Relypsa (RLYP) 10-K 12 Mar 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Mar. 03, 2015	Jun. 30, 2014
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	RLYP
Entity Registrant Name	Relypsa Inc
Entity Central Index Key	1416792
Current Fiscal Year End Date	-19
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Common Stock, Shares Outstanding		41,306,955
Entity Public Float			$464,242,623

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Current assets:
Cash and cash equivalents	$30,264	$94,759
Short-term investments	105,493
Restricted cash	268
Other receivables	425	241
Prepaid expenses and other current assets	2,756	1,657
Total current assets	139,206	96,657
Restricted cash	686
Property and equipment, net	3,467	5,624
Intangible assets		14
Other assets	8,480	3,736
Total assets	151,839	106,031
Current liabilities:
Accounts payable	3,156	4,736
Accrued payroll and related expenses	4,393	1,794
Accrued liabilities	7,778	5,947
Deferred rent, current portion	1,156	640
Line of credit	432	408
Capital loan		4,304
Total current liabilities	16,915	17,829
Long-term liabilities:
Deferred rent, long-term	983	4,394
Line of credit	267	630
Capital loan	15,213	8,328
Total liabilities	33,378	31,181
Commitments and contingencies
Stockholders’ equity:
Preferred stock: $0.001 par value; 5,000 shares authorized at December 31, 2014 and 2013, respectively; no shares issued and outstanding at December 31, 2014 and 2013
Common stock: $0.001 par value; 300,000 shares authorized at December 31, 2014 and 2013; 35,045 and 29,710 shares issued and outstanding at December 31, 2014 and 2013, respectively	35	30
Additional paid-in capital	424,175	300,660
Accumulated other comprehensive income	-21
Accumulated deficit	-305,728	-225,840
Total stockholders’ equity	118,461	74,850
Total liabilities and stockholders’ equity	$151,839	$106,031

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Statement Of Financial Position [Abstract]
Preferred stock, par value	$0.00	$0.00
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$0.00	$0.00
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares issued	35,045,000	29,710,000
Common stock, shares outstanding	35,045,000	29,710,000

Consolidated_Statements_of_Ope

Consolidated Statements of Operations (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating expenses:
Research and development	$16,596	$11,647	$11,075	$10,909	$10,914	$12,158	$13,295	$22,604	$50,227	$58,971	$36,052
General and administrative	10,486	7,311	5,322	4,795	3,732	2,685	2,988	2,535	27,914	11,940	7,285
Total operating expenses	27,082	18,958	16,397	15,704	14,646	14,843	16,283	25,139	78,141	70,911	43,337
Loss from operations	-27,082	-18,958	-16,397	-15,704	-14,646	-14,843	-16,283	-25,139	-78,141	-70,911	-43,337
Interest and other income (expense), net	69	17	36	27	13,314	-10,382	-3,549	-864	149	-1,481	-382
Interest expense	-635	-494	-376	-391	-407	-410	-388	-248	-1,896	-1,453	-6
Net loss	-27,648	-19,435	-16,737	-16,068	-1,739	-25,635	-20,220	-26,251	-79,888	-73,845	-43,725
Deemed dividend to preferred stockholders					-7,336					-7,336	-18,716
Net loss attributable to common stockholders	($27,648)	($19,435)	($16,737)	($16,068)	($9,075)	($25,635)	($20,220)	($26,251)	($79,888)	($81,181)	($62,441)
Net loss per share attributable to common stockholders, basic and diluted	($0.80)	($0.57)	($0.51)	($0.54)	($0.68)	($79.91)	($64.87)	($84.61)	($2.43)	($22.42)	($205.45)
Weighted average common shares used to compute net loss per share attributable to common stockholders, basic and diluted	34,368,776	34,064,780	33,141,384	29,710,415	13,430,244	320,802	311,703	310,275	32,837,508	3,620,235	303,927

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Loss (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement Of Income And Comprehensive Income [Abstract]
Net loss	($79,888)	($73,845)	($43,725)
Other comprehensive (loss):
Unrealized (loss) gain on available-for-sale securities, net of tax	-21	-23	15
Total comprehensive loss	($79,909)	($73,868)	($43,710)

Consolidated_Statements_of_Con

Consolidated Statements of Convertible Preferred Stock and Stockholders Equity (Deficit) (USD $)	Total	Series C-1	Series C-2	Common Stock	Additional Paid-In Capital	Additional Paid-In Capital	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit	Accumulated Deficit	Accumulated Deficit	Shelf Registration Statement	Shelf Registration Statement	Shelf Registration Statement	Convertible Preferred Stock	Convertible Preferred Stock	Convertible Preferred Stock
In Thousands, except Share data						Series C-1	Series C-2			Series C-1	Series C-2		Common Stock	Additional Paid-In Capital		Series C-1	Series C-2
Beginning balance at Dec. 31, 2011	($86,275)				$12,303			$8	($98,586)						$112,847
Beginning balance, shares at Dec. 31, 2011				270,000											8,982,000
Issuance of preferred stock for cash																64,571
Warrants deemed dividend		-18,716				-13,643				-5,073
Exercise of stock options	163				163
Stock-based compensation	1,177				1,177
Unrealized gain (loss) on investments	15							15
Net loss	-43,725								-43,725
Issuance of preferred stock for cash, shares																7,077,000
Exercise of stock options, shares				40,000
Ending balance at Dec. 31, 2012	-147,361							23	-147,384						177,418
Ending balance, shares at Dec. 31, 2012				310,000											16,059,000
Issuance of preferred stock for cash																	14,983
Warrants deemed dividend			-7,336				-2,725				-4,611
Exercise of warrants	397			3	394
Conversion of preferred stock to common stock	192,401			19	192,382										-192,401
Reclassification of warrant liability	28,647				28,647
Exercise of stock options	97				97
Stock-based compensation	3,909				3,909
Issuance of common stock, net of issuance cost	77,964			8	77,956
Unrealized gain (loss) on investments	-23							-23
Net loss	-73,845								-73,845
Issuance of preferred stock for cash, shares																	1,633,000
Exercise of warrants, shares				2,573,000
Conversion of preferred stock to common stock, shares				18,925,000											-17,692,000
Exercise of stock options, shares				24,000
Issuance of common stock, shares				7,878,000
Ending balance at Dec. 31, 2013	74,850			30	300,660				-225,840
Ending balance, shares at Dec. 31, 2013				29,710,000
Exercise of stock options	2,923				2,923
Stock-based compensation	9,695				9,695
Issuance of common stock, net of issuance cost	94,607			4	94,603							16,061	1	16,060
Unrealized gain (loss) on investments	-21							-21
Net loss	-79,888								-79,888
Issuance of warrants	234				234
Exercise of warrants, shares				12,000
Exercise of stock options, shares				677,000
Issuance of common stock, shares				4,131,000									515,000
Ending balance at Dec. 31, 2014	$118,461			$35	$424,175			($21)	($305,728)
Ending balance, shares at Dec. 31, 2014				35,045,000

Consolidated_Statements_of_Con1

Consolidated Statements of Convertible Preferred Stock and Stockholders Equity (Deficit) (Parenthetical) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Common Stock
Issuance Costs	$6,600	$8,691
Issuance of common stock, shares	4,131,000	7,878,000
Convertible Preferred Stock | Series C-1
Issuance Costs			429
Convertible Preferred Stock | Series C-2
Issuance Costs		$17

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating activities
Net loss	($79,888)	($73,845)	($43,725)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	5,492	1,088	394
Stock-based compensation	9,695	3,909	1,177
Amortization of debt discount	174	136
Amortization of debt issuance costs	24	32
Accretion of final debt payment	498	350
Accrued interest on capital loan	71
Revaluation of convertible preferred stock warrant liability		1,564	465
Changes in assets and liabilities:
Other receivables	-184	2,742	-2,983
Prepaid expenses and other current assets	-1,123	-669	9
Other assets and restricted cash	-5,698	-3,431	-84
Accounts payable	-1,580	1,264	1,483
Accrued and other liabilities	4,552	3,465	3,475
Deferred rent	-2,895	755	4,279
Net cash used in operating activities	-70,862	-62,640	-35,510
Investing activities
Purchase of investments	-163,205	-10,087	-46,189
Proceeds from maturities of short-term investments	56,965	49,198	20,363
Purchase of property and equipment	-2,595	-3,776	-2,652
Net cash provided (used in) by investing activities	-108,835	35,335	-28,478
Financing activities
Net proceeds from issuance of convertible preferred stock		14,983	64,571
Net proceeds from public offerings	94,607	77,964
Proceeds from common stock option exercises	2,923	97	152
Proceeds from warrant exercises		397
Net proceeds from capital loan	3,549	12,404
Repayment of capital loan	-1,530		-691
Net proceeds from equipment line of credit		1,225
Repayment of equipment line of credit	-408	-227	-72
Net cash provided by financing activities	115,202	106,843	63,960
Net (decrease) increase in cash and cash equivalents	-64,495	79,538	-28
Cash and cash equivalents at beginning of period	94,759	15,221	15,249
Cash and cash equivalents at end of period	30,264	94,759	15,221
Supplemental disclosures of cash flow information
Cash paid for interest	996	854	5
Cash paid for taxes	1	2	1
Noncash investing and financing activities
Unrealized gain (loss) on investments	-21	-23	15
Contingent put option liability	110
Reclassification of convertible preferred stock warrant liability due to the exercise of the warrants		28,072
Reclassification of convertible preferred stock warrant liability due to conversion to warrants to purchase common stock		575
Accounts payable related to purchase of property and equipment			2,520
Line of Credit
Noncash investing and financing activities
Issuance of warrants		25
Capital Loan
Noncash investing and financing activities
Issuance of warrants	234	193
Series C Financings
Noncash investing and financing activities
Issuance of warrants		7,336	18,716
Shelf Registration
Financing activities
Net proceeds from public offerings	$16,061

