Loxo Oncology (LOXO) 10-K 27 Mar 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014	Feb. 28, 2015	Aug. 01, 2014
Document and Entity Information
Entity Registrant Name	Loxo Oncology, Inc.
Entity Central Index Key	1581720
Document Type	10-K
Document Period End Date	31-Dec-14
Amendment Flag	FALSE
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Smaller Reporting Company
Entity Public Float			$66.30
Entity Common Stock, Shares Outstanding		16,634,063
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY

Balance_Sheets

Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
Current assets:
Cash and cash equivalents	$43,930,000	$14,994,000
Short-term investments	62,362,000
Prepaid expenses with related party and other current assets	1,484,000	17,000
Total current assets	107,776,000	15,011,000
Long-term investments	6,648,000
Property and equipment	12,000
Security deposit	23,000	11,000
Total assets	114,459,000	15,022,000
Current liabilities:
Accounts payable	239,000	221,000
Accrued expenses and other current liabilities	1,548,000	189,000
Total liabilities	1,787,000	410,000
Commitments and contingencies
Redeemable convertible preferred stock		24,843,000
Stockholders' (deficit) equity:
Common stock, $0.0001 par value; 9,375,000 and 125,000,000 shares authorized at December 31, 2013 and 2014, respectively; 452,896 and 16,644,229 shares issued and 452,896 and 16,634,063 outstanding at December 31, 2013 and 2014, respectively	2,000
Additional paid-in capital	143,660,000	59,000
Accumulated deficit	-30,962,000	-10,290,000
Other comprehensive loss	-28,000
Total stockholders' (deficit) equity	112,672,000	-10,231,000
Total liabilities, redeemable convertible preferred stock and stockholders' (deficit) equity	114,459,000	15,022,000
Redeemable convertible Series A preferred stock: Series A, $0.0001 par value; 5,126,250 and 0 shares authorized at December 31, 2013 and 2014, respectively; 2,812,497 and 0 shares issued and outstanding at December 31, 2013 and 2014, respectively; (liquidation preference of $18,000 and $0 at December 31, 2013 and 2014, respectively)
Current liabilities:
Redeemable convertible preferred stock		17,799,000
Redeemable convertible Series A-1 preferred stock: Series A-1, $0.0001 par value; 500,704 and 0 shares authorized at December 31, 2013 and 2014, respectively; 500,704 and 0 shares issued and outstanding at December 31, 2013 and 2014, respectively; (liquidation preference of $12,000 and $0 at December 31, 2013 and 2014, respectively)
Current liabilities:
Redeemable convertible preferred stock		$7,044,000

Balance_Sheets_Parenthetical

Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, except Share data, unless otherwise specified
Redeemable convertible preferred stock, par value (in dollars per share)		$0.00
Redeemable convertible preferred stock, shares authorized		5,656,954
Common stock, par value (in dollars per share)	$0.00	$0.00
Common stock, shares authorized	125,000,000	9,375,000
Common stock, shares issued	16,644,229	452,896
Common stock, shares outstanding	16,634,063	452,896
Redeemable convertible Series A preferred stock
Redeemable convertible preferred stock, par value (in dollars per share)	$0.00	$0.00
Redeemable convertible preferred stock, shares authorized	0	5,156,250
Redeemable convertible preferred stock, shares issued	0	2,812,497
Redeemable convertible preferred stock, shares outstanding	0	2,812,497
Redeemable convertible preferred stock, liquidation preference (in dollars)	$0	$18,000
Redeemable convertible Series A-1 preferred stock
Redeemable convertible preferred stock, par value (in dollars per share)	$0.00	$0.00
Redeemable convertible preferred stock, shares authorized	0	500,704
Redeemable convertible preferred stock, shares issued	0	500,704
Redeemable convertible preferred stock, shares outstanding	0	500,704
Redeemable convertible preferred stock, liquidation preference (in dollars)	$0	$12,000

Statements_of_Operations

Statements of Operations (USD $)	8 Months Ended	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2014
Operating expenses:
Research and development with related party	$9,384	$7,568
Research and development	323	6,947
General and administrative	583	6,175
Total operating expenses and loss from operations	-10,290	-20,690
Interest income, net		18
Net loss	-10,290	-20,672
Accretion of redeemable convertible preferred stock	-12	-34
Net loss attributable to common stockholders	($10,302)	($20,706)
Per share information:
Net loss per share of common stock, basic and diluted (in dollars per share)	($70.79)	($3.06)
Weighted average shares outstanding, basic and diluted	145,528	6,773,673

Statements_of_Comprehensive_Lo

Statements of Comprehensive Loss (USD $)	8 Months Ended	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2014
Statement of Comprehensive Income
Net loss	($10,290)	($20,672)
Other comprehensive loss, net of tax:
Unrealized loss on investments, net of tax		-28
Comprehensive loss	($10,290)	($20,700)

Statements_of_Redeemable_Conve

Statements of Redeemable Convertible Preferred Stock and Stockholders' (Deficit) Equity (USD $)	Jul-13	Jul-13	Mar-14	Mar-14	Series B	Preferred Stock	Redeemable convertible Series A preferred stock	Redeemable convertible Series A-1 preferred stock	Redeemable convertible Series B preferred stock	Common Stock	Additional Paid-in Capital	Deficit Accumulated During the Development Stage	Accumulated Other Comprehensive Loss	Total
In Thousands, except Share data, unless otherwise specified	Series B	Redeemable convertible Series A-1 preferred stock	Preferred Stock	Redeemable convertible Series A preferred stock
Balance at May. 08, 2013
Changes in Redeemable Securities
Issuance of redeemable convertible preferred stock	$7,044	$7,044			$17,787		$17,787
Issuance of redeemable convertible preferred stock (in shares)		500,704					2,812,497
Accretion of redeemable convertible preferred stock to redemption value					12		12
Changes Stockholders' Equity
Issuance of common stock to founders - June and July 2013 at $0.0001 per share											1			1
Issuance of common stock to founders - June and July 2013 at $0.0001 per share (in shares)										452,896
Accretion of redeemable convertible preferred stock to redemption value											-12			-12
Issuance of common stock in connection with initial public offering and private placement, net of offering costs (in shares)		500,704					2,812,497
Stock-based compensation expense											70			70
Net loss												-10,290		-10,290
Balance at Dec. 31, 2013						24,843					59	-10,290		-10,231
Balance at Dec. 31, 2013							17,799	7,044						24,843
Balance (in shares) at Dec. 31, 2013										452,896
Balance (in shares) at Dec. 31, 2013							2,812,497	500,704
Changes in Redeemable Securities
Issuance of redeemable convertible preferred stock			15,000	15,000		28,178			28,178
Issuance of redeemable convertible preferred stock (in shares)				2,343,753					3,166,233	6,134,307
Accretion of redeemable convertible preferred stock to redemption value						34	26		8
Conversion of redeemable convertible preferred stock into common stock						-68,055	-32,825	-7,044	-28,186	1	68,054			68,055
Conversion of redeemable preferred stock into common stock (in shares)							-5,156,250	-500,704	-3,166,233	9,932,042
Changes Stockholders' Equity
Issuance of common stock due to exercise of vested options											167			167
Issuance of common stock due to exercise of vested options (in shares)										114,818
Accretion of redeemable convertible preferred stock to redemption value											-34			-34
Issuance of common stock in connection with initial public offering and private placement, net of offering costs										1	72,365			72,366
Issuance of common stock in connection with initial public offering and private placement, net of offering costs (in shares)				2,343,753					3,166,233	6,134,307
Stock-based compensation expense											3,049			3,049
Other comprehensive loss													-28	-28
Net loss												-20,672		-20,672
Balance at Dec. 31, 2014										$2	$143,660	($30,962)	($28)	$112,672
Balance (in shares) at Dec. 31, 2014										16,634,063
Balance (in shares) at Dec. 31, 2014							0	0

Statements_of_Redeemable_Conve1

Statements of Redeemable Convertible Preferred Stock and Stockholders' (Deficit) Equity (Parenthetical) (USD $)	1 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 30, 2013	Jul. 31, 2013
Redeemable convertible Series A preferred stock
Debt Issuance Cost	$213	$213
Issuance of redeemable convertible preferred stock, issue price (in dollars per share)	$6.40
Redeemable convertible Series A-1 preferred stock
Issuance of redeemable convertible preferred stock, issue price (in dollars per share)		$14.07

Statements_of_Cash_Flows

Statements of Cash Flows (USD $)	8 Months Ended	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2014
Operating activities:
Net loss	($10,290)	($20,672)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization		30
Stock-based compensation	70	3,049
Issuance of redeemable convertible preferred stock in connection with collaboration agreement	7,044
Changes in operating assets and liabilities:
Prepaid expenses and other assets	-17	-1,244
Security deposits	-11	-12
Accounts payable	221	18
Accrued expenses and other current liabilities	189	1,136
Net cash used in operating activities	-2,794	-17,695
Investing activities:
Purchases of available-for-sale securities		-74,566
Proceeds from maturing available-for-sale securities		5,500
Purchase of property		-14
Net cash used in investing activities		-69,080
Financing activities:
Proceeds from issuance of redeemable convertible preferred stock, net	17,787	43,178
Proceeds from the issuance of common stock	1	74,331
Payment of deferred financing fees		-1,798
Net cash provided by financing activities	17,788	115,711
Net increase in cash and cash equivalents	14,994	28,936
Cash and cash equivalents-beginning of period		14,994
Cash and cash equivalents-end of period	14,994	43,930
Supplemental schedule of noncash financing activities:
Conversion of redeemable convertible preferred stock into common stock		$68,055

