sales Agreement
Stockholders Equity [Line Items]
Common stock shares sold					912,351
Net proceeds from sale of common stock					$ 67,600,000
Additional ATM Sales Agreement
Stockholders Equity [Line Items]
Common stock shares sold				0
Additional ATM Sales Agreement | Subsequent Event
Stockholders Equity [Line Items]
Common stock shares sold		64,468
Net proceeds from sale of common stock		$ 3,600,000
Maximum | ATM Sales Agreement
Stockholders Equity [Line Items]
Option to sell common stock for cash			$ 150,000,000
Maximum | Additional ATM Sales Agreement
Stockholders Equity [Line Items]
Option to sell common stock for cash	$ 150,000,000

Accumulated Other Comprehensi34

Accumulated Other Comprehensive Income (Loss) - Schedule of Total Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income Loss Net Of Tax [Abstract]
Unrealized loss on securities available-for-sale	$ (349)	$ (417)
Foreign currency translation adjustments	(7,055)	1,322
Total accumulated other comprehensive income (loss)	$ (7,404)	$ 905

Merger Agreement - Additional I

Merger Agreement - Additional Information (Details) - Subsequent Event - Dimension $ / shares in Units, $ in Millions	Oct. 02, 2017USD ($)$ / shares
Business Acquisition [Line Items]
Termination fee paid | $	$ 2.9
Outstanding shares of common stock, par value | $ / shares	$ 0.0001
Cash tender offer, purchase price per share | $ / shares	$ 6
Cash tender offer, amount | $	$ 151