Organization

Organization	12 Months Ended
	Dec. 31, 2014
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Organization
	1. Organization
	Relypsa, Inc. (Relypsa or the Company) is a biopharmaceutical company dedicated to the development and commercialization of new non-absorbed polymeric drugs for important applications in renal, cardiovascular and metabolic disease. Relypsa’s lead product candidate is Patiromer for Oral Suspension, or Patiromer FOS. A new drug application has been filed with the U.S. Food and Drug Administration for Patiromer FOS for the treatment of hyperkalemia and is currently under review. The Company commenced operations on October 29, 2007. The Company’s principal operations are based in Redwood City, California and it operates in one segment. On October 28, 2014, Relypsa UK LTD was incorporated as a wholly-owned subsidiary of the Company.
	Offerings
	On November 15, 2013, the Company completed its initial public offering, or IPO, which resulted in net proceeds of $78.0 million from the issuance of 7,877,500 shares of common stock. In connection with the IPO, on an as converted basis, 18,924,883 shares of the Company’s convertible preferred stock and 2,563,076 convertible preferred stock warrants were converted into 21,487,959 shares of common stock. In addition, upon completing the IPO, the liabilities related to the convertible preferred stock warrants were re-classified to additional paid-in-capital and are no longer subject to re-measurement.
	In connection with the completion of its IPO, on November 15, 2013, the Company filed an amended and restated certificate of incorporation and bylaws, which, among other things, changed the number of authorized shares of common stock to 300,000,000 shares and preferred stock to 5,000,000 shares.
	On April 16, 2014, the Company completed an underwritten public offering of 4,130,611 shares of common stock at an offering price of $24.50 per share. The Company received aggregate net proceeds of $94.6 million, after deducting the underwriting discounts and offering related transaction costs.
	On December 4, 2014, the Company filed a shelf registration statement on Form S-3, which permitted: (a) the offering, issuance and sale by the Company of up to a maximum aggregate offering price of $250.0 million of its common stock, preferred stock, debt securities, warrants and/or units; and (b) as part of the $250.0 million, the offering, issuance and sale by the Company of up to a maximum aggregate offering price of $70.0 million of its common stock that may be issued and sold under a sales agreement with Cantor Fitzgerald & Co in one or more at-the-market offerings. As of December 31, 2014, the Company had sold 514,710 shares pursuant to its at-the-market offering program at an average price of $32.17 for an aggregate offering price of $16.6 million and the Company received aggregate net proceeds of $16.1 million. During January and February 2015, the Company sold an additional 1,549,910 shares at an average price of $34.48 for an aggregate offering price of $53.4 million. The Company received aggregate net proceeds of $51.8 million.
	On March 3, 2015, the Company completed an underwritten public offering of 4,485,000 shares of common stock at an offering price of $38.50 per share for gross proceeds of $172.3 million. The Company estimates net proceeds from the offering to be $161.9 million, after deducting the underwriting discounts and estimated expenses.

Summary_of_Significant_Account

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies
	Basis of Presentation
	The accompanying consolidated financial statements include the accounts of Relypsa and its wholly-owned subsidiary, Relypsa UK LTD, and have been prepared in accordance with U.S. generally accepted accounting principles (“US GAAP”). Intercompany transactions and balances have been eliminated in consolidation.
	Use of Estimates
	The preparation of consolidated financial statements in conformity with US GAAP requires management to make estimates and judgments that affect the amounts reported in the consolidated financial statements and accompanying notes. On an ongoing basis, management evaluates its estimates, including those related to clinical trial accruals, manufacturing accruals, fair value of assets and liabilities, common stock, income taxes, and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could differ from those estimates.
	Liquidity
	The Company has never been profitable and, as of December 31, 2014, the Company has an accumulated deficit of $305.7 million. The Company has incurred net losses of approximately $79.9 million, $73.8 million and $43.7 million in the years ended December 31, 2014, 2013 and 2012, respectively. The Company expects to continue to incur net operating losses for at least the next two years, as the Company continues the development of, seeks regulatory approval for, and if approved, begins to commercialize, Patiromer FOS. The Company will need additional funding to support its future operating activities and adequate funding may not be available to the Company on acceptable terms, or at all. The Company’s failure to obtain sufficient funds on acceptable terms when needed could have a material adverse effect on the business, results of operations, and financial condition. The Company will need to generate significant revenues to achieve profitability and may never do so.
	Cash Equivalents
	The Company considers all highly liquid investments purchased with original maturities of 90 days or less at the date of purchase to be cash equivalents.
	Investments
	Investments consist of debt securities classified as available-for-sale and have maturities greater than 90 days from the date of acquisition. Short-term investments have maturities than 365 days as of the balance sheet date. Long-term investments have maturities greater than 365 days as of the balance sheet date. Investments are carried at fair value based upon quoted market prices or pricing models for similar securities. Unrealized gains and losses on available-for-sale securities are excluded from earnings and are reported as a component of accumulated comprehensive income (loss). Realized gains or losses on the sale of all such securities are reported in interest and other income (expense), net and computed using the specific identification method.
	Concentrations of Credit Risk
	Financial instruments that potentially subject the Company to a concentration of credit risk consist primarily of cash, cash equivalents and short-term investments. The Company invests its excess cash in money market accounts, U.S. Treasury notes, municipal bonds, agency bonds, corporate notes, certificates of deposit and commercial paper. Other than for obligations of the U.S. government, the Company’s policy is that no more than 5% of its investments may be concentrated in a single issuer. Bank deposits are held by a single financial institution having a strong credit rating and these deposits may at times be in excess of insured limits. The Company is exposed to credit risk in the event of a default by the financial institutions holding its cash, cash equivalents and short-term investments and issuers of investments to the extent recorded on the balance sheets. The Company’s investment policy limits investments to certain types of debt securities issued by the U.S. government, its agencies and institutions with investment-grade credit ratings and places restrictions on maturities and concentration by type and issuer.
	Restricted Cash
	Restricted cash related to the Company’s leases consist of irrevocable letters of credit that are collateralized by restricted deposits held at the Company’s bank over a term that is consistent with the corresponding lease.
	Property and Equipment
	Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the related assets, ranging from three to seven years. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful lives of the respective assets.
	Intangible Assets
	Intangible assets consist of acquired patent rights that arose from the Company’s acquisition of assets from Ilypsa (See Note 9 for further details). Acquired patent rights were assigned an estimated useful life of seven years and are being amortized using the straight-line method. The carrying value of these patent rights has been fully amortized as of December 31, 2014.
	Impairment of Long-Lived Assets
	The Company identifies and records impairment losses on long-lived assets used in operations when events and changes in circumstances indicate that the carrying amount of an asset might not be recoverable. Recoverability is measured by comparing the anticipated undiscounted future net cash flows to the related asset’s carrying value. If an asset is considered impaired, the asset is written down to fair value, which is determined based either on discounted cash flows or appraised value, depending on the nature of the asset. The Company has not experienced any impairment during 2014, 2013, and 2012.
	Research and Development Expenses
	Research and development costs are charged to expense as incurred and consist of costs incurred to further the Company’s research and development activities including salaries and related employee benefits, costs associated with clinical trials, costs related to pre-commercialization manufacturing activities such as manufacturing process validation activities and the manufacturing of commercial supply prior to approval, nonclinical research and development activities, regulatory activities, research-related overhead expenses and fees paid to external service providers and contract research and manufacturing organizations that conduct certain research and development activities on behalf of the Company.
	Clinical Trial Accruals
	Clinical trial costs are a component of research and development expenses. The Company accrues and expenses clinical trial activities performed by third parties based upon actual work completed in accordance with agreements established with clinical research organizations and clinical sites. The Company determines the actual costs through discussions with internal personnel and external service providers as to the progress or stage of completion of trials or services and the agreed-upon fee to be paid for such services.
	Contract Manufacturing Accruals
	Contract Manufacturing costs are a component of research and development expenses. The Company accrues and expenses contract manufacturing activities performed by third parties based upon the actual or the estimated amount of work completed in accordance with agreements established with contract manufacturing organizations. The Company determines the actual costs or it estimates the costs through discussions with internal personnel and external service providers as to the manufacturing activities that were performed or completed and the agreed-upon fee to be paid for such services.
	Income Taxes
	The Company uses the liability method to account for income taxes. Under this method, deferred tax assets and liabilities are determined based on differences between the financial statement carrying amounts of existing assets and liabilities and their tax bases. Deferred tax assets and liabilities are measured using enacted tax rates applied to taxable income in the years in which those temporary differences are expected to be recovered or settled. A valuation allowance is established when necessary to reduce deferred tax assets to the amount expected to be realized. Financial statement effects of uncertain tax positions are recognized when it is more–likely-than-not, based on the technical merits of the position, that they will be sustained upon examination. Interest and penalties related to unrecognized tax benefits are included within the provision for income tax.
	Stock-Based Compensation
	The Company accounts for stock-based compensation based on the fair value of the share-based awards that are ultimately expected to vest. The fair value of employee stock options granted is estimated on the date of grant using the Black-Scholes option pricing model, and is recognized in expense over the service period using the straight-line method, net of estimated forfeitures. Forfeiture estimates are adjusted to the extent that actual forfeitures differ from the prior estimates. The fair value of restricted stock unit awards used in the Company’s expense recognition method is based on the number of shares granted and the closing price of the Company’s common stock on the date of the grant. Such value is recognized as an expense over the service period using the straight-line method, net of estimated forfeitures. The Company adopted an employee stock purchase plan (ESPP) on November 2013 and in August 2014, the ESPP was initiated, pursuant to which eligible employees can purchase shares of the Company’s common stock at a price per share equal to the lesser of 85% of the fair market value on the first day of the offering period or 85% of the fair market value on the purchase date. The fair value of each award is estimated on the first day of the offering period using the Black-Scholes option pricing model and is recognized in expense over the service period using the straight-line method, net of estimated forfeitures.
	The Company records the expense attributable to nonemployee services paid with share-based awards based on the estimated fair value of the awards determined using the Black-Scholes option pricing model. The measurement of stock-based compensation for nonemployees is subject to periodic adjustments as the options vest, and the expense is recognized over the period during which services are received.
	Convertible Preferred Stock
	Prior to the Company’s IPO, the Company recorded all shares of convertible preferred stock at their respective fair values, net of issuance costs, on the dates of issuance. Upon completing the IPO, all shares of the Company’s convertible preferred stock then outstanding converted into 18,924,883 shares of our common stock.
	Convertible Preferred Stock Warrant Liability
	Prior to the Company’s IPO, freestanding warrants for shares that were either puttable or redeemable were classified as liabilities on the balance sheets and were carried at their estimated fair value. The preferred stock underlying the warrants was redeemable in certain circumstances. At the end of each reporting period, changes in the estimated fair value during the period were recorded in interest and other income (expense), net. The consummation of the Company’s IPO resulted in the conversion of all classes of the Company’s preferred stock into common stock. Upon such conversion of the underlying classes of preferred stock, the warrants were reclassified as a component of equity and were no longer subject to re-measurement.
	Deferred Rent
	Rent expense is recognized on a straight-line basis over the noncancelable term of the Company’s operating lease and, accordingly, the Company records the difference between cash rent payments and the recognition of rent expense as a deferred rent liability. In accordance with the terms of the lease for the Company’s office and laboratory space as of December 31, 2014, the Company also recorded lessor-funded lease incentives, such as reimbursable leasehold improvements, as a deferred rent liability, which is amortized as a reduction of rent expense over the noncancelable term of its operating lease. During June 2014, the lease was modified, see Note 8 for further details.
	Net Loss per Common Share Attributable to Common Stockholders
	Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of common shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share of common stock is the same as basic net loss per share of common stock, since the effects of potentially dilutive securities are antidilutive. The net loss per share of common stock attributable to common stockholders is computed using the two-class method required for participating securities. All series of the Company’s convertible preferred stock are considered to be participating securities as they are entitled to participate in undistributed earnings with shares of common stock. Due to the Company’s net loss, there is no impact on earnings per share calculation in applying the two-class method since the participating securities have no legal requirement to share in any losses.
	The following outstanding shares of common stock equivalents were excluded from the computations of diluted net loss per common share attributable to common stockholders for the periods presented as the effect of including such securities would be antidilutive (in thousands):
		Year Ended December 31,
		2014		2013		2012
	Convertible preferred stock—as converted to common stock		—		—		17,281
	Warrants to purchase convertible preferred stock—as converted to common stock		—		—		2,236
	Warrants to purchase common stock		56		64		—
	Options to purchase common stock		3,813		3,545		2,163
	Restricted stock units		123		—		—
	Common stock subject to repurchase		1		5		7
			3,993		3,614		21,687
	Recent Accounting Pronouncements
	In June 2014, FASB issued Accounting Standards Update No. 2014-10, Development Stage Entities: Elimination of Certain Financial Reporting Requirements. The update removes the definition of a development stage entity from FASB ASC 915 and eliminates the requirement for development stage entities to present inception-to-date information on the statements of operations, cash flows and stockholders’ equity (deficit). The Company early adopted this standard for the period covered by the report herein.
	In August 2014, FASB issued Accounting Standards Update No. 2014-15, Presentation of Financial Statements Going Concern – Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern. The amendments require management to assess an entity’s ability to continue as a going concern by incorporating and expanding upon certain principles that are currently in U.S. auditing standards. Specifically, the amendments: (1) provide a definition of the term substantial doubt; (2) require an evaluation every reporting period including interim periods; (3) provide principles for considering the mitigating effect of management’s plans; (4) require certain disclosures when substantial doubt is alleviated as a result of consideration of management’s plans; (5) require an express statement and other disclosures when substantial doubt is not alleviated; and (6) require an assessment for a period of one year after the date that the financial statements are issued (or available to be issued). ASU 2014-15 will be effective for annual periods ending after December 15, 2016 and interim periods within annual periods beginning after December 15, 2016 with early adoption permitted. ASU 2014-15 will be effective for the Company beginning with its annual report for fiscal 2016 and interim periods thereafter. The Company has not yet determined the effect of the adoption of this standard on the Company’s consolidated financial statements.