Organization_and_Description_o

Organization and Description of the Business	12 Months Ended
	Dec. 31, 2014
Organization and Description of the Business
Organization and Description of the Business
	1.Organization and Description of the Business
	Loxo Oncology, Inc. (the “Company”) was incorporated on May 9, 2013 in the State of Delaware. The Company is committed to the discovery, development, and commercialization of targeted cancer therapies with best-in-class potential. Loxo Oncology, Inc.’s diverse pipeline reflects the convergence of proven therapeutic technologies with emerging insights into the underlying susceptibilities of cancer and drug resistance. The Company operates in one segment and has its principal office in Stamford, Connecticut.
	Initial Public Offering
	On July 31, 2014, the Company’s registration statements on Form S-1 (File Nos. 333-197123 and 333-197779) relating to its initial public offering of its common stock were declared effective by the Securities and Exchange Commission (“SEC”). The shares began trading on The NASDAQ Global Select Market on August 1, 2014.  The initial public offering closed on August 6, 2014, and 5,261,538 shares of common stock were sold at an initial public offering price of $13.00 per share, for aggregate gross proceeds to the Company of $68.4 million. Concurrent with the close of the offering, New Enterprise Associates 14, L.P., or NEA, an existing stockholder, purchased 230,769 shares of common stock at the initial public offering price in a private placement and the Company received gross proceeds of $3.0 million.  In addition, upon the closing of the initial public offering, all of the Company’s outstanding convertible preferred stock was converted into an aggregate total of 9,932,042 shares of common stock.
	On August 29, 2014, the underwriters of the Company’s initial public offering gave notification that they would partially exercise the over-allotment option granted to them and on September 4, 2014, 642,000 additional shares of common stock were sold on the Company’s behalf at the initial public offering price of $13.00 per share, for aggregate gross proceeds of approximately $8.3 million.
	The Company paid to the underwriters underwriting discounts and commissions of approximately $5.6 million in connection with the offering, including the private placement and over-allotment.  In addition, the Company incurred expenses of approximately $1.7 million in connection with the offering. Thus, the net offering proceeds to the Company, after deducting underwriting discounts and commissions and offering expenses, were approximately $72.4 million.
	Stock Splits
	In July 2013, the Company’s Board of Directors (the “Board”) and stockholders approved a 2.07 to 1 reverse stock split of the Company’s common stock. The reverse stock split became effective on July 2, 2013. Subsequently, in July 2014, the Board and stockholders approved a 1.5625-for-1 forward stock split of the Company’s common stock. The forward stock split became effective on July 21, 2014.  All share and per share amounts in the financial statements and notes thereto have been adjusted to give effect to this reverse stock split.
	Liquidity
	At December 31, 2014, the Company had working capital of $106.0 million, an accumulated deficit of $31.0 million, cash and cash equivalents of $43.9 million, and investments of $69.0 million, of which $62.4 million and $6.6 million were classified as short-term and long-term, respectively. Upon consummation of its initial public offering and the concurrent private placement on August 6, 2014, and the over-allotment exercise on September 5, 2014, the Company received cash proceeds, net of underwriting discounts and commissions, and initial public offering costs of approximately $72.4 million. The Company has not generated any product revenues and has not achieved profitable operations. There is no assurance that profitable operations will ever be achieved, and, if achieved, could be sustained on a continuing basis. In addition, development activities, clinical and pre-clinical testing, and commercialization of the Company’s products will require significant additional financing.
	The Company believes that its existing cash, cash equivalents, and investments, will be sufficient to enable the Company to continue as a going-concern for a reasonable period of time beyond December 31, 2014. However, the Company will need to secure additional funding in the future, from one or more equity or debt financings, collaborations, or other sources, in order to carry out all of its planned research and development activities. If the Company is unable to obtain additional financing or generate license or product revenue, the lack of liquidity could have a material adverse effect on the Company’s future prospects.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	2.Summary of Significant Accounting Policies
	Significant Accounting Policies
	The accompanying financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”).
	Use of Estimates
	Management considers many factors in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the financial statements if these results differ from historical experience, or other assumptions do not turn out to be substantially accurate, even if such assumptions are reasonable when made. In preparing these financial statements, management used significant estimates in the following areas, among others: stock-based compensation expense, the determination of the fair value of stock-based awards, the accounting for research and development costs, and the recoverability of the Company’s net deferred tax assets and related valuation allowance.
	Prior to its initial public offering in August 2014, the Company utilized significant estimates and assumptions in determining the fair value of its common stock and convertible preferred stock. The Board determined the estimated fair value of the Company’s common stock based on a number of objective and subjective factors, including external market conditions affecting the biotechnology industry and the prices at which the Company sold shares of convertible preferred stock, the superior rights and preferences of securities senior to the Company’s common stock at the time, and the likelihood of achieving a liquidity event, such as the sale of the Company.
	Cash and Cash Equivalents
	The Company considers all highly-liquid investments that have maturities of three months or less when acquired to be cash equivalents. As of December 31, 2013, the Company’s cash and cash equivalents consisted of cash deposited in a business checking account and a $20,000 certificate of deposit. As of December 31, 2014, the Company’s cash and cash equivalents consisted of $18.0 million deposited in a business checking account, a $20,000 certificate of deposit, $8.9 million in a money market fund and $17.0 million in government sponsored enterprise debt securities that had maturities of three month or less when acquired. Cash equivalents are valued at cost, which approximates their fair market value.
	Investments
	At the time of purchase, the Company classifies investments in marketable securities as either available-for-sale securities, held to maturity securities, or trading securities, depending on its intent at that time.
	Investments available-for-sale are carried at fair value with unrealized holding gains and losses recorded within other comprehensive income. Fair value is determined based on observable market quotes or valuation models using assessments of counterparty credit worthiness, credit default risk or underlying security and overall capital market liquidity. The Company reviews unrealized losses associated with available-for-sale investments to determine the classification as a “temporary” or “other-than-temporary” impairment. A temporary impairment results in an unrealized loss being recorded in other comprehensive income. An impairment that is viewed as other-than-temporary is recognized in the statement of operations. The Company considers various factors in determining the classification, including the length of time and extent to which the fair value has been less than the Company’s cost basis, the financial condition and near-term prospects of the issuer or investee, and the Company’s ability to hold the investment for a period of time sufficient to allow for any anticipated recovery in market value. As of December 31, 2014, the Company held $69.0 million in investments of which $62.4 million and $6.6 million were classified as short-term and long-term, respectively.
	Concentration of Credit Risk
	Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents and available for sale securities. The Company maintains its cash and cash equivalents at financial institutions, which at times, exceed federally insured limits.  At December 31, 2013 and 2014, the Company’s cash and cash equivalents were held by two financial institutions and the amounts on deposit were in excess of FDIC insurance limits. The Company has not recognized any losses on our cash and cash equivalents.
	At December 31, 2014, the available for sale securities are invested in U.S. government sponsored enterprise debt securities that had a maturity date of three months or greater when acquired.  As noted in Note 4 to the Financial Statements, the fair value of these securities was $69.0 million, $28,000 less than their original par value purchase price.
	Property and Equipment
	Property and equipment are depreciated using the straight-line method over the estimated useful lives of the assets, which are generally three to five years. Maintenance and repairs are expensed as incurred. Upon disposal, retirement, or sale, the related cost and accumulated depreciation is removed from the accounts and any resulting gain or loss is included in the results of operations.
	Research and Development Expenses with a Related Party
	Research and development expenses with a related party consist primarily of the estimated fair value of Series A-1 convertible preferred stock issued in connection with the Array BioPharma, Inc. (“Array”) collaboration agreement (see Note 8) of approximately $7.0 million and $2.4 million in expenses incurred in relation to the conduct of the discovery and preclinical development programs by Array for the period from May 9, 2013 (Date of Inception) to December 31, 2013. Research and development expenses with a related party consist primarily of $7.6 million in expenses incurred in relation to the conduct of the discovery and preclinical development programs by Array for the year ended December 31, 2014.
	Research and Development Expenses
	Research and development costs are charged to expense as incurred. These costs include, but are not limited to, employee-related expenses, including salaries, benefits, stock-based compensation and travel as well as expenses related to third-party collaborations and contract research agreements; expenses incurred under agreements with contract research organizations and investigative sites that conduct preclinical studies; the cost of acquiring, developing and manufacturing clinical trial materials; facilities, depreciation and other expenses, which include direct and allocated expenses for rent and maintenance of facilities, insurance and other supplies; and costs associated with preclinical activities and regulatory operations.
	Costs for certain development activities, such as preclinical studies, are recognized based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, preclinical site activations, or information provided to the Company by its vendors with respect to their actual costs incurred. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the financial statements as prepaid or accrued research and development expense, as the case may be.
	Comprehensive Loss
	Comprehensive loss is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from non-owner sources. Comprehensive loss is comprised of operating net losses and unrealized gains or losses on investments.
	Income Taxes
	Income taxes are recorded in accordance with ASC Topic 740, Income Taxes (“ASC 740”), which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Deferred tax assets and liabilities are determined based on the differences between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are provided, if based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized.
	The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. As of December 31, 2014, the Company does not have any uncertain tax positions.
	Accretion of Redeemable Convertible Preferred Stock
	The Company accounted for the discount due to issuance costs on its Series A redeemable convertible preferred stock using the straight-line method, which approximates the effective interest method, accreting such amounts to preferred stock from the date of issuance to the date of redemption.
	Stock-Based Compensation
	The Company’s stock-based compensation plans are more fully described in Note 7 to the Financial Statements. The Company accounts for stock-based compensation in accordance with the provisions of ASC Topic 718, Compensation-Stock Compensation (“ASC 718”), which requires the recognition of expense related to the fair value of stock-based compensation awards in the Statement of Operations.
	For stock options issued to employees and members of the Board for their services on the Board, the Company estimates the grant date fair value of each option using the Black-Scholes option-pricing model. The use of the Black-Scholes option pricing model requires management to make assumptions with respect to the expected term of the option, the expected volatility of the common stock consistent with the expected term of the option, risk-free interest rates, the value of the common stock and expected dividend yield of the common stock. For awards subject to service-based vesting conditions, the Company recognizes stock-based compensation expense equal to the grant date fair value of stock options on a straight-line basis over the requisite service period, which is generally the vesting term. For awards subject to both performance and service-based vesting conditions, the Company recognizes stock-based compensation expense using the straight-line recognition method when it is probable that the performance condition will be achieved. Forfeitures are required to be estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates.
	Share-based payments issued to non-employees are recorded at fair value, and are periodically revalued as the equity instruments vest and are recognized as expense over the related service period. See Note 7 to the Financial Statements for a discussion of the assumptions used by the Company in determining the grant date fair value of options granted under the Black-Scholes option pricing model, as well as a summary of the stock option activity under the Company’s stock-based compensation plan for the period from May 9, 2013 (Date of Inception) to December 31, 2013 and the year ended December 31, 2014.
	Basic and Diluted Net Loss Per Share of Common Stock
	Basic net loss per share of common stock is computed by dividing net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period, excluding the dilutive effects of convertible preferred stock, unvested restricted stock and stock options. Diluted net loss per share of common stock is computed by dividing the net loss attributable to common stockholders by the sum of the weighted-average number of shares of common stock outstanding during the period plus the potential dilutive effects of convertible preferred stock, unvested restricted stock and stock options outstanding during the period calculated in accordance with the treasury stock method, although these shares and options are excluded if their effect is anti-dilutive. Because the impact of these items is anti-dilutive during periods of net loss, there was no difference between basic and diluted net loss per share of common stock for the period from May 9, 2013 (Date of Inception) to December 31, 2013 or the year ended December 31, 2014.
	Recent Accounting Pronouncements
	On August 27, 2014, the FASB issued ASU 2014-15, which provides guidance on determining when and how reporting entities must disclose going-concern uncertainties in their financial statements. The new standard requires management to perform interim and annual assessments of an entity’s ability to continue as a going concern within one year of the date of issuance of the entity’s financial statements (or within one year after the date on which the financial statements are available to be issued, when applicable). Further, an entity must provide certain disclosures if there is “substantial doubt about the entity’s ability to continue as a going concern.” The FASB believes that requiring management to perform the assessment will enhance the timeliness, clarity, and consistency of related disclosures and improve convergence with International Financial Reporting Standards (“IFRS”) (which emphasize management’s responsibility for performing the going-concern assessment). However, the time horizon for the assessment (look-forward period) and the disclosure thresholds under U.S. GAAP and IFRSs will continue to differ.  The Company does not anticipate that the adoption of this standard will have a material impact on its financial statements.
	On June 10, 2014, the FASB issued ASU No. 2014-10, Development Stage Entities (Topic 915): Elimination of Certain Financial Reporting Requirements, Including an Amendment to Variable Interest Entities Guidance in Topic 810, Consolidation.  The guidance is intended to reduce the overall cost and complexity associated with financial reporting for development stage entities without reducing the availability of relevant information.  The Board also believes the changes will simplify the consolidation accounting guidance by removing the differential accounting requirements for development stage entities. As a result of these changes, there no longer will be any accounting or reporting differences in GAAP between development stage entities and other operating entities.  For organizations defined as public business entities the presentation and disclosure requirements in Topic 915 will no longer be required starting with the first annual period beginning after December 15, 2014, including interim periods therein.  Early application is permitted for any annual reporting period or interim period for which the entity’s financial statements have not yet been issued (public business entities) or made available for issuance (other entities).  The Company early adopted this guidance during the year ended December 31, 2014 and, as a result, the Company no longer presents inception-to-date information about the statements of operations, cash flows, and stockholders’ (deficit) equity.
	In June 2014, the FASB issued ASU No. 2014-12, Compensation - Stock Compensation (Topic 718): Accounting for Share-Based Payments When the Terms of an Award Provide that a Performance Target Could be Achieved after the Requisite Service Period, (“ASU 2014-12”). ASU 2014-12 requires that a performance target that affects vesting, and that could be achieved after the requisite service period, be treated as a performance condition. As such, the performance target should not be reflected in estimating the grant date fair value of the award. This update further clarifies that compensation cost should be recognized in the period in which it becomes probable that the performance target will be achieved and should represent the compensation cost attributable to the period(s) for which the requisite service has already been rendered. The Company does not anticipate that the adoption of this standard will have a material impact on its financial statements.
	Segment Information
	Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision-making group, in making decisions on how to allocate resources and assess performance. The Company’s chief operating decision maker is the chief executive officer. The Company and the chief executive officer view the Company’s operations and manage its business as one operating segment. All long-lived assets of the Company reside in the United States.
	Reclassifications
	Certain expenses that had been previously classified as general and administrative were reclassified into research and development.