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements
	Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value should maximize the use of observable inputs and minimize the use of unobservable inputs. The accounting guidance for fair value establishes a three-level hierarchy for disclosure of fair value measurements, as follows:
	Level 1—Inputs are unadjusted, quoted prices in active markets for identical assets or liabilities at the measurement date.
	Level 2—Inputs (other than quoted market prices included in Level 1) that are either directly or indirectly observable, 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 instrument’s anticipated life.
	Level 3—Unobservable inputs that are supported by little or no market activity and reflect management’s best estimate of what market participants would use in pricing the asset or liability at the measurement date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model.
	A financial instrument’s categorization within the valuation hierarchy is based upon the lowest level of input that is significant to the fair value measurement.
	The following tables set forth the fair value of the Company’s consolidated financial instruments that were measured at fair value on a recurring basis as of December 31, 2014 and 2013 (in thousands):
			Level 1		Level 2		Level 3		Total
	31-Dec-14
	Financial assets:
	Money market funds		$	10,202	$	—	$	—	$	10,202
	Corporate bonds			—		71,763		—		71,763
	Agency bonds			—		15,162		—		15,162
	Commercial paper			—		30,846		—		30,846
	Total financial assets		$	10,202	$	117,771	$	—	$	127,973
	Liabilities:
	Contingent put option liability		$	—	$	—	$	233	$	233
	Total liabilities		$	—	$	—	$	233	$	233
			Level 1		Level 2		Level 3		Total
	31-Dec-13
	Financial assets:
	Money market funds		$	93,523	$	—	$	—	$	93,523
	Total financial assets		$	93,523	$	—	$	—	$	93,523
	As of December 31, 2014, the Company’s Level 3 liability is comprised of a contingent put option liability. The following table sets forth a summary of the changes in the estimated fair value of the Company’s level 3 instruments, which are measured at fair value on a recurring basis (in thousands):
	Balance as of December 31, 2012	$	19,529
	Issuance of preferred stock warrants		7,554
	Net increase in fair value included in other income (expense)		1,564
	Reclassification of warrant liability to equity		(28,647	)
	Balance as of December 31, 2013		—
	Issuance of contingent put option liability		110
	Increase in fair value included in other income (expense)		123
	Balance as of December 31, 2014	$	233
	Where quoted prices are available in an active market, securities are classified as Level 1. The Company classifies money market funds as Level 1. When quoted market prices are not available for the specific security, then the Company estimates fair value by using benchmark yields, reported trades, broker/dealer quotes, and issuer spreads; these securities are classified as Level 2. The Company classifies corporate bonds, commercial paper, agency bonds and certificates of deposit as Level 2. In certain cases where there is limited activity or less transparency around inputs to valuation, securities are classified as Level 3. The Company classified a contingent put option liability as a Level 3 liability. See Note 7 “Borrowings,” for further description. The fair value of the contingent put option liability was determined by using a risk-neutral valuation model, wherein the fair value of the underlying debt facility is estimated both with and without the presence of the default provisions, holding all other assumptions constant. The resulting difference between the two estimated fair values is the estimated fair value of the default provisions, or the contingent put option. The fair value of the underlying debt facility is estimated by calculating the expected cash flows in consideration of an estimated probability of default and expected recovery rate in default, and discounting such cash flows back to the reporting date using a risk-free rate. The carrying value of the underlying debt facility or capital loan approximates fair value. Changes to the estimated fair value of the contingent put option will be recorded in interest and other income (expense), net in the consolidated statements of operations.
	The carrying values of the Company’s financial instruments, such as cash equivalents, other receivables, accounts payable, accrued liabilities, and line of credit approximate fair value due to the short-term nature of these items.
	The fair values of the Series A and B convertible preferred stock warrants were measured using the Black-Scholes option-pricing model. The Series C-1 convertible preferred stock warrants issued in conjunction with the capital loan and line of credit were also measured using the Black-Scholes option-pricing model. Inputs used to determine estimated fair value include the estimated fair value of the underlying stock at the valuation measurement date, volatility of the price of the underlying stock, the remaining contractual term of the warrants, risk-free interest rates and expected dividends. In connection with the Series C financing the Company issued warrants to purchase Series C-1 and Series C-2 convertible preferred stock at an exercise price of $0.17 per share. The Series C-1 and C-2 convertible preferred stock were valued using the probability-weighted expected return method (PWERM).
	The Company’s C-2 convertible preferred stock was valued on October 10, 2013 using PWERM to allocate the enterprise value to the Company’s various equity components. Under this method, the per share value of each class of stock can be estimated based upon the probability-weighted present value of expected future equity values, under various possible future liquidity event scenarios. The future liquidity event scenarios and the probability of each were as follows: 1) initial public offering 65%; 2) remain private 10%; 3) strategic merger or sale 20%; or 4) dissolution or liquidation 5%. The estimated time to a liquidity event of two months and likelihood of the future liquidity event scenarios was determined based primarily on input from management of the Company.
	Upon the Company’s IPO during November 2013, the Series A and B convertible preferred stock warrants were re-measured using the Black-Scholes option-pricing model and the Series C-1 and C-2 convertible preferred stock warrants were re-measured with the intrinsic value on the date of the IPO.
	There were no transfers between Level 1, Level 2 and Level 3 during the periods presented.

Cash_Cash_Equivalents_and_Shor

Cash, Cash Equivalents, and Short-Term Investments	12 Months Ended
	Dec. 31, 2014
Cash Cash Equivalents And Short Term Investments [Abstract]
Cash, Cash Equivalents, and Short-term Investments	4. Cash, Cash Equivalents, and Short-Term Investments
	The following is a summary of cash, cash equivalents, and short-term investments (in thousands).
		Amortized		Gross		Gross			Estimated
		Cost		Unrealized		Unrealized			Fair
				Gains		Losses			Value
	31-Dec-14
	Cash and money market funds	$	17,986	$	—	$	—		$	17,986
	Corporate bonds		71,794		2		(33	)		71,763
	Agency bonds		15,163		2		(3	)		15,162
	Commercial paper		30,835		11		—			30,846
	Total financial assets	$	135,778	$	15	$	(36	)	$	135,757
	Reported as:
	Cash and cash equivalents								$	30,264
	Short-term Investments									105,493
									$	135,757
		Amortized		Gross		Gross			Estimated
		Cost		Unrealized		Unrealized			Fair
				Gains		Losses			Value
	31-Dec-13
	Cash and money market funds	$	94,759	$	—	$	—		$	94,759
	Total financial assets	$	94,759	$	—	$	—		$	94,759
	Reported as:
	Cash and cash equivalents								$	94,759
									$	94,759
	For the years ended December 31, 2014 and 2013, there were no realized gains or losses on the available-for-sale securities.
	All of the Company’s available-for-sale securities are subject to a periodic impairment review. The Company considers a debt security to be impaired when its fair value is less than its carrying cost, in which case the Company would further review the investment to determine whether it is other-than-temporarily impaired. When the Company evaluates an investment for other-than-temporary impairment, the Company reviews factors such as the length of time and extent to which fair value has been below cost basis, the financial condition of the issuer and any changes thereto, intent to sell, and whether it is more likely than not the Company will be required to sell the investment before the recovery of its cost basis. If an investment is other-than-temporarily impaired, the Company writes it down through earnings to its impaired value and establishes that as a new cost basis for the investment. The Company did not identify any of its available-for-sale securities as other-than-temporarily impaired in any of the periods presented. As of December 31, 2014, no investment was in a continuous unrealized loss position for more than one year, the unrealized losses were not due to change in credit risk and the Company believes that is more likely than not the investments will be held until maturity or a forecasted recovery of fair value.

Property_and_Equipment

Property and Equipment	12 Months Ended
	Dec. 31, 2014
Property Plant And Equipment [Abstract]
Property and Equipment	5. Property and Equipment
	The following table is a summary of property and equipment (in thousands):
		December 31,
		2014			2013
	Machinery and equipment	$	2,829		$	2,315
	Computers and software		657			126
	Leasehold improvements		6,135			4,998
	Furniture and fixtures		1,009			596
			10,630			8,035
	Less accumulated depreciation and amortization		(7,163	)		(2,411	)
		$	3,467		$	5,624
	Property and equipment depreciation and amortization expense was $4.8 million, $1.0 million and $0.4 million for the years ended December 31, 2014, 2013 and 2012, respectively. The increase in depreciation and amortization expense for 2014 is primarily attributable to the accelerated amortization of the leasehold improvements caused by the early termination of the related lease. See Note 8 for further details.

Accrued_Liabilities

Accrued Liabilities	12 Months Ended
	Dec. 31, 2014
Payables And Accruals [Abstract]
Accrued Liabilities	6. Accrued Liabilities
	Accrued liabilities consisted of the following (in thousands):
		December 31,
		2014		2013
	Accrued contract manufacturing	$	6,655	$	3,617
	Accrued accounts payable		795		771
	Accrued professional and consulting services		184		85
	Other		103		104
	Accrued clinical trial expenses		41		1,370
		$	7,778	$	5,947

Borrowings

Borrowings	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Borrowings	7. Borrowings
	Line of Credit
	In July 2008, the Company entered into a $1.5 million equipment line of credit. The terms of repayment range from 36 months to 42 months, and the interest rate is fixed at 8%. During the year ended December 31, 2012, the equipment line of credit was fully repaid in accordance with the contractual terms.
	In connection with the equipment line of credit, the Company issued warrants to purchase 2,616 shares of Series A-1 convertible preferred stock at an exercise price of $17.20. The warrants are exercisable upon issuance and expire ten years from the date of grant (July 2008). The aggregate value of the warrants on the date of issuance was $17,000, which was recorded as a liability and as debt issuance costs, which were amortized to interest expense over the term of the line of credit using the effective interest rate method. Upon the Company’s IPO during November 2013, the warrants for convertible preferred stock were converted to warrants for common stock and the related liability was re-classified to equity. As of December 31, 2014, these warrants were still outstanding.
	Capital Loan
	In 2008, the Company entered into a Capital Loan and Security Agreement and was advanced a total of $9.0 million. This debt took priority over other debts, with certain exceptions such as the line of credit. The term of the loan was 42 months, with payments of interest only over the first twelve months, followed by 30 equal monthly payments of principal and interest. The interest rate during the interest only period was fixed at 10% and during the remainder of the term the interest rate was fixed at 9.85%.
	During the year ended December 31, 2010, the loan was amended, whereby the monthly installments were reduced and the interest rate increased to 11.85% for the April through August 2010 installments, and the subsequent monthly installments were increased. The end of term payment due on January 31, 2012, originally $0.1 million, was increased to $0.3 million. These changes were considered to be a loan modification and were accounted for prospectively as a yield adjustment. During the year ended December 31, 2012 the loan was fully repaid in accordance with the contractual terms.
	In connection with the Capital Loan and Security Agreement, the Company issued warrants to purchase 26,162 shares of Series A convertible preferred stock. The aggregate value of the warrants of $0.2 million was recorded as a liability and as debt issuance costs, which were amortized to interest expense over the term of the agreement using the effective interest rate method. Upon the Company’s IPO during November 2013, the warrants to purchase convertible preferred stock were converted to warrants to purchase common stock. As a result of this conversion, the warrants ceased to be re-measured and the warrant liability was reclassified to equity. These were exercised during 2013, resulting in the net issuance of 9,800 shares of common stock.
	Convertible Promissory Notes
	On April 27, 2010 and July 27, 2010, the Company entered into convertible note and warrant purchase agreements with certain existing investors under which the Company issued convertible notes in an aggregate amount of $6.0 million at an interest rate of 8% per annum, due on September 30, 2010. The convertible notes could not be prepaid without the consent of the holders. The principal and the accrued interest under the convertible notes were automatically redeemable into the securities sold in the Company’s next equity financing which occurred in September 2010 when the Company sold shares of Series B-1 convertible preferred stock at a price of $13.502 per share. Concurrent with the closing of the financing and in accordance with the terms of the notes, the aggregate note principal and accrued interest totaling $6.1 million was converted into 453,953 shares of Series B-1 convertible preferred stock at $13.502 per share. Upon the Company’s IPO during November 2013, the Series B-1 convertible preferred stock were converted to common stock.
	In connection with the issuance of convertible notes, the Company also issued warrants to purchase 88,868 shares of Series B-1 convertible preferred stock at an exercise price of $13.502 per share and with an aggregate fair value at issuance of $0.3 million. This amount was recorded as a convertible preferred stock warrant liability and as debt discount, which was fully amortized to interest expense during the year ended December 31, 2010. Upon the Company’s IPO during November 2013, the warrants to purchase convertible preferred stock were converted to warrants to purchase common stock. As a result of this conversion, the warrants ceased to be re-measured and the warrant liability was reclassified to equity. All of these warrants were exercised for cash or net exercised during 2013, resulting in the issuance of 44,713 shares of common stock.
	Line of Credit
	In May 2013, the Company entered into a $1.6 million equipment line of credit with a bank. The equipment line will be repaid in 36 equal monthly installments, and the interest rate is fixed at 5.75%. The Company will be required to make a final payment of 9.25% of the amounts advanced. The outstanding balance on the credit line is secured by a first priority lien over all equipment purchased using the line of credit. In connection with the Company’s equipment line of credit, the Company issued a warrant to purchase 6,968 shares of Series C-1 convertible preferred stock at an exercise price of $9.1848. Upon the Company’s IPO in November 2013, the warrants to purchase convertible preferred stock were converted to warrants to purchase common stock.
	The fair value of the warrants of $25,000 at the issuance date was estimated using the Black-Scholes model with the following assumptions: a risk-free interest rate of 1.07%, a life of 7 years, a volatility factor of 91.6%, and no dividend yield. This amount is recorded as a debt discount, which is being amortized into interest expense over the repayment period. Interest expense was approximately $0.1 million, for the years ended December 31, 2014, and 2013. These were exercised during 2014, resulting in the net issuance of 3,998 shares of common stock.
	The equipment line of credit includes customary operating but non-financial covenants, including limitations on the Company’s ability to incur additional indebtedness, issue dividends and transfer or encumber any collateral securing the debt, as well as a cross-default provision with regard to the Company’s term loan. The Company utilized $1.3 million of the line of credit in the second quarter of 2013, and the remaining $0.3 million balance available under the line of credit expired unused on June 30, 2013. As of December 31, 2014, principal of $0.6 million was outstanding under the line of credit, and the Company was in compliance with all required covenants.
	Capital Loan
	On May 30, 2014, the Company entered into an Amended and Restated Loan and Security Agreement (the Amended Loan Agreement) with Oxford Finance LLC and Silicon Valley Bank to amend and restate in its entirety the Loan and Security Agreement, (the Original Agreement), dated January 31, 2013. Per the terms of the Amended Loan Agreement, upon closing, the Company immediately drew $15.0 million, of which approximately $11.0 million was used to repay the outstanding debt under the Original Agreement, and approximately $0.4 million was used to satisfy the accrued portion of the final payment fee under the Original Agreement. Payments on the new $15.0 million loan will be interest only through January 2016, followed by 30 months of equal monthly payments of interest and principal. Due to the acceptance of the company’s New Drug Application for Patiromer FOS by the U.S. Food and Drug Administration in December 2014, the interest only payment period was extended until January 2016 and the Company may, at its option, draw an additional $20.0 million between July 1, 2015 and December 31, 2015. The Company will be required to make a final payment upon loan maturity of 8.5% of the amounts advanced and the unaccrued portion of the final payment fee under the Original Agreement in the amount of approximately $0.4 million. The interest rate for the first tranche is 7.40% and the interest rate on the second tranche will be equal to the greater of (i) the three month LIBOR rate plus 7.17% or (ii) 7.40%.
	On January 31, 2013, the Company issued warrants to purchase 54,437 shares of Series C-1 convertible preferred stock at an exercise price of $9.1848 in connection with the Original Agreement. Upon the Company’s IPO in November 2013, the warrants to purchase convertible preferred stock were converted to warrants to purchase common stock. 13,609 warrants were exercised during 2014, resulting in the net issuance of 7,808 shares of common stock.
	On May 30, 2014, the Company issued warrants to purchase 12,661 shares of common stock at an exercise price of $23.69 in connection with the first $15.0 million tranche under the Amended Loan Agreement. The fair value of the warrants was $0.2 million at the issuance date and was estimated using the Black-Scholes model with the following assumptions: a risk-free interest rate of 2.06%, a life of 7 years, a volatility factor of 91.67%, and no dividend yield. This amount is recorded as a debt discount, which will be amortized into interest expense over the repayment period. Total interest expense was $1.7 million, and $1.3 million for the years ended December 31, 2014, and 2013.
	Upon an event of default, Oxford Finance LLC and Silicon Valley Bank have the option to accelerate repayment of the Amended Loan Agreement, including payment of any applicable prepayment charges, which range from 1%-3% of the outstanding loan balance and accrued interest, as well as a final payment fee. This option is considered a contingent put option liability, as the holder of the loan may exercise the option in the event of default, and is considered an embedded derivative, which must be valued and separately accounted for in the Company’s financial statements. As of December 31, 2014 the estimated fair value of the contingent put option liability was $0.2 million, which was determined by using a risk-neutral valuation model, see Note 3 for details on fair value measurement of the option.
	The Amended Loan Agreement includes customary operating but non-financial covenants, including limitations on the Company’s ability to incur additional indebtedness, issue dividends and transfer or encumber any collateral securing the debt, as well as a cross-default provision with regard to the Company’s equipment line of credit. As of December 31, 2014, principal of $15.0 million was outstanding under the Amended Loan Agreement, and the Company was in compliance with all required covenants.