Net_Loss_Per_Common_Share

Net Loss Per Common Share	12 Months Ended
	Dec. 31, 2014
Net Loss Per Common Share
Net Loss Per Common Share
	3.Net Loss Per Common Share
	The following table sets forth the computation of basic and diluted net loss per share for the periods indicated (in thousands, except share and per share data):
		Period from May 9,			Year Ended
		2013 (Date of			December 31, 2014
		Inception) to
		December 31, 2013
	Basic and diluted net loss per common share calculation:
	Net loss	$	(10,290	)	$	(20,672	)
	Accretion of redeemable convertible preferred stock	(12		)	(34		)
	Net loss attributable to common stockholders	$	(10,302	)	$	(20,706	)
	Weighted average common shares outstanding	145,528			6,773,673
	Net loss per share of common stock—basic and diluted	$	(70.79	)	$	(3.06	)
	The following outstanding securities at December 31, 2013 and 2014 have been excluded from the computation of diluted weighted average shares outstanding, as they would have been anti-dilutive:
		Period from May 9,		Year Ended
		2013 (Date of		December 31, 2014
		Inception) to
		December 31, 2013
	Redeemable convertible preferred stock	3,313,201		—
	Unvested restricted stock	264,190		170,622
	Stock options	681,056		2,011,005
		4,258,447		2,181,627

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2014
Fair Value Measurements.
Fair Value Measurements	4.Fair Value Measurements
	Financial Instruments
	The financial instruments recorded in the Company’s balance sheets include cash and cash equivalents, investments, and accounts payable. Included in cash and cash equivalents are money market funds representing a type of mutual fund required by law to invest in low-risk securities (for example, U.S. government bonds, U.S. Treasury Bills and commercial paper). Money market funds are structured to maintain the fund’s net asset value at $1.00 per unit, which assists in providing adequate liquidity upon demand by the holder. Money market funds pay dividends that generally reflect short-term interest rates. Thus, only the dividend yield fluctuates. Also included in cash and cash equivalents are the U.S. government sponsored enterprise debt securities that have a maturity of 3 months or less from their original acquisition date.  Due to their short-term maturity, the carrying amounts of cash and cash equivalents (including money market funds), and accounts payable approximate their fair values.
	For investments, the Company records unrealized gains or losses resulting from changes in fair value between measurement dates as a component of other comprehensive income (loss). There were no investments in money market funds or government enterprise debt securities during the period May 9, 2013 (Date of Inception) to December 31, 2013.
	(amounts in thousands)		Amortized		Gross		Gross			Fair Value
			Cost		Unrealized		Unrealized
					Gains		(Losses)
	December 31, 2014
	Government enterprise debt securities		$	17,009	$	—	$	—		$	17,009
	Money market funds		8,846		—		—			8,846
	Total included in cash and cash equivalents		25,855		—		—			25,855
	Government enterprise debt securities		69,038		—		(28		)	69,010
	Short-term available-for-sale securities		62,387		—		(25		)	62,362
	Long-term available-for-sale securities		6,651		—		(3		)	6,648
	Total fair value financial instruments		$	94,893	$	—	$	(28	)	$	94,865
	Fair value guidance establishes a three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value. These tiers include:
		·		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.
	The Company’s financial assets and liabilities measured at fair value on a recurring basis at December 31, 2014 were as follows (in thousands):
			Fair Value Measurements at Measurement Date:
			Quoted Prices in Active		Significant Other		Significant Unobservable			Total for the Year Ended
			Markets for Identical Assets		Observable Inputs		Inputs			December 31, 2014
			(Level 1)		(Level 2)		(Level 3)
	Assets:
	Government enterprise debt securities		$	—	$	86,019	$	—		$	86,019
	Money market funds		8,846		—		—			8,846
	Totals		$	8,846	$	86,019	$	—		$	94,865
	There were no items that were accounted for at fair value on a non-recurring basis during the period May 9, 2013 (Date of Inception) to December 31, 2013.

Accrued_Expenses_and_Other_Cur

Accrued Expenses and Other Current Liabilities	12 Months Ended
	Dec. 31, 2014
Accrued Expenses and Other Current Liabilities
Accrued Expenses and Other Current Liabilities
	5.Accrued Expenses and Other Current Liabilities
	Accrued expenses and other current liabilities consisted of the following (in thousands):
		December 31,		December 31,
		2013		2014
	Research and development expenses	$	81	$	742
	General and administrative expenses	108		769
	Share repurchase obligation	—		37
		$	189	$	1,548

Capital_Stock_Redeemable_Conve

Capital Stock, Redeemable Convertible Preferred Stock, and Stockholders' (Deficit) Equity Capitalization	12 Months Ended
	Dec. 31, 2014
Capital Stock, Redeemable Convertible Preferred Stock, and Stockholders' (Deficit) Equity Capitalization.
Capital Stock, Redeemable Convertible Preferred Stock, and Stockholders' (Deficit) Equity Capitalization
	6.Capital Stock, Redeemable Convertible Preferred Stock, and Stockholders’ (Deficit) Equity
	Capitalization
	As of December 31, 2013, the Company’s amended and restated certificate of incorporation reflected the following authorized shares: 15,031,954 shares of capital stock, consisting of 9,375,000 shares of common stock, par value of $0.0001 per share, and 5,656,954 shares of preferred stock, par value of $0.0001 per share of which (i) 5,156,250 shares are designated Series A Redeemable Convertible Preferred Stock (“Series A”) and (ii) 500,704 shares are designated Series A-1 Redeemable Convertible Preferred Stock (“Series A-1”).
	On July 3, 2013, the Company entered into a stock purchase agreement, which was subsequently amended on September 19, 2013, pursuant to which the Company agreed to sell to certain investors, upon the satisfaction of certain conditions, up to 2,812,497 shares of Series A. The initial closing occurred on July 3, 2013 and 1,562,500 shares of Series A were issued. The remaining 1,249,997 shares were issued on September 19, 2013. Upon completing the July and September closings, the Company received net proceeds of approximately $17.8 million. Additionally on July 3, 2013, the Company issued 500,704 shares, with an estimated fair value of approximately $7.0 million, of Series A-1 to Array in connection with entering into a collaboration agreement (see Note 8) to the Financial Statements). The estimated fair value of these shares has been recognized as research and development expense-related party in the accompanying statements of operations.
	On February 28, 2014, the Company filed with the United States Food and Drug Administration an Investigational New Drug Application for a tyrosine kinase inhibitor targeted to the TRK family of receptors. As a result and in accordance with the provisions of the stock purchase agreement entered into on July 3, 2013, the Company issued 2,343,753 shares of Series A at a price of $6.40 per share and received net proceeds of $15.0 million on March 18, 2014.
	On April 24, 2014 and June 24, 2014, the Company entered into stock purchase agreements pursuant to which the Company agreed to sell 2,664,343 and 501,890 shares, respectively, of Series B, $0.0001 par value, at a purchase price of $8.9661 per share. Upon completing the April and June offerings, the Company received gross proceeds of approximately $28.4 million.
	As previously discussed in Note 1 to the Financial Statements, the Company completed its initial public offering in August 2014.  As part of that offering, all of the Company’s outstanding convertible preferred stock was converted into an aggregate total of 9,932,042 shares of common stock.
	Upon the completion of the initial public offering, the Company’s authorized capital stock consisted of 125,000,000 shares of common stock, $0.0001 par value per share, and 5,000,000 shares of undesignated preferred stock, $0.0001 par value per share.