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	8. Commitments and Contingencies
	Lanxess Corporation
	On January 9, 2014, the Company entered into a Manufacturing and Supply Agreement (the Lanxess Supply Agreement) with Lanxess that supersedes an earlier agreement, the Memorandum of Understanding that was executed on November 27, 2012. Under the Supply Agreement, Lanxess has agreed to manufacture and supply for commercial sale the active pharmaceutical ingredient (API) for Patiromer FOS. The Lanxess Supply Agreement terminates on December 31, 2020 unless terminated earlier. The Company may extend the Lanxess Supply Agreement for an additional five years with notice. The Lanxess Supply Agreement may be terminated (i) by the Company with notice if it abandons development or manufacturing of Patiromer FOS or fails to obtain FDA approval or the parties disagree regarding the feasibility or price of certain specification changes, or with notice at least 24 months in advance, which may be given after October 1, 2015, (ii) by Lanxess with notice if the Company fails to issue certain purchase orders, or with notice at least 24 months in advance, which may be given after October 1, 2015 and (iii) by either party with notice in the event of certain delays by the other party in performing its material obligations, or with notice if the parties fail to timely agree on certain price terms beginning in September 2015. Under the Lanxess Supply Agreement, Lanxess is obligated to manufacture the Company’s commercial supply of API for Patiromer FOS, and the Company is obligated to purchase from Lanxess such products manufactured, pursuant to the terms and conditions of the Lanxess Supply Agreement.
	During 2013 and 2014, the Company paid Lanxess for plant modifications that are required to support the manufacturing scale up of API for Patiromer FOS in anticipation of commercial launch. These payments are recorded in other assets and are currently being amortized to research and development expenses in accordance with the period over which the Company expects to derive future economic value. Upon receiving FDA approval, the amortization cost will no longer be recognized as a research and development cost expense, but as an increase to the cost of inventory. Lanxess will have full ownership of the purchased equipment and may manufacture other products with the modified plant when not occupied by the production of API for Patiromer FOS.
	DPx Fine Chemicals
	On May 14, 2014, the Company entered into a Manufacturing and Supply Agreement (the DPx Supply Agreement) with DPx Fine Chemicals Austria GmbH & Co KG (DPx Fine Chemicals), formally DSM Fine Chemicals Austria NFG GMBH & Co. KG. Under the DPx Supply Agreement, DPx Fine Chemicals has agreed to manufacture and supply for commercial sale the API for Patiromer FOS. Under the DPx Supply Agreement, the Company is obligated to make certain minimum purchases of API. The DPx Supply Agreement may be terminated (i) by the Company with notice if it abandons development or commercialization of Patiromer FOS or fails to obtain FDA approval, or with 12 months’ notice and without cause after DPx Fine Chemicals’ manufacture and release of certain quantity of API; (ii) by DPx Fine Chemicals within 24 months’ notice and without cause after its manufacture and release of a certain quantity of API; and (iii) by either party with notice for the other party’s uncured material breach, insolvency, liquidation, bankruptcy or dissolution.
	DPx Fine Chemicals has agreed to make plant modifications under the DPx Supply Agreement and will be the exclusive owner of the purchased equipment. DPx Fine Chemicals may manufacture other products with the modified plant when not occupied by the API for Patiromer FOS. Under the DPx Supply Agreement, the Company has agreed to reimburse DPx Fine Chemicals up to a specified amount for plant modifications. These payments are recorded in other assets and upon being placed into service, these payments will be amortized to research and development expenses in accordance with the period over which the Company expects to derive future economic value. Upon receiving FDA approval, the amortization cost will no longer be recognized as a research and development cost expense, but as an increase to the cost of inventory. DPx Fine Chemicals will have full ownership of the purchased equipment and may manufacture other products with the modified plant when not occupied by the production of API for Patiromer FOS.
	Patheon
	On September 5, 2014, the Company entered into a multi-year supply agreement (the Patheon Supply Agreement) with Patheon, Inc. (Patheon). Under the Patheon Supply Agreement, Patheon has agreed to manufacture and supply for commercial sale the bulk and finished drug product, referred to as Patiromer FOS. The Company has agreed to supply API for Patiromer FOS to Patheon for use in the manufacture and supply of the bulk and finished drug product, and has agreed, subject to certain conditions, to purchase certain quantities of bulk and finished drug product from Patheon. The Patheon Supply Agreement may be terminated by the Company with written notice under certain provisions.
	Patheon has agreed to make facility improvements under the Patheon Supply Agreement and will be the exclusive owner of the purchased equipment and facility improvements. Patheon may manufacture other products with the facility improvements when not occupied by manufacturing Patiromer FOS. Under the Patheon Supply Agreement, the Company has agreed to reimburse Patheon up to a specified amount for plant modifications.  These payments are recorded in other assets and upon being placed in service, these payments will be amortized to research and development expenses in accordance with the period over which the Company expects to derive future economic value.  Upon receiving FDA approval, the amortization cost will no longer be recognized as a research and development cost expense, but as an increase to the cost of inventory.
	The following are the combined future minimum payments under the terms of the Lanxess Supply Agreement, the DPx Supply Agreement, and Patheon Supply Agreement as of December 31, 2014.  These amounts are based on forecasts that include estimates of future market demand, quantity discounts and manufacturing efficiencies (in thousands):
	2015	$	45,266
	2016		41,174
	2017		37,800
	2018		42,400
	2019		42,201
	Thereafter		22,000
	Total future minimum payments	$	230,841
	Operating Lease
	On June 26, 2014, the Company entered into a new lease (the Lease) for office and laboratory facilities in Redwood City, California that will serve as the Company’s new principal executive offices. The Company currently leases office and laboratory space located in Redwood City, California pursuant to a lease dated September 7, 2012 (the Existing Lease) that commenced on January 1, 2013.
	The Existing Lease provided the Company with a tenant improvement allowance in order for the Company to complete an office renovation and a lab build out. The Company accounted for the aggregate tenant improvement allowance received as leasehold improvement assets and as a deferred rent liability. These leasehold improvement assets were being amortized to depreciation expense over the shorter of the period from which the improvements were placed into service until the end of their useful life or until the end of the lease term.  The deferred rent liability attributable to the leasehold improvement allowance was being amortized over the lease term as a credit to rent expense. Upon the execution of the Lease, the remaining net book value of the leasehold improvement assets and other assets to be abandoned upon the termination of the Existing Lease and the remaining deferred rent liability attributable to the improvement allowance were amortized or depreciated on a straight-line basis through February 28, 2015, which is the date the Company estimates it will have ceased using the premises. As of December 31, 2014, the remaining $1.5 million net book value of the leasehold improvements and other assets to be abandoned upon the termination of the Existing Lease and the remaining deferred rent liability of $1.2 million will be amortized or depreciated over the estimated remaining two months of the Existing Lease.
	The Lease commenced on February 1, 2015 following the landlord’s construction of certain improvements to the premises required under the Lease.  The Existing Lease terminates 60 days after the lease commencement date, and the Company will not be obligated to pay any base rent, expenses or other fees under the Existing Lease during the 60-day period after the Lease commences. The Lease terminates 10 years after commencement and the Company has an option to extend the Lease for an additional five years upon written notice to landlord. The long-term deferred rent of $1.0 million will be amortized over the Lease term of 10 years.
	The Lease contains rent escalation provisions over the term of the Lease. In accordance with the terms of the Lease the Company provided a security deposit in the form of a $0.7 million irrevocable letter of credit that is collateralized by a $0.7 million restricted deposit at the Company’s bank. The total rent obligation will be expensed ratably over the term of the Lease.
	Future minimum operating lease payments under the Lease at December 31, 2014, assuming the lease commences February 1, 2015 are as follows (in thousands):
	2015	$	2,178
	2016		3,178
	2017		3,340
	2018		3,440
	2019		3,543
	Thereafter		20,064
	Total future minimum payments	$	35,743
	The Company will be entitled to a one-time allowance of approximately $0.6 million for costs related to relocation, cabling, furniture, fixtures and equipment. The Company will also be entitled, at its election, to an additional allowance of approximately $2.2 million for certain move and tenant improvement related costs incurred by the Company prior to the second anniversary of the Lease commencement. In the event the Company elects to use all or any portion of the additional allowance, the base rent will increase.
	Rent expense was $(2.1) million, $1.1 million and $1.5 million for the years ended December 31, 2014, 2013 and 2012, respectively. The Company recognized rent expense on a straight-line basis over the lease term.  The negative rent expense of $2.1 million incurred for 2014 is primarily attributable to the accelerated amortization of the deferred rent liability attributable to the leasehold improvement allowance caused by the termination of the Existing Lease.
	Indemnification
	In the normal course of business, the Company enters into contracts and agreements that contain a variety of representations and warranties and provide for general indemnifications. The Company’s exposure under these agreements is unknown because it involves claims that may be made against the Company in the future, but have not yet been made. To date, the Company has not paid any claims or been required to defend any action related to its indemnification obligations. However, the Company may record charges in the future as a result of these indemnification obligations. The Company also has indemnification obligations to its officers and directors for specified events or occurrences, subject to some limits, while they are serving at the Company’s request in such capacities. There have been no claims to date and the Company has director and officer insurance that may enable the Company to recover a portion of any amounts paid for future potential claims. The Company believes the fair value of these indemnification agreements is minimal. Accordingly, the Company has not recorded any liabilities for these agreements as of December 31, 2014.

Technology_Agreement

Technology Agreement	12 Months Ended
	Dec. 31, 2014
Goodwill And Intangible Assets Disclosure [Abstract]
Technology Agreement	9. Technology Agreement
	During 2007, the Company acquired certain assets from Ilypsa, Inc. (Ilypsa), a wholly owned subsidiary of Amgen, Inc. (Amgen), consisting of certain property and equipment and intangible assets primarily relating to Ilypsa’s nonabsorbed polymer therapeutics clinical development programs.
	During November 2009, the Company purchased from Ilypsa certain rights Ilypsa had retained in one of the clinical development programs that the Company had acquired in 2007 under an amended intellectual property agreement (the Amended Agreement).
	The Amended Agreement also obligated the Company to make milestone payments to Ilypsa (Amgen) upon occurrence of certain events related to this program. In February 2013, the Company dosed its first patient in a pivotal clinical trial that is evaluating patiromer for the treatment of hyperkalemia and in March 2013, the Company made a $12.5 million milestone payment to Ilypsa (Amgen) in accordance with the Amended Agreement. Further, upon a change in control transaction, an additional payment could be owed to Ilypsa (Amgen) ranging from 6.7% to 10% of the purchase price, less certain expenses, up to a total of $30.0 million. The Company has no obligation to pay royalties on future sales of patiromer to Ilypsa (Amgen).