Stock_Based_Compensation

Stock Based Compensation	12 Months Ended
	Dec. 31, 2014
Stock-Based Compensation
Stock-Based Compensation
	7.Stock-Based Compensation
	2013 Equity Incentive Plan
	Effective July 2, 2013, the Company adopted the 2013 Equity Incentive Plan, which was amended in November 2013 (the “2013 Plan”). The 2013 Plan provides for the granting of incentive stock options, non-statutory stock options and the issuance of restricted stock awards. As of December 31, 2013 and December 31, 2014, there were 905,796 and 1,544,615 shares, respectively, of common stock authorized for issuance in connection with the Plan, of which there were 224,734 and 0 shares available for future issuance, respectively.
	Certain options are eligible for exercise prior to vesting. Exercised but unvested shares are subject to repurchase by the Company at the initial exercise price. The proceeds from the shares subject to repurchase are classified as a liability and reclassified to equity as the shares vest. Under the 2013 Plan’s early exercise feature, the Company could be required to repurchase 10,166 shares as of December 31, 2014. The Company records cash received from early exercised shares as a liability. As of December 31, 2014, $37,000 has been recorded as a liability and included in accrued expenses.  In connection with the Company’s initial public offering, no further grants will be made under this plan and all remaining shares available for grant were transferred to the 2014 Equity Incentive Plan.
	2014 Equity Incentive Plan
	The Company adopted a 2014 Equity Incentive Plan (the “2014 Plan”) that became effective on July 30, 2014 and serves as the successor to the 2013 Equity Incentive Plan. The 2014 Equity Incentive Plan provides for the grant of awards to employees, directors, consultants, independent contractors and advisors, provided the consultants, independent contractors, directors and advisors are natural persons that render services not in connection with the offer and sale of securities in a capital-raising transaction. The exercise price of stock options must be at least equal to the fair market value of our common stock on the date of grant.
	The Company has reserved 1,092,085 shares of its common stock to be issued under the 2014 Equity Incentive Plan of which 510,863 shares were available for future issuance as of December 31, 2014.  Shares will increase automatically on January 1 of each of 2015 through 2024 by the number of shares equal to 3.0% of the aggregate number of outstanding shares of our common stock as of the immediately preceding December 31. The Company’s Board may reduce the amount of the increase in any particular year.
	The 2014 Equity Incentive Plan authorizes the award of stock options, restricted stock awards, or RSAs, stock appreciation rights, or SARs, restricted stock units, or RSUs, performance awards and stock bonuses.
	The following table summarizes stock option activity under the 2013 and 2014 Plans for the period from May 9, 2013 (Date of Inception) to December 31, 2013 and for the year ended December 31, 2014:
								Average
						Weighted-		Remaining	Aggregate
			Number			Average		Contractual	Intrinsic Value
			of Shares			Exercise Price		Term (in years)	(in thousands)
	Outstanding at May 9, 2013 (Date of Inception)		—			$	—	—
	Granted		681,056			1.18
	Exercised		—			—
	Cancelled		—			—
	Forfeited		—			—
	Outstanding at December 31, 2013		681,056			$	1.18	9.92
	Granted		1,579,970			8.5
	Exercised		(114,818		)	1.45
	Cancelled		—			—
	Forfeited		(135,203		)	3.65
	Outstanding at December 31, 2014		2,011,005			$	6.75	9.47	$	11,375
	Vested and expected to vest at December 31, 2014		1,914,005			$	6.71	9.47	$	10,891
	Exercisable at December 31, 2014		504,751			$	2.19	9.06	$	4,863
	Weighted-average grant date fair value of options granted during the year ended December 31, 2014		$	9.23
	As of December 31, 2014, there was $13.5 million of total unrecognized compensation expense related to options granted but not yet vested of which $2.2 million is attributable to non-employee awards and subject to re-measurement until vested.  The total unrecognized compensation expense of $13.5 million will be recognized as expense over a weighted-average period of 3.48 years.
	The Company uses the Black-Scholes option pricing model to estimate the fair value of option awards with the following weighted-average assumptions, which are based on industry comparative information, for the period indicated:
			Period from May 9,		Year Ended
			2013 (Date of		December 31, 2014
			Inception) to
			December 31, 2013
	Risk-free interest rate		2.14	%	1.90	%
	Expected dividend yield		0	%	0	%
	Expected stock price volatility		84.76	%	86.22	%
	Expected term of options (in years)		7.44		6.50
	Expected forfeiture rate		0	%	5.98	%
	The weighted-average valuation assumptions were determined as follows:
		·		Risk-free interest rate: The Company bases the risk-free interest rate on the interest rate payable on U.S. Treasury securities in effect at the time of grant for a period that is commensurate with the assumed expected option term.
		·		Expected annual dividends: The estimate for annual dividends is 0%, because the Company has not historically paid, and does not expect for the foreseeable future to pay, a dividend.
		·		Expected stock price volatility: The expected volatility used is based on historical volatilities of similar entities within the Company’s industry which were commensurate with the Company’s expected term assumption.
		·		Expected term of options: The expected term of options represents the period of time options are expected to be outstanding. The expected term of the options granted to employees is derived from the “simplified” method as described in Staff Accounting Bulletin 107 relating to stock-based compensation. The expected term for options granted to non-employees is equal to the contractual term of the awards.
		·		Expected forfeiture rate: The Company’s estimated annual forfeiture rate was 0% and 5.98% for the period from May 9, 2013 (Date of Inception) to December 31, 2013 and for the year ended December 31, 2014, respectively. Stock-based compensation expense is reduced for estimated future forfeitures.
		·		The estimated fair value of the Company’s stock-based awards is amortized on a straight-line basis over the awards’ service period for those awards with graded vesting and which contain only a service condition.  For awards with graded vesting and a performance and service condition, when achievement of the performance condition is deemed probable, the Company recognizes compensation cost using the accelerated recognition method over the awards’ service period.
	Share-based compensation expense recognized was as follows (in thousands):
			Period from May 9,			Year Ended December
			2013 (Date of			31, 2014
			Inception) to
			December 31, 2013
	Research and development		$	15		$	2,002
	General and administrative		55			1,047
			$	70		$	3,049
	Restricted Stock
	The stock-based compensation expense for restricted stock is determined based on the estimated fair value of the Company’s common stock on the grant date of the awards applied to the total number of awards that are anticipated to vest. During the period from May 9, 2013 (Date of Inception) to December 31, 2013, the Company granted 264,189 restricted stock awards.  During the year ended December 31, 2014, 93,567 restricted shares vested and the remaining 170,622 shares are expected to vest over the next three years.  Stock-based compensation was de minimis for the period from May 9, 2013 (Date of Inception) to December 31, 2013 and for the year ended December 31, 2014.
	2014 Employee Stock Purchase Plan
	The Company adopted a 2014 Employee Stock Purchase Plan (“ESPP”) that became effective on July 31, 2014, which was the effective date of the Company’s registration statement.  The ESPP provides employees of the Company, including any parent or subsidiary companies that the Board designates from time to time as a corporation that shall participate in the plan, with a means of acquiring an equity interest in the Company and to provide an incentive for continued employment.
	There are 149,600 shares of common stock reserved for future issuances under the ESPP. Any employee regularly employed by the Company for six months or more on a full-time or part-time basis (20 hours or more per week on a regular schedule) will be eligible to participate in the plan. The ESPP will operate in successive six month offering periods. Each eligible employee who has elected to participate may purchase up to 1,000 shares or $25,000 during each offering period. The purchase price will be the lower of (i) 85% of the fair market value of a share of common stock on the first trading day of the offering period or (ii) 85% of the fair market value of a share of common stock on the last trading day of the offering period. The ESPP will continue for a period of ten years from the first purchase date under the plan unless otherwise terminated by the Board. As of March 26, 2015, no commencement date for the first offering period has been approved by the Board or compensation committee and no shares have been issued under the ESPP.

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies.
Commitments and Contingencies
	8.Commitments and Contingencies
	Operating Leases
	The Company leases office space under two operating leases for its locations in South San Francisco, California and Stamford, Connecticut.  The Company’s lease agreements contain escalation clauses, accordingly, the Company straight-lines the rent expense over the lease term.  Rent expense under operating leases during the period from May 9, 2013 (Date of Inception) to December 31, 2013 and during the year ended December 31, 2014 was $1,529 and $55,614, respectively.
	Future minimum lease payments as of December 31, 2014 are as follows (in thousands):
		Operating
		Lease
	2015	$	131
	2016	133
	2017	98
	2018	26
		$	388
	Array Collaboration
	On July 3, 2013, the Company signed a multi-year strategic collaboration agreement with Array, and this agreement was subsequently amended on November 26, 2013, April 10, 2014 and October 13, 2014. Under the terms of the collaboration agreement, the Company obtained certain rights to Array’s TRK inhibitor program, as well as additional novel oncology targets. The Company has worldwide commercial rights to each product candidate from the collaboration and Array participates in any potential successes through milestones, royalties, and an equity ownership in the Company.
	With respect to the discovery and preclinical program, the collaboration agreement runs through July 3, 2016, and the Company has the option to extend the term for up to two additional one-year renewal periods by providing written notice to Array at least three months before the end of the initial discovery and preclinical development programs or the renewal period, if applicable. In addition to LOXO-101, the parties designated 12 discovery targets, of which seven were selected for additional study in January 2015, which will be reduced to four on or before January 2016. The Company has the option to increase the total candidate selection number to five for a modest additional payment.
	As part of the agreement the Company agreed to pay Array a fixed amount per month, based on Array’s commitment to provide full-time equivalents and other support relating to the conduct of the discovery and preclinical development programs. The Company recorded related-party research and development expenses of $9.4 million and $7.6 million the period from May 9, 2013 (Date of Inception) to December 31, 2013 and for the year ended December 31, 2014, respectively.
	Milestones
	With respect to product candidates directed to TRK, including LOXO-101 and its back-up compounds, the Company could be required to pay Array up to $222 million in milestone payments, the substantial majority of which are due upon the achievement of commercial milestones. With respect to product candidates directed to targets other than TRK, the Company could be required to pay Array up to $213 million in milestone payments, the substantial majority of which are due upon the achievement of commercial milestones.
	Royalties
	The Company is required to pay Array mid-single digit royalties on worldwide net sales of products directed to TRK and directed to targets. With respect to the royalty on products directed to targets, the Company has the right to credit certain milestone payments against royalties on sales of products directed to such target.
	Convertible preferred stock issuance
	In connection with this agreement, the Company issued Array 500,704 shares of Series A-1 convertible preferred stock, par value $0.0001. The Company recognized, as a component of research and development expense with related party approximately $7.0 million related to the estimated fair value of the shares issued during the period from May 9, 2013 (Date of Inception) to December 31, 2013.  As previously discussed in Note 1 to the Financial Statements, the Company completed its initial public offering in August 2014.  As part of that offering, all of the convertible preferred stock was converted into shares of common stock.
	Research and Development Arrangements
	In the course of normal business operations, the Company enters into agreements with contract service organizations, or CROs, to assist in the performance of research and development and preclinical activities. Expenditures to CROs may represent a significant cost in preclinical development for the Company in future periods. The Company can elect to discontinue the work under these agreements at any time. The Company could also enter into additional collaborative research, contract research, manufacturing, and supplier agreements in the future, which may require upfront payments and even long-term commitments of cash.
	Legal Proceedings
	The Company is not involved in any legal proceeding that it expects to have a material effect on its business, financial condition, results of operations and cash flows.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Taxes.
Income Taxes
	9.Income Taxes
	The Company provides for income taxes under ASC 740. Under ASC 740, the liability method is used in accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on differences between financial reporting and tax bases of assets and liabilities, and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse.
	The Company did not record a current or deferred income tax expense or benefit since its inception.
	The Company’s loss before income taxes was $10.3 million and $20.7 million for the period from May 9, 2013 (Date of Inception) to December 31, 2013 and for the year ended December 31, 2014, respectively, and was generated entirely in the United States.
	Deferred taxes are recognized for temporary differences between the basis of assets and liabilities for financial statement and income tax purposes. The significant components of the Company’s deferred tax assets are comprised of the following (in thousands):
		Period from May 9,			Year Ended
		2013 (Date of			December 31, 2014
		Inception) to
		December 31, 2013
	Net operating losses	$	1,248		$	7,526
	Accrued expenses	—			194
	Acquired research and development	2,479			2,348
	Research and development credits	108			688
	Stock options	—			967
	Other temporary differences	13			283
	Gross deferred tax assets	3,848			12,006
	Deferred tax valuation allowance	(3,848		)	(12,006		)
		$	—		$	—
	The Company has evaluated the positive and negative evidence bearing upon the realizability of its deferred tax assets. Based on the Company’s history of operating losses since inception, the Company has concluded that it is more likely than not that the benefit of its deferred tax assets will not be realized. Accordingly, the Company has provided a full valuation allowance for deferred tax assets as of December 31, 2013 and 2014. The valuation allowance increased by $3.8 million and $8.2 million for the period from May 9, 2013 (Date of Inception) to December 31, 2013 and for the year ended December 31, 2014, respectively, due primarily to the generation of net operating losses during the periods.
	A reconciliation of income tax benefit computed at the statutory federal income tax rate to income taxes as reflected in the financial statements is as follows:
		December 31, 2013		December 31, 2014
	U.S. statutory income tax rate	34	%	34	%
	State income taxes, net of federal benefit	2.4		3
	Permanent differences	(0.1	)	(0.7	)
	Provision to return true-up	—		(0.1	)
	R&D credit carryforwards	1.1		3.3
	Valuation allowance	(37.4	)	(39.5	)
	Effective tax rate	—	%	—	%
	As of December 31, 2013 and 2014, the Company had U.S. federal net operating loss carryforwards of $3.4 million and $20.3 million, respectively, which may be available to offset future income tax liabilities and will begin to expire at various dates starting in 2033. As of December 31, 2013 and 2014, the Company also had U.S. state net operating loss carryforwards of $1.4 million and $10.6 million, respectively, which may be available to offset future income tax liabilities and will begin to expire at various dates starting in 2033.  The NOL carry forwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. NOL and tax credit carry forwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant shareholders over a three-year period in excess of 50%, as defined under Sections 382 and 383 of the Internal Revenue Code of 1986, as amended, as well as similar state tax provisions. This could limit the amount of NOLs that the Company can utilize annually to offset future taxable income or tax liabilities. The amount of the annual limitation, if any, will be determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years.
	The Company files income tax returns in the United States, and various state jurisdictions. The federal and state income tax returns are generally subject to tax examinations for the period from May 9, 2013 (Date of Inception) to December 31, 2013 and for the year ended December 31, 2014.  To the extent the Company has tax attribute carryforwards, the tax years in which the attribute was generated may still be adjusted upon examination by the Internal Revenue Service or state tax authorities to the extent utilized in a future period.

Related_Party_Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2014
Related Party Transactions
Related Party Transactions
	10.Related Party Transactions
	The Company recorded related-party research and development expenses of $9.4 million and $7.6 million for the period from May 9, 2013 (Date of Inception) to December 31, 2013 and the year ended December 31, 2014, respectively, as a component of research and development with a related party for services provided by Array under the collaboration agreement as described in further detail above in Note 8 to the Financial Statements.  As of December 31, 2014, the Company had $0.6 million in prepaid expenses to Array under the collaboration agreement for services that will be provided in subsequent periods.
	On October 23, 2014, the Board appointed Dr. Lori Kunkel as a Class II director.  Dr. Kunkel currently holds 26,840 shares of common stock of Loxo and options representing the right to purchase 109,374 shares of common stock of Loxo, together representing approximately $1.6 million.  Dr. Kunkel has a consulting agreement with the Company to assist in the Company’s drug development process.  Dr. Kunkel is eligible to receive a maximum of $15,000 monthly for her consulting work.  Payments are expensed as incurred and recorded as a component of research and development expenses.  During the year ended December 31, 2014, the Company recognized expense of $100,127 in accordance with the terms of the consulting agreement.  As of December 31, 2014, there was $15,000 included in accounts payable to Dr. Kunkel.

Subsequent_Events

Subsequent Events	12 Months Ended
	Dec. 31, 2014
Subsequent Events
Subsequent Events
	11.Subsequent Events
	On January 12, 2015, the Company and Dr. Goldstein announced that concurrently with his appointment as Partner at Aisling Capital, Dr. Goldstein would transition from the role of full-time Chief Financial Officer to Acting CFO on part-time basis, effective as of February 1, 2015. Also on January 12, 2015, the Board appointed Alan Fuhrman as an independent Class III director and the Chair of the Audit Committee, with a term that will expire at the annual stockholder meeting in 2017.
	In 2015, the Board granted 43,625 stock option awards with exercise prices ranging from $11.35 to $13.75 per share and pursuant to the 2014 Plan. The vesting terms of each award may vest over three to four years as defined in each award.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policy)	12 Months Ended
	Dec. 31, 2014
Summary of Significant Accounting Policies
Use of Estimates	Use of Estimates
	Management considers many factors in developing the estimates and assumptions that are used in the preparation of these financial statements. Management must apply significant judgment in this process. In addition, other factors may affect estimates, including expected business and operational changes, sensitivity and volatility associated with the assumptions used in developing estimates, and whether historical trends are expected to be representative of future trends. The estimation process often may yield a range of potentially reasonable estimates of the ultimate future outcomes and management must select an amount that falls within that range of reasonable estimates. This process may result in actual results differing materially from those estimated amounts used in the preparation of the financial statements if these results differ from historical experience, or other assumptions do not turn out to be substantially accurate, even if such assumptions are reasonable when made. In preparing these financial statements, management used significant estimates in the following areas, among others: stock-based compensation expense, the determination of the fair value of stock-based awards, the accounting for research and development costs, and the recoverability of the Company’s net deferred tax assets and related valuation allowance.
	Prior to its initial public offering in August 2014, the Company utilized significant estimates and assumptions in determining the fair value of its common stock and convertible preferred stock. The Board determined the estimated fair value of the Company’s common stock based on a number of objective and subjective factors, including external market conditions affecting the biotechnology industry and the prices at which the Company sold shares of convertible preferred stock, the superior rights and preferences of securities senior to the Company’s common stock at the time, and the likelihood of achieving a liquidity event, such as the sale of the Company.
Cash and Cash Equivalents	Cash and Cash Equivalents
	The Company considers all highly-liquid investments that have maturities of three months or less when acquired to be cash equivalents. As of December 31, 2013, the Company’s cash and cash equivalents consisted of cash deposited in a business checking account and a $20,000 certificate of deposit. As of December 31, 2014, the Company’s cash and cash equivalents consisted of $18.0 million deposited in a business checking account, a $20,000 certificate of deposit, $8.9 million in a money market fund and $17.0 million in government sponsored enterprise debt securities that had maturities of three month or less when acquired. Cash equivalents are valued at cost, which approximates their fair market value.
Investments
	Investments
	At the time of purchase, the Company classifies investments in marketable securities as either available-for-sale securities, held to maturity securities, or trading securities, depending on its intent at that time.
	Investments available-for-sale are carried at fair value with unrealized holding gains and losses recorded within other comprehensive income. Fair value is determined based on observable market quotes or valuation models using assessments of counterparty credit worthiness, credit default risk or underlying security and overall capital market liquidity. The Company reviews unrealized losses associated with available-for-sale investments to determine the classification as a “temporary” or “other-than-temporary” impairment. A temporary impairment results in an unrealized loss being recorded in other comprehensive income. An impairment that is viewed as other-than-temporary is recognized in the statement of operations. The Company considers various factors in determining the classification, including the length of time and extent to which the fair value has been less than the Company’s cost basis, the financial condition and near-term prospects of the issuer or investee, and the Company’s ability to hold the investment for a period of time sufficient to allow for any anticipated recovery in market value. As of December 31, 2014, the Company held $69.0 million in investments of which $62.4 million and $6.6 million were classified as short-term and long-term, respectively.
Concentration of Credit Risk	Concentration of Credit Risk
	Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents and available for sale securities. The Company maintains its cash and cash equivalents at financial institutions, which at times, exceed federally insured limits.  At December 31, 2013 and 2014, the Company’s cash and cash equivalents were held by two financial institutions and the amounts on deposit were in excess of FDIC insurance limits. The Company has not recognized any losses on our cash and cash equivalents.
	At December 31, 2014, the available for sale securities are invested in U.S. government sponsored enterprise debt securities that had a maturity date of three months or greater when acquired.  As noted in Note 4 to the Financial Statements, the fair value of these securities was $69.0 million, $28,000 less than their original par value purchase price.
Property and Equipment	Property and Equipment
	Property and equipment are depreciated using the straight-line method over the estimated useful lives of the assets, which are generally three to five years. Maintenance and repairs are expensed as incurred. Upon disposal, retirement, or sale, the related cost and accumulated depreciation is removed from the accounts and any resulting gain or loss is included in the results of operations.
Research and Development Expenses with a Related Party
	Research and Development Expenses with a Related Party
	Research and development expenses with a related party consist primarily of the estimated fair value of Series A-1 convertible preferred stock issued in connection with the Array BioPharma, Inc. (“Array”) collaboration agreement (see Note 8) of approximately $7.0 million and $2.4 million in expenses incurred in relation to the conduct of the discovery and preclinical development programs by Array for the period from May 9, 2013 (Date of Inception) to December 31, 2013. Research and development expenses with a related party consist primarily of $7.6 million in expenses incurred in relation to the conduct of the discovery and preclinical development programs by Array for the year ended December 31, 2014.
Research and Development Expenses
	Research and Development Expenses
	Research and development costs are charged to expense as incurred. These costs include, but are not limited to, employee-related expenses, including salaries, benefits, stock-based compensation and travel as well as expenses related to third-party collaborations and contract research agreements; expenses incurred under agreements with contract research organizations and investigative sites that conduct preclinical studies; the cost of acquiring, developing and manufacturing clinical trial materials; facilities, depreciation and other expenses, which include direct and allocated expenses for rent and maintenance of facilities, insurance and other supplies; and costs associated with preclinical activities and regulatory operations.
	Costs for certain development activities, such as preclinical studies, are recognized based on an evaluation of the progress to completion of specific tasks using data such as patient enrollment, preclinical site activations, or information provided to the Company by its vendors with respect to their actual costs incurred. Payments for these activities are based on the terms of the individual arrangements, which may differ from the pattern of costs incurred, and are reflected in the financial statements as prepaid or accrued research and development expense, as the case may be.
Comprehensive Loss
	Comprehensive Loss
	Comprehensive loss is defined as the change in equity of a business enterprise during a period from transactions and other events and circumstances from non-owner sources. Comprehensive loss is comprised of operating net losses and unrealized gains or losses on investments.
Income Taxes
	Income Taxes
	Income taxes are recorded in accordance with ASC Topic 740, Income Taxes (“ASC 740”), which provides for deferred taxes using an asset and liability approach. The Company recognizes deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements or tax returns. Deferred tax assets and liabilities are determined based on the differences between the financial statement and tax bases of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. Valuation allowances are provided, if based upon the weight of available evidence, it is more likely than not that some or all of the deferred tax assets will not be realized.
	The Company accounts for uncertain tax positions in accordance with the provisions of ASC 740. When uncertain tax positions exist, the Company recognizes the tax benefit of tax positions to the extent that the benefit will more likely than not be realized. The determination as to whether the tax benefit will more likely than not be realized is based upon the technical merits of the tax position as well as consideration of the available facts and circumstances. As of December 31, 2014, the Company does not have any uncertain tax positions.
Accretion of Redeemable Convertible Preferred Stock
	Accretion of Redeemable Convertible Preferred Stock
	The Company accounted for the discount due to issuance costs on its Series A redeemable convertible preferred stock using the straight-line method, which approximates the effective interest method, accreting such amounts to preferred stock from the date of issuance to the date of redemption.
Stock-Based Compensation
	Stock-Based Compensation
	The Company’s stock-based compensation plans are more fully described in Note 7 to the Financial Statements. The Company accounts for stock-based compensation in accordance with the provisions of ASC Topic 718, Compensation-Stock Compensation (“ASC 718”), which requires the recognition of expense related to the fair value of stock-based compensation awards in the Statement of Operations.
	For stock options issued to employees and members of the Board for their services on the Board, the Company estimates the grant date fair value of each option using the Black-Scholes option-pricing model. The use of the Black-Scholes option pricing model requires management to make assumptions with respect to the expected term of the option, the expected volatility of the common stock consistent with the expected term of the option, risk-free interest rates, the value of the common stock and expected dividend yield of the common stock. For awards subject to service-based vesting conditions, the Company recognizes stock-based compensation expense equal to the grant date fair value of stock options on a straight-line basis over the requisite service period, which is generally the vesting term. For awards subject to both performance and service-based vesting conditions, the Company recognizes stock-based compensation expense using the straight-line recognition method when it is probable that the performance condition will be achieved. Forfeitures are required to be estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates.
	Share-based payments issued to non-employees are recorded at fair value, and are periodically revalued as the equity instruments vest and are recognized as expense over the related service period. See Note 7 to the Financial Statements for a discussion of the assumptions used by the Company in determining the grant date fair value of options granted under the Black-Scholes option pricing model, as well as a summary of the stock option activity under the Company’s stock-based compensation plan for the period from May 9, 2013 (Date of Inception) to December 31, 2013 and the year ended December 31, 2014.
Basic and Diluted Net Loss Per Share of Common Stock
	Basic and Diluted Net Loss Per Share of Common Stock
	Basic net loss per share of common stock is computed by dividing net loss attributable to common stockholders by the weighted-average number of shares of common stock outstanding during the period, excluding the dilutive effects of convertible preferred stock, unvested restricted stock and stock options. Diluted net loss per share of common stock is computed by dividing the net loss attributable to common stockholders by the sum of the weighted-average number of shares of common stock outstanding during the period plus the potential dilutive effects of convertible preferred stock, unvested restricted stock and stock options outstanding during the period calculated in accordance with the treasury stock method, although these shares and options are excluded if their effect is anti-dilutive. Because the impact of these items is anti-dilutive during periods of net loss, there was no difference between basic and diluted net loss per share of common stock for the period from May 9, 2013 (Date of Inception) to December 31, 2013 or the year ended December 31, 2014.
Recent Accounting Pronouncements
	Recent Accounting Pronouncements
	On August 27, 2014, the FASB issued ASU 2014-15, which provides guidance on determining when and how reporting entities must disclose going-concern uncertainties in their financial statements. The new standard requires management to perform interim and annual assessments of an entity’s ability to continue as a going concern within one year of the date of issuance of the entity’s financial statements (or within one year after the date on which the financial statements are available to be issued, when applicable). Further, an entity must provide certain disclosures if there is “substantial doubt about the entity’s ability to continue as a going concern.” The FASB believes that requiring management to perform the assessment will enhance the timeliness, clarity, and consistency of related disclosures and improve convergence with International Financial Reporting Standards (“IFRS”) (which emphasize management’s responsibility for performing the going-concern assessment). However, the time horizon for the assessment (look-forward period) and the disclosure thresholds under U.S. GAAP and IFRSs will continue to differ.  The Company does not anticipate that the adoption of this standard will have a material impact on its financial statements.
	On June 10, 2014, the FASB issued ASU No. 2014-10, Development Stage Entities (Topic 915): Elimination of Certain Financial Reporting Requirements, Including an Amendment to Variable Interest Entities Guidance in Topic 810, Consolidation.  The guidance is intended to reduce the overall cost and complexity associated with financial reporting for development stage entities without reducing the availability of relevant information.  The Board also believes the changes will simplify the consolidation accounting guidance by removing the differential accounting requirements for development stage entities. As a result of these changes, there no longer will be any accounting or reporting differences in GAAP between development stage entities and other operating entities.  For organizations defined as public business entities the presentation and disclosure requirements in Topic 915 will no longer be required starting with the first annual period beginning after December 15, 2014, including interim periods therein.  Early application is permitted for any annual reporting period or interim period for which the entity’s financial statements have not yet been issued (public business entities) or made available for issuance (other entities).  The Company early adopted this guidance during the year ended December 31, 2014 and, as a result, the Company no longer presents inception-to-date information about the statements of operations, cash flows, and stockholders’ (deficit) equity.
	In June 2014, the FASB issued ASU No. 2014-12, Compensation - Stock Compensation (Topic 718): Accounting for Share-Based Payments When the Terms of an Award Provide that a Performance Target Could be Achieved after the Requisite Service Period, (“ASU 2014-12”). ASU 2014-12 requires that a performance target that affects vesting, and that could be achieved after the requisite service period, be treated as a performance condition. As such, the performance target should not be reflected in estimating the grant date fair value of the award. This update further clarifies that compensation cost should be recognized in the period in which it becomes probable that the performance target will be achieved and should represent the compensation cost attributable to the period(s) for which the requisite service has already been rendered. The Company does not anticipate that the adoption of this standard will have a material impact on its financial statements.
Segment Information
	Segment Information
	Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision-making group, in making decisions on how to allocate resources and assess performance. The Company’s chief operating decision maker is the chief executive officer. The Company and the chief executive officer view the Company’s operations and manage its business as one operating segment. All long-lived assets of the Company reside in the United States.
Reclassifications
	Reclassifications
	Certain expenses that had been previously classified as general and administrative were reclassified into research and development.