Convertible_Preferred_Stock_an

Convertible Preferred Stock and Stockholders' Equity (Deficit)	12 Months Ended
	Dec. 31, 2014
Stockholders Equity Note [Abstract]
Convertible Preferred Stock and Stockholders’ Equity (Deficit)	10. Convertible Preferred Stock and Stockholders’ Equity (Deficit)
	Convertible Preferred Stock
	During the year ended December 31, 2012, the Company issued 7,076,901 shares of Series C-1 convertible stock at a price of $9.1848 per share for aggregate proceeds of $65.0 million. Per the terms of the Series C Stock and Warrant Purchase Agreement, as amended (the “Series C Agreements”) if purchasers of Series C-1 shares purchased in excess of 100% of their pro rata share, they were issued warrants to purchase two shares of Series C-1 for each Series C-1 share purchased in excess of their pro rata share. The Company issued warrants to purchase 2,076,643 shares of Series C-1 convertible stock that were immediately exercisable, have an exercise price of $0.17 per share and have a five year contractual life.
	The terms of the Series C Agreements also required the holders of Series C-1 shares to purchase Series C-2 shares for $9.1848 per share in a second financing (the Series C-2 Financing) which would occur upon the Company’s cash balance falling below a defined level, or upon the election of holders of more than 50% of the Series C-1 shares. Notwithstanding this, each purchaser’s obligation to purchase shares at the Series C-2 closing shall terminate if holders of shares of convertible preferred stock representing at least 55% of the total number of shares of common stock into which the outstanding convertible preferred stock could be converted, voting together as a single class elect to terminate such obligation.
	During October 2013, the Company issued 1,633,126 shares of Series C-2 convertible preferred stock at a price of $9.1848 per share for aggregate gross proceeds of $15.0 million. Per the terms of the Series C Preferred Stock and Warrant Purchase Agreement, as amended, if purchasers of Series C convertible preferred stock purchased in excess of 100% of their pro rata share, they were issued warrants to purchase two shares of Series C for each Series C share purchased in excess of their pro rata share. The Company issued warrants to purchase 479,221 shares of Series C-2 convertible preferred stock that were immediately exercisable, have an exercise price of $0.17 per share and had a five year contractual life. The warrants were valued at $15.308 per share at the time of issuance based on their intrinsic value. The warrants were determined to be a deemed dividend to preferred stockholders, and the $7.3 million fair value was been classified as an increase to stockholders’ equity (deficit) and as a convertible preferred stock warrant liability. Upon the Company’s IPO during November 2013, the warrants to purchase convertible preferred stock were exercised into 472,187 shares of common stock. As a result of this conversion, the warrants ceased to be re-measured and the warrant liability was reclassified to equity.
	As of December 31, 2013, the Company had no outstanding convertible preferred stock. Upon the Company’s IPO during November 2013, all outstanding shares of convertible preferred stock converted into 18,924,883 shares of common stock.
	Dividends
	The holders of convertible preferred stock are entitled to receive cumulative annual dividends at the rate of 8% of the applicable original issue price per annum, payable when, as, and if declared by the Board of Directors, prior and in preference to any declaration or payment of any dividend on the common stock of the Company. No dividends have been declared or paid through the date of conversion
	Liquidation
	Upon liquidation, dissolution, or winding up of the Company, whether voluntary or involuntary, before any payment to holders of common stock, the holders of convertible preferred stock, are entitled to be paid out of the assets of the Company legally available for distribution up to the full preferential amounts (if any) to which the preferred holders are entitled.
	Conversion
	In accordance with the Amended and Restated Certificate of Incorporation, upon the Company’s IPO during November 2013, all shares of Series A, Series A-1, Series B-2, Series C-1 Series C-2 preferred stock converted into common stock on a one-for-one basis. The shares of Series B-1 preferred stock were converted into common stock on a one-for-1.47 basis.
	Shelf Registration Statement
	On December 4, 2014, the Company filed a shelf registration statement on Form S-3, which permitted: (a) the offering, issuance and sale by the Company of up to a maximum aggregate offering price of $250.0 million of its common stock, preferred stock, debt securities, warrants and/or units; and (b) as part of the $250.0 million, the offering, issuance and sale by the Company of up to a maximum aggregate offering price of $70.0 million of its common stock that may be issued and sold under a sales agreement with Cantor Fitzgerald & Co in one or more at-the-market offerings. As of December 31, 2014, the Company had sold 514,710 shares pursuant to its at-the-market offering program at an average price of $32.17 for an aggregate offering price of $16.6 million and the Company received aggregate net proceeds of $16.1 million.
	Common Stock
	The Company has issued options to purchase common stock that are subject to vesting and allow for early exercise. Unvested common stock is issued upon the early exercise of the options. The Company reserves the right to repurchase the unvested common stock in the event of employee termination. At the Company’s option, the repurchase price equals the lower of the share price paid by the employees or the fair market value as of the date of repurchase. The shares purchased by the employees pursuant to the early exercise of stock options are not deemed, for accounting purposes, to be issued until those shares vest. The cash received in exchange for exercised and unvested shares related to stock options granted is recorded as a liability for the early exercise of stock options on the accompanying balance sheets and will be transferred into common stock and additional paid-in capital as the shares vest. As of December 31, 2014, 2013 and 2012, there are 1,276, 5,125 and 6,627 shares respectively, of unvested common stock with an aggregate purchase price of $6,000, $23,000 and $27,000, respectively that are subject to repurchase at prices ranging from $3.96 to $7.40 per share, and are classified as a component of other accrued liabilities. The following table summarizes information about unvested common stock (in thousands except per share amounts):
		Number of			Weighted-
		Shares			Average
					Purchase Price
					($)
	Unvested shares at December 31, 2011		11		$	3.96
	Shares sold		36			4.13
	Shares vested		(40	)		4.13
	Shares repurchased		—			—
	Unvested shares at December 31, 2012		7			4.13
	Shares sold		23			4.12
	Shares vested		(25	)		4.01
	Shares repurchased		—			—
	Unvested shares at December 31, 2013		5			3.78
	Shares sold		—			—
	Shares vested		(3	)		4.49
	Shares repurchased		-1			5.27
	Unvested shares at December 31, 2014		1		$	4.49

Equity_Compensation_Plans_and_

Equity Compensation Plans and Stock Based Compensation	12 Months Ended
	Dec. 31, 2014
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Equity Compensation Plans and Stock Based Compensation	11. Equity Compensation Plans and Stock Based Compensation
	The Company adopted the 2007 Equity Incentive Plan (the 2007 Plan) which provides for the granting of stock options to employees, directors and consultants of the Company. Options granted under the 2007 Plan may be either incentive stock options or nonqualified stock options. Incentive stock options may be granted only to Company employees. Nonqualified stock options may be granted to Company employees, outside directors and consultants. Options under the 2007 Plan may be granted for periods of up to ten years and are exercisable immediately, subject to rights of repurchase by the Company, which lapse over the period the applicable shares vest. Employee options granted by the Company generally vest over four years.
	In November 2013, the Company’s stockholders approved the 2013 Equity Incentive Plan (the 2013 Plan). The number of shares which may be issued or transferred under the 2013 Plan shall be equal to the sum of 1,276,587 shares and any of the shares available for issuance under the 2007 Plan. Options granted under the 2013 Plan may be granted for periods of up to ten years and are generally exercisable when vested. As of December 31, 2014, the Company has reserved 531,213 shares of common stock for issuance under the 2013 Plan. In June 2014, the Company’s board of directors approved the 2014 Employment Commencement Incentive Plan (the Inducement Plan) under which 1,000,000 shares were reserved. As of December 31, 2014, 363,800 shares of the Company’s common stock were subject to inducement grants that were issued pursuant to the Inducement Plan. The awards were made pursuant to the NASDAQ inducement grant exception as a component of the Company’s new hires’ employment compensation.
	The Company adopted the ESPP on November 2013 and in August 2014, the ESPP was initiated. A total of 255,317 shares of the Company’s common stock were originally reserved for issuance under the ESPP, which permits eligible employees to purchase common stock at a discount through payroll deductions. The ESPP is a compensatory plan as defined by the authoritative guidance for stock compensation. As such stock-based compensation expense of $0.1 million has been recorded during the twelve months ended December 31, 2014.
	The following table summarizes option activity under the 2007 Plan, the 2013 Plan, and the Inducement Plan and related information (in thousands, except per share and contractual term amounts):
		Shares			Shares			Weighted-			Weighted-		Aggregate
		Available			Subject to			Average			Average		Intrinsic
		for Grant			Outstanding			Exercise			Remaining		Value
					Options			Price Per			Contractual
								Share			Term
											(in years)
	Options outstanding at December 31, 2011		57			1,491			4.13
	Additional shares reserved under plan		718			—			—
	Options granted		(921	)		921			3.96
	Options exercised		—			(36	)		4.13
	Options forfeited		213			(213	)		4.3
	Options outstanding at December 31, 2012		67			2,163			4.13
	Additional shares reserved under plan		2,571			—			—
	Options granted		(1,444	)		1,444			7.91
	Options exercised		—			(23	)		4.12
	Options forfeited		39			(39	)		4.12
	Options outstanding at December 31, 2013		1,233			3,545			5.63
	Additional shares reserved under the Inducement plan		1,000			—			—
	Options granted		(1,004	)		1,004			25.63
	Restricted stock units granted		-130			—			—
	Options exercised		—			(675	)		4.31
	Options forfeited		61			(61	)		10.95
	Restricted stock units cancelled		7			—			—
	Balances outstanding at December 31, 2014		1,167			3,813		$	11.05			8.3	$	75,837
	Vested and expected to vest at December 31, 2014		—			3,658		$	10.79			8.3	$	73,701
	Vested at December 31, 2014		—			1,427		$	5.5			7.5	$	36,148
	The aggregate intrinsic values of options outstanding and exercisable, vested and expected to vest were calculated as the difference between the exercise price of the options and the NASDAQ listing fair value of the Company’s common stock as of December 31, 2014.
	The following table summarizes information about stock options outstanding at December 31, 2014:
		Options Outstanding									Options Exercisable
	Exercise Price	Number			Weighted			Weighted			Number		Weighted
		Outstanding			Average			Average			Exercisable		Average
					Remaining			Exercise					Exercise
					Contractual			Price					Price
					Life (Years)
	$2.76 - $4.30		1,285,437			7.3		$	3.94			1,285,437	$	3.94
	$4.65 - $11.00		1,503,836			8.3			7.24			1,447,892		7.09
	$19.41 - $22.52		336,750			9.7			20.69			—		—
	$23.24 - $23.96		94,153			9.3			23.63			6,072		23.29
	$25.70 - $41.40		592,535			9.6			28.65			12,499		33.76
	Total		3,812,711			8.3		$	11.05			2,751,900	$	5.78
	The weighted-average grant-date fair value of options granted during the years ended December 31, 2014, 2013, and 2012 was $19.77, $6.23, and $3.10, respectively. The total fair value of options that vested during the years ended December 31, 2014, 2013, 2012 was $9.1 million, $5.0 million, and $1.1 million, respectively. The intrinsic value (the difference between the fair value of the Company’s common stock at the exercise date and the exercise price) of the options exercised was $14.9 million, $158,000 and $8,000 during the years ended December 31, 2014, 2013 and 2012, respectively.
	Stock-Based Compensation Expense
	Stock-based compensation expense included in the Company’s consolidated statements of operations is as follows (in thousands):
		Year Ended December 31,
		2014			2013			2012
	Research and development:
	Employees	$	1,657		$	776		$	368
	Non-employee consultants		4,045			1,659			117
	General and administrative:
	Employees		3,980			1,474			692
	Non-employee consultants		13			—			—
	Total stock-based compensation expense	$	9,695		$	3,909		$	1,177
	The total unrecognized stock-based compensation expense related to stock options and ESPP stock purchase rights at December 31, 2014, was $29.7 million, and is expected to be recognized over a weighted-average period of approximately 2.9 years.
	The fair value of each option award is estimated on the date of grant using the Black-Scholes option valuation model. Since the Company has only been publicly traded for a short period and does not have adequate trading history for its common stock, the expected stock price volatility was calculated based on the average historical volatility for comparable publicly traded pharmaceutical companies. The Company selected companies with comparable characteristics, including enterprise value, risk profiles, position within the industry, and with historical share price information sufficient to meet the expected life of Relypsa’s stock-based awards.
	The expected term of the options is based on the average period the stock options are expected to remain outstanding. As the Company does not have sufficient historical information to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior, the expected term is calculated as the midpoint between the weighted-average vesting term and the contractual expiration period also known as the simplified method. The risk-free rate is based on U.S. Treasury zero coupon issues with remaining terms consistent with the expected terms of the stock options, as determined at the time of grant. To date, the Company has not declared or paid any cash dividends and does not have any plans to do so in the future. Therefore, the Company used an expected divided yield of zero.
	The following table illustrates the assumptions for the Black-Scholes option-pricing model used in determining the fair value of options granted to employees:
		Year Ended December 31,
		2014			2013			2012
	Expected volatility		87-109	%		95-110	%		87-95	%
Expected dividends		0	%		0	%		0	%
Expected term (in years)		6			6			6
Risk-free rate		1.6-2.0	%		1.1-1.8	%		0.8-1.1	%
In the years ended December 31, 2013 and 2012, the Company granted 290 and 29,651 common stock options, respectively, with exercise prices of $7.40 and $3.96, respectively, per share in exchange for services from non-employees. No common stock options were granted to non-employees in the year ended December 31, 2014.  Outstanding non-employee options are primarily related to employees who converted to consultants in 2014 and 2013. Stock-based compensation expense related to stock options granted to non-employees is measured and recognized as the stock options are earned. The Company believes that the estimated fair value of the stock options is more readily measurable than the fair value of the services rendered.
The fair value of non-employee stock options was calculated using the Black-Scholes valuation model, based on the following assumptions:
	Year Ended December 31,
	2014			2013			2012
Expected volatility		92	%		91%-122	%		83	%
Expected dividends		0	%		0	%		0	%
Expected term (in years)		9-Jul			10-May			10-Sep
Risk-free rate		1.8-2.0	%		1.3-2.7	%		1.6-1.7	%
Restricted Stock Units
During 2014, the Company granted restricted stock unit (“RSUs”) awards subject to performance vesting criteria and subject to time vesting criteria from the 2013 Plan. RSUs subject to performance vesting criteria consist of the right to receive shares of common stock, subject to achievement of time-based criteria and certain corporate performance-related goals over a specified period. RSUs subject to time vesting criteria entitle holders to receive shares of common stock at the end of a specified period of time. For RSUs subject to time vesting criteria, vesting is based on continuous employment or service of the holder. The fair value of RSUs is measured based on the number of shares granted and the closing market price of our common stock on the date of grant.
The following table summarizes RSU activity for the twelve months ended December 31, 2014 (in thousands, except per share amounts):
	Restricted Stock Units
	Number of Shares			Weighted Average
				Grant Date Fair
				Value Per Share
Nonvested at December 31, 2013		—		$	—
Granted		130			27.07
Vested		—			—
Canceled		-7			30.96
Nonvested at December 31, 2014		123		$	26.85
As of December 31, 2014, none of the RSUs granted by the Company have vested.
The total unrecognized stock-based compensation expense related to non-vested RSUs at December 31, 2014, was $2.9 million, and is expected to be recognized over a weighted-average period of approximately 3.5 years.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income Taxes	12. Income Taxes
	The Company did not record a provision or benefit for income taxes during the years ended December 31, 2014, 2013, and 2012.
	The reconciliation of the statutory federal income tax rate to the Company’s effective tax rate is as follows:
		Year Ended December 31,
		2014			2013			2012
	Federal statutory rate	34	%		34	%		34	%
	State statutory rate	5.8			5.8			5.8
	Research and development tax credit	2.3			1.7			1.4
	Permanent item	(1.0	)		(1.0	)		(0.8	)
	Valuation allowance	(41.1	)		(40.5	)		(40.4	)
	Provision for income taxes	—	%		—	%		—	%
	At December 31, 2014, the Company had total deferred tax assets of $108.8 million. Due to uncertainties surrounding its ability to generate future taxable income to realize these tax assets, a full valuation has been established to offset total deferred tax assets.
	The tax effects of temporary differences and carryforwards that give rise to significant portions of the deferred tax assets are as follows (in thousands):
		December 31,
		2014				2013				2012
	Federal and state net operating loss carryforwards	$	91,604			$	64,768			$	40,323
	Federal and state research and development credits		6,253				4,703				3,550
	Depreciation		3,036				1,538				1,421
	Intangible assets		—				(6	)			(12	)
	Accruals and reserves		7,900				4,942				2,718
			108,793				75,945				48,000
	Valuation allowance		(108,793	)			(75,945	)			(48,000	)
	Net deferred tax assets	$	—			$	—			$	—
	No tax benefit has been recorded through December 31, 2014, 2013 and 2012, because of the Company’s history of operating losses, and a full valuation allowance has been provided. The Company’s valuation allowance increased by $32.8 million, $27.9 million and $7.5 million for the years ended December 31, 2014, 2013 and 2012, respectively.
	As of December 31, 2014, the Company had net operating loss carryforwards of approximately $240.1 million and $234.3 million that may be available, subject to the limitations described below, to reduce future taxable income, if any, for federal and California state income tax purposes, respectively. The federal and state net operating loss carryforwards will begin to expire in 2027 and 2017, respectively. Included in the gross amount, approximately $9.3 million of net operating loss is created by excess stock option deductions. A credit to additional paid-in capital will be recorded when the excess stock option deduction reduces the income tax payable.
	Additionally, the future utilization of the net operating loss carry forwards to offset future taxable income may be subject to an annual limitation, pursuant to Internal Revenue Code Section 382, as a result of ownership changes that may have occurred previously or that could occur in the future. The Company performed an initial analysis under Section 382. As a result of this analysis, the Company removed the deferred tax assets for net operating losses of $28.6 million generated through December 31, 2014 from its deferred tax asset schedule and have recorded a corresponding decrease to the valuation allowance. When this analysis is finalized, the deferred tax asset-schedule and associated valuation allowance will be updated. Due to the existence of the valuation allowance, future changes in unrecognized tax benefits will not impact the Company’s effective tax rate.
	As of December 31, 2014, the Company had research and development credit carryforwards of approximately $5.2 million and $5.6 million available to reduce future tax expense, if any, for federal and California state income tax purposes, respectively. The federal credits expire beginning in 2027, and the California credits carry forward indefinitely. Further, the future utilization of the research and development credit carryforwards are also subject to the limitations as discussed above.
	The Company recognizes uncertain tax positions when it is more-likely-than-not, based on the technical merits, that the position will not be sustained upon examination. A reconciliation of the beginning and ending amounts of unrecognized tax benefits is as follows (in thousands):
		2014				2013				2012
	Balance at January 1	$	2,476			$	1,836			$	1,455
	Increases related to current period tax positions		713				588				350
	Increases related to prior period tax positions		—				52			31
	Balance as of December 31	$	3,189			$	2,476			$	1,836
	As of December 31, 2014, due to a valuation allowance against the Company’s deferred tax assets, none of the unrecognized tax benefits, if recognized, would affect the Company’s effective tax rate. The Company is subject to U.S. federal and state income tax examination for fiscal tax years ending 2011 through 2014. The U.S. federal and U.S. state taxing authorities may choose to audit tax returns for tax years beyond the statute of limitation period due to significant tax attribute carryforwards from prior years, making adjustments only to carryforward attributes. The Company’s policy is to include interest and penalties related to unrecognized tax benefits within the Company’s provision for income taxes and to date have not recorded any interest or penalties. The Company is not currently under an examination and does not expect material changes to its unrecognized tax benefits in the next twelve months.