Net_Loss_Per_Common_Share_Tabl

Net Loss Per Common Share (Table)	12 Months Ended
	Dec. 31, 2014
Net Loss Per Common Share
Schedule of computation of basic and diluted net loss per share	The following table sets forth the computation of basic and diluted net loss per share for the period indicated (in thousands, except share and per share data):
		Period from May 9,			Year Ended
		2013 (Date of			December 31, 2014
		Inception) to
		December 31, 2013
	Basic and diluted net loss per common share calculation:
	Net loss	$	(10,290	)	$	(20,672	)
	Accretion of redeemable convertible preferred stock	(12		)	(34		)
	Net loss attributable to common stockholders	$	(10,302	)	$	(20,706	)
	Weighted average common shares outstanding	145,528			6,773,673
	Net loss per share of common stock—basic and diluted	$	(70.79	)	$	(3.06	)
Schedule of outstanding securities excluded from the computation of diluted weighted average shares outstanding, as they would have been anti-dilutive		Period from May 9,		Year Ended
		2013 (Date of		December 31, 2014
		Inception) to
		December 31, 2013
	Redeemable convertible preferred stock	3,313,201		—
	Unvested restricted stock	264,190		170,622
	Stock options	681,056		2,011,005
		4,258,447		2,181,627

Fair_Value_Measurements_Table

Fair Value Measurements (Table)	12 Months Ended
	Dec. 31, 2014
Fair Value Measurements.
Schedule of available-for-sale securities unrealized gains and losses from changes in fair value
	(amounts in thousands)	Amortized		Gross		Gross			Fair Value
		Cost		Unrealized		Unrealized
				Gains		(Losses)
	December 31, 2014
	Government enterprise debt securities	$	17,009	$	—	$	—		$	17,009
	Money market funds	8,846		—		—			8,846
	Total included in cash and cash equivalents	25,855		—		—			25,855
	Government enterprise debt securities	69,038		—		(28		)	69,010
	Short-term available-for-sale securities	62,387		—		(25		)	62,362
	Long-term available-for-sale securities	6,651		—		(3		)	6,648
	Total fair value financial instruments	$	94,893	$	—	$	(28	)	$	94,865
Schedule of financial assets and liabilities measured at fair value on a recurring basis
	The Company’s financial assets and liabilities measured at fair value on a recurring basis at December 31, 2014 were as follows (in thousands):
		Fair Value Measurements at Measurement Date:
		Quoted Prices in Active		Significant Other		Significant Unobservable			Total for the Year Ended
		Markets for Identical Assets		Observable Inputs		Inputs			December 31, 2014
		(Level 1)		(Level 2)		(Level 3)
	Assets:
	Government enterprise debt securities	$	—	$	86,019	$	—		$	86,019
	Money market funds	8,846		—		—			8,846
	Totals	$	8,846	$	86,019	$	—		$	94,865

Accrued_Expenses_and_Other_Lia

Accrued Expenses and Other Liabilities (Table)	12 Months Ended
	Dec. 31, 2014
Accrued Expenses and Other Current Liabilities
Schedule of Accrued Expenses and Other Current Liabilities
	Accrued expenses and other current liabilities consisted of the following (in thousands):
		December 31,		December 31,
		2013		2014
	Research and development expenses	$	81	$	742
	General and administrative expenses	108		769
	Share repurchase obligation	—		37
		$	189	$	1,548

Stock_Based_Compensation_Table

Stock Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2014
Stock-Based Compensation
Summary of stock option activity under the 2013 and 2014 Plans
							Average
					Weighted-		Remaining	Aggregate
		Number			Average		Contractual	Intrinsic Value
		of Shares			Exercise Price		Term (in years)	(in thousands)
	Outstanding at May 9, 2013 (Date of Inception)	—			$	—	—
	Granted	681,056			1.18
	Exercised	—			—
	Cancelled	—			—
	Forfeited	—			—
	Outstanding at December 31, 2013	681,056			$	1.18	9.92
	Granted	1,579,970			8.5
	Exercised	(114,818		)	1.45
	Cancelled	—			—
	Forfeited	(135,203		)	3.65
	Outstanding at December 31, 2014	2,011,005			$	6.75	9.47	$	11,375
	Vested and expected to vest at December 31, 2014	1,914,005			$	6.71	9.47	$	10,891
	Exercisable at December 31, 2014	504,751			$	2.19	9.06	$	4,863
	Weighted-average grant date fair value of options granted during the year ended December 31, 2014	$	9.23
Schedule of weighted-average assumptions under the Black-Scholes option pricing model estimate the fair value of option awards
		Period from May 9,		Year Ended
		2013 (Date of		December 31, 2014
		Inception) to
		December 31, 2013
	Risk-free interest rate	2.14	%	1.90	%
	Expected dividend yield	0	%	0	%
	Expected stock price volatility	84.76	%	86.22	%
	Expected term of options (in years)	7.44		6.50
	Expected forfeiture rate	0	%	5.98	%
Schedule of Share-based compensation expense recognized
	Share-based compensation expense recognized was as follows (in thousands):
		Period from May 9,			Year Ended December
		2013 (Date of			31, 2014
		Inception) to
		December 31, 2013
	Research and development	$	15		$	2,002
	General and administrative	55			1,047
		$	70		$	3,049

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies.
Schedule of future minimum lease payments
	Future minimum lease payments as of December 31, 2014 are as follows (in thousands):
		Operating
		Lease
	2015	$	131
	2016	133
	2017	98
	2018	26
		$	388