401k_Savings_Plan

401(k) Savings Plan	12 Months Ended
	Dec. 31, 2014
Compensation And Retirement Disclosure [Abstract]
401(k) Savings Plan	13. 401(k) Savings Plan
	The Company sponsors a 401(k) Plan to provide retirement and incidental benefits for its employees. Employees may contribute from 1% to 90% of their annual compensation to the 401(k) Plan, limited to a maximum annual amount as set periodically by the Internal Revenue Service. The Company does not make matching contributions to the 401(k) Plan.

Related_Party_Transaction

Related Party Transaction	12 Months Ended
	Dec. 31, 2014
Related Party Transactions [Abstract]
Related Party Transaction	14. Related Party Transaction
	Consulting Agreement with Dr. Klaus Veitinger
	In October 2010, the Company entered into a Consulting and Independent Contractor Agreement with an entity controlled by Dr. Klaus Veitinger, a member of its board of directors, under which Dr. Veitinger provides certain consulting services to the Company in connection with patiromer. This agreement was terminated in conjunction with the Company’s IPO that was completed in November 2013. Pursuant to this agreement the Company paid approximately $0.1 million and $0.2 million in 2013 and 2012, respectively.

Subsequent_Events

Subsequent Events	12 Months Ended
	Dec. 31, 2014
Subsequent Events [Abstract]
Subsequent Events	15. Subsequent Events
	During January and February 2015, the Company sold 1,549,910 shares pursuant to its at-the-market offering program at an average price of $34.48. The Company received aggregate net proceeds of $51.8 million.
	On March 3, 2015, the Company completed an underwritten public offering of 4,485,000 shares of common stock at an offering price of $38.50 per share for gross proceeds of $172.3 million. The Company estimates net proceeds from the offering to be $161.9 million, after deducting the underwriting discounts and estimated expenses.
	The financial statements as of December 31, 2014, including share and per share amounts, do not include the effects of either offering.
	The table below shows, on a pro forma basis, the impact of the Company’s offering on certain condensed balance sheet items. The as adjusted condensed balance sheet data below gives effect to the sale of 6,034,910 shares of securities.
		As of December 31, 2014
		Actual			As Adjusted
			(unaudited)
			(in thousands)
	Balance Sheet Data:
	Cash, cash equivalents and short-term investments	$	135,757		$	349,509
	Working capital		122,291			336,043
	Total assets		151,839			365,591
	Accumulated deficit		(305,728	)		(305,728	)
	Total stockholders’ equity		118,461			332,213