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Taxes.
Schedule of Deferred Tax Assets and Liabilities	The significant components of the Company’s deferred tax assets are comprised of the following (in thousands):
		Period from May 9,			Year Ended
		2013 (Date of			December 31, 2014
		Inception) to
		December 31, 2013
	Net operating losses	$	1,248		$	7,526
	Accrued expenses	—			194
	Acquired research and development	2,479			2,348
	Research and development credits	108			688
	Stock options	—			967
	Other temporary differences	13			283
	Gross deferred tax assets	3,848			12,006
	Deferred tax valuation allowance	(3,848		)	(12,006		)
		$	—		$	—
Schedule of Effective Income Tax Rate Reconciliation
		December 31, 2013		December 31, 2014
	U.S. statutory income tax rate	34	%	34	%
	State income taxes, net of federal benefit	2.4		3
	Permanent differences	(0.1	)	(0.7	)
	Provision to return true-up	—		(0.1	)
	R&D credit carryforwards	1.1		3.3
	Valuation allowance	(37.4	)	(39.5	)
	Effective tax rate	—	%	—	%

Organization_and_Description_o1

Organization and Description of the Business (Details) (USD $)	1 Months Ended		8 Months Ended	9 Months Ended	12 Months Ended		0 Months Ended
	Jul. 31, 2014	Jul. 31, 2013	Dec. 31, 2013	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Aug. 29, 2014	Jul. 31, 2014
				item	item	item
Organization and Description of the Business
Number of operating segments				1	2	2
Organization and description of the business
Stock split ratio of the common stock	1.5625	2.07
Aggregate gross proceeds from shares sold			$1,000		$74,331,000
Working capital					106,000,000
Accumulated deficit			10,290,000		30,962,000	10,290,000
Cash and cash equivalents			14,994,000		43,930,000	14,994,000
Investments					69,000,000
Short-term investments					62,362,000
Long-term investments					6,648,000
Minimum number of future funding sources sought to continue as a going concern					1
Convertible Preferred Stock [Member]
Organization and description of the business
Aggregate shares of common stock issued upon conversion of all outstanding convertible preferred stock	9,932,042							9,932,042
Initial public offering
Organization and description of the business
Shares of common stock sold							642,000	5,261,538
Initial public offering price (in dollars per share)	13						$13	$13
Aggregate gross proceeds from shares sold							8,300,000	68,400,000
Underwriters underwriting discounts and commissions paid					5,600,000
Offering expense					1,700,000
Aggregate gross proceeds from shares sold					72,400,000
Proceeds from initial public offering							8,300,000	68,400,000
Initial public offering | NEA
Organization and description of the business
Shares of common stock sold								230,769
Aggregate gross proceeds from shares sold								3,000,000
Proceeds from initial public offering								3,000,000
Business Checking Account
Organization and description of the business
Cash and cash equivalents					18,000,000
Certificates of Deposit
Organization and description of the business
Cash and cash equivalents			20,000		20,000	20,000
Money Market Funds
Organization and description of the business
Cash and cash equivalents					$8,900,000

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Detail) (USD $)	8 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2013	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
		item	item	item
Cash and Cash Equivalents
Cash and cash equivalents	$14,994,000		$43,930,000	$14,994,000
Investments
Investments			69,000,000
Short-term investments			62,362,000
Long-term investments			6,648,000
Concentration of Credit Risk
Number of Operating Segments		1	2	2
Fair value of available-for-sale securities			69,000,000
Property and equipment
Depreciation expense			30,000
Research and Development Expenses with a Related Party
Research and development expenses with a related party	323,000		6,947,000
Minimum
Property and equipment
Estimated useful lives of the assets			3 years
Maximum
Property and equipment
Estimated useful lives of the assets			5 years
Business Checking Account
Cash and Cash Equivalents
Cash and cash equivalents			18,000,000
Certificates of Deposit
Cash and Cash Equivalents
Cash and cash equivalents	20,000		20,000	20,000
Money Market Funds
Cash and Cash Equivalents
Cash and cash equivalents			8,900,000
US Government-sponsored Enterprises Debt Securities
Cash and Cash Equivalents
Cash and cash equivalents			17,000,000
Investments
Investments			69,000,000
Concentration of Credit Risk
Fair value of available-for-sale securities			69,000,000
Amount less than original par value			28,000
Array | Research and development
Research and Development Expenses with a Related Party
Research and development expenses with a related party	2,400,000		7,600,000
Array | Redeemable convertible Series A-1 preferred stock | Research and development
Research and Development Expenses with a Related Party
Issuance of redeemable convertible preferred stock	$7,000,000

Net_Loss_Per_Common_Share_Deta

Net Loss Per Common Share (Detail) (USD $)	3 Months Ended	8 Months Ended	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2013	Dec. 31, 2014
Basic and diluted net loss per common share calculation:
Net loss		($10,290)	($20,672)
Accretion of redeemable convertible preferred stock		-12	-34
Net loss attributable to common stockholders		($10,302)	($20,706)
Weighted average common shares outstanding		145,528	6,773,673
Net loss per share of common stock-basic and diluted (in dollars per share)		($70.79)	($3.06)
Outstanding securities excluded from computation of diluted weighted average shares outstanding, as they would have been anti-dilutive
Total (in shares)	4,258,447		2,181,627
Redeemable convertible preferred stock
Outstanding securities excluded from computation of diluted weighted average shares outstanding, as they would have been anti-dilutive
Total (in shares)	3,313,201
Unvested restricted stock
Outstanding securities excluded from computation of diluted weighted average shares outstanding, as they would have been anti-dilutive
Total (in shares)	264,190		170,622
Stock options
Outstanding securities excluded from computation of diluted weighted average shares outstanding, as they would have been anti-dilutive
Total (in shares)	681,056		2,011,005

Fair_Value_Measurements_Detail

Fair Value Measurements (Detail) (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014
Money Market Funds
Net asset value for money market funds (in dollars per unit)	$1
Amortized Cost	$8,846
Fair Value	8,846
Cash and cash equivalents
Amortized Cost	25,855
Fair Value	25,855
US Government-sponsored Enterprises Debt Securities
Amortized Cost	17,009
Fair Value	17,009
Government enterprise AFS securities
Amortized Cost	69,038
Gross Unrealized (Losses)	-28
Fair Value	69,010
Available-for-sale securities
Amortized Cost	94,893
Gross Unrealized (Losses)	-28
Fair Value	94,865
Short-term available-for-sale securities
Amortized Cost	62,387
Gross Unrealized (Losses)	-25
Fair Value	62,362
Long-term available-for-sale securities
Amortized Cost	6,651
Gross Unrealized (Losses)	-3
Fair Value	$6,648

Fair_Value_Measurements_Detail1

Fair Value Measurements (Detail 2) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Fair Value, Measurements, Recurring
Available-for-sale	$94,865
Fair Value, Measurements, Non-recurring
Available-for-sale		0
Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Available-for-sale	8,846
Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Available-for-sale	86,019
Money Market Funds
Investments		0
Money Market Funds | Fair Value, Measurements, Recurring
Available-for-sale	8,846
Money Market Funds | Fair Value, Inputs, Level 1 | Fair Value, Measurements, Recurring
Available-for-sale	8,846
US Government-sponsored Enterprises Debt Securities
Investments		0
US Government-sponsored Enterprises Debt Securities | Fair Value, Measurements, Recurring
Available-for-sale	86,019
US Government-sponsored Enterprises Debt Securities | Fair Value, Inputs, Level 2 | Fair Value, Measurements, Recurring
Available-for-sale	$86,019

Accrued_Expenses_and_Other_Cur1

Accrued Expenses and Other Current Liabilities (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Accrued Expenses and Other Current Liabilities
Accrued expenses and other current liabilities	$1,548	$189
Research and development
Accrued Expenses and Other Current Liabilities
Accrued expenses and other current liabilities	742	81
General and administrative
Accrued Expenses and Other Current Liabilities
Accrued expenses and other current liabilities	769	108
Shares repurchase obligation
Accrued Expenses and Other Current Liabilities
Accrued expenses and other current liabilities	$37

Redeemable_Convertible_Preferr

Redeemable Convertible Preferred Stock and Stockholders' Deficit (Detail) (USD $)	12 Months Ended	0 Months Ended		1 Months Ended	8 Months Ended	0 Months Ended		3 Months Ended	0 Months Ended	2 Months Ended
	Dec. 31, 2014	Jul. 31, 2014	Aug. 29, 2014	Aug. 31, 2014	Dec. 31, 2013	Mar. 18, 2014	Sep. 19, 2013	Sep. 19, 2013	Jul. 03, 2013	Jun. 30, 2014	Jun. 30, 2013	31-May-13	Mar. 31, 2014	Sep. 30, 2013	Jul. 31, 2013	Jun. 24, 2014	Apr. 24, 2014
Capitalization
Capital stock, shares authorized					15,031,954						15,031,954	15,031,954
Common stock, shares authorized	125,000,000			125,000,000	9,375,000						9,375,000	9,375,000
Common stock, par value (in dollars per share)	$0.00			0.0001	$0.00						$0.00	$0.00
Preferred stock, shares authorized				5,000,000	5,656,954						5,656,954	5,656,954
Preferred stock, par value (in dollars per share)				0.0001	$0.00						$0.00	$0.00
Conversion of redeemable convertible preferred stock into common stock	$68,055,000
2013 Equity Incentive Plan
2013 and 2014 Equity Incentive Plan
Shares authorized for issuance	1,544,615				905,796						905,796	905,796
Shares available for future issuance	0				224,734
Shares that could be required to be repurchased	10,166
Shares subject to repurchase liability, amount recorded in accrued expenses	37,000
2014 Equity Incentive Plan
2013 and 2014 Equity Incentive Plan
Shares authorized for issuance	1,092,085
Shares available for future issuance	510,863
Increase in common stock reserved for issuance as percentage of the aggregate number of outstanding shares of common stock	3.00%
2014 Employee Stock Purchase Plan
2013 and 2014 Equity Incentive Plan
Employment period required to participate in the plan		6 months
Maximum shares which may be purchased by eligible employee		1,000
Maximum value of shares which may be purchased by eligible employee		25,000
Award term		P10Y
Shares issued under the plan		0
Initial public offering
Capitalization
Shares issued		5,261,538	642,000
Number of shares of common stock into which outstanding convertible preferred stock are converted				9,932,042
Minimum | 2014 Employee Stock Purchase Plan
2013 and 2014 Equity Incentive Plan
Period per week on a regular schedule required to participate in the plan		P20H
Common Stock
Capitalization
Share purchase price (in dollars per share)		$0.00								$0.00
Redeemable convertible Series A preferred stock
Capitalization
Preferred stock, shares authorized	0				5,156,250						5,156,250	5,156,250
Preferred stock, par value (in dollars per share)	$0.00				$0.00
Shares issued					2,812,497
Shares issued	0				2,812,497
Share purchase price (in dollars per share)													$6.40	$6.40
Redeemable convertible Series A preferred stock | Investors | Stock purchase agreement
Capitalization
Preferred stock, shares authorized									2,812,497
Shares issued						2,343,753	1,249,997
Shares issued									1,562,500
Net proceeds						15,000,000		17,800,000
Share purchase price (in dollars per share)						$6.40
Redeemable convertible Series A-1 preferred stock
Capitalization
Preferred stock, shares authorized	0				500,704						500,704	500,704
Preferred stock, par value (in dollars per share)	$0.00				$0.00
Shares issued	0				500,704
Share purchase price (in dollars per share)															$14.07
Redeemable convertible Series A-1 preferred stock | Collaboration agreement | Array
Capitalization
Shares issued									500,704
Conversion of redeemable convertible preferred stock into common stock									7,000,000
Redeemable convertible Series B preferred stock
Capitalization
Shares issued	3,166,233
Share purchase price (in dollars per share)										$8.97
Redeemable convertible Series B preferred stock | Stock purchase agreement
Capitalization
Preferred stock, par value (in dollars per share)																$0.00	$0.00
Shares issued																501,890	2,664,343
Net proceeds										$28,400,000
Share purchase price (in dollars per share)																$8.97	$8.97

Stock_Based_Compensation_Detai

Stock Based Compensation (Details) (USD $)	12 Months Ended	0 Months Ended
	Dec. 31, 2014	Jul. 31, 2014	Dec. 31, 2013	Jun. 30, 2013	31-May-13
2013 Equity Incentive Plan
2013 and 2014 Equity Incentive Plan
Shares authorized for issuance	1,544,615		905,796	905,796	905,796
Shares available for future issuance	0		224,734
Shares that could be required to be repurchased	10,166
Shares subject to repurchase liability, amount recorded in accrued expenses	$37,000
2014 Equity Incentive Plan
2013 and 2014 Equity Incentive Plan
Shares authorized for issuance	1,092,085
Shares available for future issuance	510,863
Increase in common stock reserved for issuance as percentage of the aggregate number of outstanding shares of common stock	3.00%
2014 Employee Stock Purchase Plan
2013 and 2014 Equity Incentive Plan
Employment period required to participate in the plan		6 months
Maximum shares which may be purchased by eligible employee		1,000
Maximum value of shares which may be purchased by eligible employee		$25,000
Award term		P10Y
Shares issued under the plan		0
2014 Employee Stock Purchase Plan | Minimum
2013 and 2014 Equity Incentive Plan
Period per week on a regular schedule required to participate in the plan		P20H

Stock_Based_Compensation_Detai1

Stock Based Compensation (Details 2) (USD $)	8 Months Ended	12 Months Ended		3 Months Ended	0 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2013	Jul. 31, 2014	Jun. 30, 2013	31-May-13
2013 and 2014 Equity Incentive Plans | Stock option
Number of Shares
Outstanding at the beginning of the period (in shares)		681,056
Granted (in shares)	681,056	1,579,970
Exercised (in shares)		-114,818
Forfeited (in shares)		-135,203
Outstanding at the end of the period (in shares)	681,056	2,011,005	681,056	681,056
Vested and expected to vest at the end of the period (in shares)		1,914,005
Exercisable at the end of the period (in shares)		504,751
Weighted average grant date fair value of options granted (in dollars per share)		$9.23
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)		$1.18
Granted (in dollars per share)	$1.18	$8.50
Exercised (in dollars per share)		$1.45
Forfeited (in dollars per share)		$3.65
Outstanding at the end of the period (in dollars per share)	$1.18	$6.75	$1.18	1.18
Vested and expected to vest at the end of the period (in dollars per share)		$6.71
Exercisable at the end of the period (in dollars per share)		$2.19
Average Remaining Contractual Term
Outstanding at the beginning of the period		9 years 5 months 19 days	9 years 11 months 1 day
Outstanding at the end of the period		9 years 5 months 19 days	9 years 11 months 1 day
Vested and expected to vest at the end of the period		9 years 5 months 19 days
Exercisable at the end of the period		9 years 22 days
Aggregate Intrinsic value
Outstanding at the end of the period		$11,375,000
Vested and expected to vest at the end of the period		10,891,000
Exercisable at the end of the period		4,863,000
2013 Equity Incentive Plan
2013 and 2014 Equity Incentive Plan
Shares authorized for issuance	905,796	1,544,615	905,796	905,796		905,796	905,796
Shares available for future issuance	224,734	0	224,734	224,734
Shares that could be required to be repurchased		10,166
Shares subject to repurchase liability, amount recorded in accrued expenses		$37,000
2013 Equity Incentive Plan | Stock option
Weighted average assumptions used to estimate fair value of option awards using Black Scholes option pricing model
Risk-free interest rate	2.14%	1.90%
Expected dividend yield	0.00%	0.00%		0.00%
Expected stock price volatility	84.76%	86.22%
Expected term of options (in years)	7 years 5 months 9 days	6 years 6 months
Expected forfeiture rate	0.00%	5.98%
2014 Equity Incentive Plan
2013 and 2014 Equity Incentive Plan
Shares authorized for issuance		1,092,085
Shares available for future issuance		510,863
Increase in common stock reserved for issuance as percentage of the aggregate number of outstanding shares of common stock		3.00%
2014 Employee Stock Purchase Plan
2013 and 2014 Equity Incentive Plan
Employment period required to participate in the plan					6 months
Maximum shares which may be purchased by eligible employee					1,000
Award term					P10Y

Stock_Based_Compensation_Detai2

Stock Based Compensation (Details 3) (USD $)	12 Months Ended	8 Months Ended	0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Jul. 31, 2014
2013 and 2014 Equity Incentive Plans
Stock based compensation expense
Unrecognized compensation expense	$13,500,000
Weighted-average period for recognition	3 years 5 months 23 days
2013 Equity Incentive Plan
Stock based compensation expense
Share-based compensation expense	3,049,000	70,000
2013 Equity Incentive Plan | Research and development
Stock based compensation expense
Share-based compensation expense	2,002,000	15,000
2013 Equity Incentive Plan | General and administrative
Stock based compensation expense
Share-based compensation expense	1,047,000	55,000
2014 Employee Stock Purchase Plan
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Right to purchase common stock (in shares)	149,600
Employment period required to participate in the plan			6 months
Maximum value of shares which may be purchased by eligible employee			25,000
Maximum shares which may be purchased by eligible employee			1,000
Award term			P10Y
Shares issued under the plan			0
2014 Employee Stock Purchase Plan | First day of trading
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Purchase price as percentage of fair market value of a share of common stock			85.00%
2014 Employee Stock Purchase Plan | Last day of trading in offering period
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Purchase price as percentage of fair market value of a share of common stock			85.00%
2014 Employee Stock Purchase Plan | Minimum
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Period per week on a regular schedule required to participate in the plan			P20H
Stock option | 2013 and 2014 Equity Incentive Plans
Stock based compensation expense
Unrecognized compensation expense	13,500,000
Stock option | Non-employees | 2013 and 2014 Equity Incentive Plans
Stock based compensation expense
Unrecognized compensation expense	$2,200,000
Restricted Stock Awards
Restricted Stock
Granted (in shares)		264,189
Vested (in shares)	93,567
Expected to vest (in shares)	170,622
Vesting period	3 years

Commitments_and_Contingencies_1

Commitments and Contingencies (Detail) (USD $)	8 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Aug. 31, 2014	Jun. 30, 2013	31-May-13
Commitments and contingencies
2015		$131,000
2016		133,000
2017		98,000
2018		26,000
Total		388,000
Rent expense under operating leases	1,529,000	55,614,000
Related party research and development expenses	9,384,000	7,568,000
Research and development	323,000	6,947,000
Redeemable convertible preferred stock, par value (in dollars per share)	$0.00		$0.00	$0.00	$0.00
Redeemable convertible Series A-1 preferred stock
Commitments and contingencies
Shares issued	500,704	0
Redeemable convertible preferred stock, par value (in dollars per share)	$0.00	$0.00
Collaboration agreement | Array
Commitments and contingencies
Number of additional renewal periods options available to the entity to extend agreement term (in periods)		2
Term of additional renewal periods options available to the entity to extend agreement term (in options)		1 year
Number of discovery targets designated by the parties (in targets)		12
Number of discovery targets which are to be selected for additional study on or before January 2015 (in targets)		7
Number of discovery targets which are to be selected for additional study on or before January 2016		4
Maximum number of candidates which can be selected by the entity for modest additional payment (in candidates)		5
Related party research and development expenses	9,400,000	7,600,000
Collaboration agreement | Product candidates directed to TRK | Array
Commitments and contingencies
Maximum milestone payments which the entity could be required to pay		222,000,000
Collaboration agreement | Product candidates directed to targets other than TRK | Array
Commitments and contingencies
Maximum milestone payments which the entity could be required to pay		213,000,000
Collaboration agreement | Related party | Product candidates directed to targets other than TRK | Array
Commitments and contingencies
Research and development	$7,000,000

Income_Taxes_Details

Income Taxes (Details) (USD $)	8 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014
Income Taxes.
Loss before income taxes	($10,300,000)	($20,700,000)
Increase in valuation allowance	3,800,000	8,200,000
Components of Deferred Tax Assets [Abstract]
Net operating losses	1,248,000	7,526,000
Accrued expenses		194,000
Acquired research and development	2,479,000	2,348,000
Research and development credits	108,000	688,000
Stock options		967,000
Other temporary differences	13,000	283,000
Gross deferred tax assets	3,848,000	12,006,000
Deferred tax asset valuation allowance	($3,848,000)	($12,006,000)

Income_Taxes_Details_2

Income Taxes (Details 2) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Income Taxes.
U.S. federal net operating loss carryforwards	$20.30	$3.40
U. S. state net operating loss carryforwards	$10.60	$1.40
Effective Income Tax Rate Reconciliation, Percent [Abstract]
U. S. statutory income tax rate	34.00%	34.00%
State income taxes, net of federal benefit	3.00%	2.40%
Permanent differences	-0.70%	-0.10%
Provision to return true-up	-0.10%
R&D credit carryforwards	3.30%	1.10%
Valuation allowance	-39.50%	-37.40%
Effective tax rate	0.00%	0.00%

Related_Party_Transactions_Det

Related Party Transactions (Detail) (USD $)	8 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014
Related party transactions
Related party research and development expenses	$9,384,000	$7,568,000
Common stock, shares outstanding	452,896	16,634,063
Collaboration agreement | Array
Related party transactions
Related party research and development expenses	9,400,000	7,600,000
Prepaid expenses		600,000
Consulting Letter Agreement | Dr. Lori Kunkel
Related party transactions
Common stock, shares outstanding		26,840
Shares available for purchase via stock options		109,374
Value of common stock held and shares available via stock options		1,600,000
Maximum monthly fees for consulting work		15,000
Expenses recognized for consulting work		100,127,000
Accounts payable to related parties		$15,000,000

Forward_Stock_Split_Detail

Forward Stock Split (Detail)	1 Months Ended
	Jul. 31, 2014	Jul. 31, 2013
Class of Stock Disclosures [Abstract]
Stock split ratio of the common stock	1.5625	2.07

Subsequent_Events_Detail

Subsequent Events (Detail) (Subsequent event, Stock options, 2014 Equity Incentive Plan, USD $)	1 Months Ended
	Mar. 31, 2015	Jan. 31, 2015
Subsequent events
Granted (in shares)		43,625
Maximum
Subsequent events
Granted (in dollars per share)		$13.75
Vesting period	4 years	4 years
Minimum
Subsequent events
Granted (in dollars per share)		$11.35
Vesting period	3 years	3 years