Selected_Quarterly_Financial_D

Selected Quarterly Financial Data (unaudited)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data	16. Selected Quarterly Financial Data (unaudited)
	The following table contains quarterly financial information for 2014 and 2013. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period.
			December 31,			September 30,			June 30,			March 31,			December 31,			September 30,			June 30,			March 31,
			2014			2014			2014			2014			2013			2013			2013			2013
			(In thousands, except share and per share data)
	Operating expenses:
	Research and		$	16,596		$	11,647		$	11,075		$	10,909		$	10,914		$	12,158		$	13,295		$	22,604
	development(1)
	General and			10,486			7,311			5,322			4,795			3,732			2,685			2,988			2,535
	administrative
	Total operating			27,082			18,958			16,397			15,704			14,646			14,843			16,283			25,139
	expenses
	Loss from			(27,082	)		(18,958	)		(16,397	)		(15,704	)		(14,646	)		(14,843	)		(16,283	)		(25,139	)
	operations
	Interest and other			69			17			36			27			13,314			(10,382	)		(3,549	)		(864	)
	income (expense),
	net(2)
	Interest expense			(635	)		(494	)		(376	)		(391	)		(407	)		(410	)		(388	)		(248	)
	Net loss			(27,648	)		(19,435	)		(16,737	)		(16,068	)		(1,739	)		(25,635	)		(20,220	)		(26,251	)
	Deemed dividend to			—			—			—			—			(7,336	)		—			—			—
	preferred
	stockholders(3)
	Net loss attributable		$	(27,648	)	$	(19,435	)	$	(16,737	)	$	(16,068	)	$	(9,075	)	$	(25,635	)	$	(20,220	)	$	(26,251	)
	to common
	stockholders
	Net loss per share		$	(0.80	)	$	(0.57	)	$	(0.51	)	$	(0.54	)	$	(0.68	)	$	(79.91	)	$	(64.87	)	$	(84.61	)
	attributable to
	common
	stockholders basic
	and diluted
	Weighted average common			34,368,776			34,064,780			33,141,384			29,710,415			13,430,244			320,802			311,703			310,275
	shares used to compute
	net loss per share
	attributable to common
	stockholders, basic and
	diluted(4)
	-1	During the quarter ended March 31, 2013, a $12.5 million milestone payment was made to Amgen pursuant to the amended and restated IP license and assignment agreement, resulting from the initiation of dosing in the pivotal Phase 3 trial for patiromer.
	-2	During 2013 interest and other income (expense), net is primarily related to the change in value of warrants to acquire convertible preferred stock that were issued in connection with preferred stock financings, capital loan and equipment line of credit.
	-3	During 2013, the Company recognized deemed dividends to preferred stockholders as a result of the issuance of warrants as part of the C-2 financing.
	-4	During the quarter ended December 31, 2013, shares outstanding increased as a result of the initial public offering of the Company’s common stock and the conversion of convertible preferred stock and warrants into common stock.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation
	The accompanying consolidated financial statements include the accounts of Relypsa and its wholly-owned subsidiary, Relypsa UK LTD, and have been prepared in accordance with U.S. generally accepted accounting principles (“US GAAP”). Intercompany transactions and balances have been eliminated in consolidation.
Use of Estimates	Use of Estimates
	The preparation of consolidated financial statements in conformity with US GAAP requires management to make estimates and judgments that affect the amounts reported in the consolidated financial statements and accompanying notes. On an ongoing basis, management evaluates its estimates, including those related to clinical trial accruals, manufacturing accruals, fair value of assets and liabilities, common stock, income taxes, and stock-based compensation. Management bases its estimates on historical experience and on various other market-specific and relevant assumptions that management believes to be reasonable under the circumstances. Actual results could differ from those estimates.
Liquidity	Liquidity
	The Company has never been profitable and, as of December 31, 2014, the Company has an accumulated deficit of $305.7 million. The Company has incurred net losses of approximately $79.9 million, $73.8 million and $43.7 million in the years ended December 31, 2014, 2013 and 2012, respectively. The Company expects to continue to incur net operating losses for at least the next two years, as the Company continues the development of, seeks regulatory approval for, and if approved, begins to commercialize, Patiromer FOS. The Company will need additional funding to support its future operating activities and adequate funding may not be available to the Company on acceptable terms, or at all. The Company’s failure to obtain sufficient funds on acceptable terms when needed could have a material adverse effect on the business, results of operations, and financial condition. The Company will need to generate significant revenues to achieve profitability and may never do so.
Cash Equivalents	Cash Equivalents
	The Company considers all highly liquid investments purchased with original maturities of 90 days or less at the date of purchase to be cash equivalents.
Investments	Investments
	Investments consist of debt securities classified as available-for-sale and have maturities greater than 90 days from the date of acquisition. Short-term investments have maturities than 365 days as of the balance sheet date. Long-term investments have maturities greater than 365 days as of the balance sheet date. Investments are carried at fair value based upon quoted market prices or pricing models for similar securities. Unrealized gains and losses on available-for-sale securities are excluded from earnings and are reported as a component of accumulated comprehensive income (loss). Realized gains or losses on the sale of all such securities are reported in interest and other income (expense), net and computed using the specific identification method.
Concentrations of Credit Risk	Concentrations of Credit Risk
	Financial instruments that potentially subject the Company to a concentration of credit risk consist primarily of cash, cash equivalents and short-term investments. The Company invests its excess cash in money market accounts, U.S. Treasury notes, municipal bonds, agency bonds, corporate notes, certificates of deposit and commercial paper. Other than for obligations of the U.S. government, the Company’s policy is that no more than 5% of its investments may be concentrated in a single issuer. Bank deposits are held by a single financial institution having a strong credit rating and these deposits may at times be in excess of insured limits. The Company is exposed to credit risk in the event of a default by the financial institutions holding its cash, cash equivalents and short-term investments and issuers of investments to the extent recorded on the balance sheets. The Company’s investment policy limits investments to certain types of debt securities issued by the U.S. government, its agencies and institutions with investment-grade credit ratings and places restrictions on maturities and concentration by type and issuer.
Restricted Cash	Restricted Cash
	Restricted cash related to the Company’s leases consist of irrevocable letters of credit that are collateralized by restricted deposits held at the Company’s bank over a term that is consistent with the corresponding lease.
Property and Equipment	Property and Equipment
	Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the related assets, ranging from three to seven years. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful lives of the respective assets.
Intangible Assets	Intangible Assets
	Intangible assets consist of acquired patent rights that arose from the Company’s acquisition of assets from Ilypsa (See Note 9 for further details). Acquired patent rights were assigned an estimated useful life of seven years and are being amortized using the straight-line method. The carrying value of these patent rights has been fully amortized as of December 31, 2014.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets
	The Company identifies and records impairment losses on long-lived assets used in operations when events and changes in circumstances indicate that the carrying amount of an asset might not be recoverable. Recoverability is measured by comparing the anticipated undiscounted future net cash flows to the related asset’s carrying value. If an asset is considered impaired, the asset is written down to fair value, which is determined based either on discounted cash flows or appraised value, depending on the nature of the asset. The Company has not experienced any impairment during 2014, 2013, and 2012.
Research and Development Expenses	Research and Development Expenses
	Research and development costs are charged to expense as incurred and consist of costs incurred to further the Company’s research and development activities including salaries and related employee benefits, costs associated with clinical trials, costs related to pre-commercialization manufacturing activities such as manufacturing process validation activities and the manufacturing of commercial supply prior to approval, nonclinical research and development activities, regulatory activities, research-related overhead expenses and fees paid to external service providers and contract research and manufacturing organizations that conduct certain research and development activities on behalf of the Company.
Clinical Trial Accruals	Clinical Trial Accruals
	Clinical trial costs are a component of research and development expenses. The Company accrues and expenses clinical trial activities performed by third parties based upon actual work completed in accordance with agreements established with clinical research organizations and clinical sites. The Company determines the actual costs through discussions with internal personnel and external service providers as to the progress or stage of completion of trials or services and the agreed-upon fee to be paid for such services.
Contract Manufacturing Accruals	Contract Manufacturing Accruals
	Contract Manufacturing costs are a component of research and development expenses. The Company accrues and expenses contract manufacturing activities performed by third parties based upon the actual or the estimated amount of work completed in accordance with agreements established with contract manufacturing organizations. The Company determines the actual costs or it estimates the costs through discussions with internal personnel and external service providers as to the manufacturing activities that were performed or completed and the agreed-upon fee to be paid for such services.
Income Taxes	Income Taxes
	The Company uses the liability method to account for income taxes. Under this method, deferred tax assets and liabilities are determined based on differences between the financial statement carrying amounts of existing assets and liabilities and their tax bases. Deferred tax assets and liabilities are measured using enacted tax rates applied to taxable income in the years in which those temporary differences are expected to be recovered or settled. A valuation allowance is established when necessary to reduce deferred tax assets to the amount expected to be realized. Financial statement effects of uncertain tax positions are recognized when it is more–likely-than-not, based on the technical merits of the position, that they will be sustained upon examination. Interest and penalties related to unrecognized tax benefits are included within the provision for income tax.
Stock-Based Compensation	Stock-Based Compensation
	The Company accounts for stock-based compensation based on the fair value of the share-based awards that are ultimately expected to vest. The fair value of employee stock options granted is estimated on the date of grant using the Black-Scholes option pricing model, and is recognized in expense over the service period using the straight-line method, net of estimated forfeitures. Forfeiture estimates are adjusted to the extent that actual forfeitures differ from the prior estimates. The fair value of restricted stock unit awards used in the Company’s expense recognition method is based on the number of shares granted and the closing price of the Company’s common stock on the date of the grant. Such value is recognized as an expense over the service period using the straight-line method, net of estimated forfeitures. The Company adopted an employee stock purchase plan (ESPP) on November 2013 and in August 2014, the ESPP was initiated, pursuant to which eligible employees can purchase shares of the Company’s common stock at a price per share equal to the lesser of 85% of the fair market value on the first day of the offering period or 85% of the fair market value on the purchase date. The fair value of each award is estimated on the first day of the offering period using the Black-Scholes option pricing model and is recognized in expense over the service period using the straight-line method, net of estimated forfeitures.
	The Company records the expense attributable to nonemployee services paid with share-based awards based on the estimated fair value of the awards determined using the Black-Scholes option pricing model. The measurement of stock-based compensation for nonemployees is subject to periodic adjustments as the options vest, and the expense is recognized over the period during which services are received.
Convertible Preferred Stock	Convertible Preferred Stock
	Prior to the Company’s IPO, the Company recorded all shares of convertible preferred stock at their respective fair values, net of issuance costs, on the dates of issuance. Upon completing the IPO, all shares of the Company’s convertible preferred stock then outstanding converted into 18,924,883 shares of our common stock.
Convertible Preferred Stock Warrant Liability	Convertible Preferred Stock Warrant Liability
	Prior to the Company’s IPO, freestanding warrants for shares that were either puttable or redeemable were classified as liabilities on the balance sheets and were carried at their estimated fair value. The preferred stock underlying the warrants was redeemable in certain circumstances. At the end of each reporting period, changes in the estimated fair value during the period were recorded in interest and other income (expense), net. The consummation of the Company’s IPO resulted in the conversion of all classes of the Company’s preferred stock into common stock. Upon such conversion of the underlying classes of preferred stock, the warrants were reclassified as a component of equity and were no longer subject to re-measurement.
Deferred Rent	Deferred Rent
	Rent expense is recognized on a straight-line basis over the noncancelable term of the Company’s operating lease and, accordingly, the Company records the difference between cash rent payments and the recognition of rent expense as a deferred rent liability. In accordance with the terms of the lease for the Company’s office and laboratory space as of December 31, 2014, the Company also recorded lessor-funded lease incentives, such as reimbursable leasehold improvements, as a deferred rent liability, which is amortized as a reduction of rent expense over the noncancelable term of its operating lease. During June 2014, the lease was modified, see Note 8 for further details.
Net Loss per Common Share Attributable to Common Stockholders	Net Loss per Common Share Attributable to Common Stockholders
	Basic net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted-average number of common shares outstanding during the period, without consideration for common stock equivalents. Diluted net loss per share of common stock is the same as basic net loss per share of common stock, since the effects of potentially dilutive securities are antidilutive. The net loss per share of common stock attributable to common stockholders is computed using the two-class method required for participating securities. All series of the Company’s convertible preferred stock are considered to be participating securities as they are entitled to participate in undistributed earnings with shares of common stock. Due to the Company’s net loss, there is no impact on earnings per share calculation in applying the two-class method since the participating securities have no legal requirement to share in any losses.
	The following outstanding shares of common stock equivalents were excluded from the computations of diluted net loss per common share attributable to common stockholders for the periods presented as the effect of including such securities would be antidilutive (in thousands):
		Year Ended December 31,
		2014		2013		2012
	Convertible preferred stock—as converted to common stock		—		—		17,281
	Warrants to purchase convertible preferred stock—as converted to common stock		—		—		2,236
	Warrants to purchase common stock		56		64		—
	Options to purchase common stock		3,813		3,545		2,163
	Restricted stock units		123		—		—
	Common stock subject to repurchase		1		5		7
			3,993		3,614		21,687
Recent Accounting Pronouncements	Recent Accounting Pronouncements
	In June 2014, FASB issued Accounting Standards Update No. 2014-10, Development Stage Entities: Elimination of Certain Financial Reporting Requirements. The update removes the definition of a development stage entity from FASB ASC 915 and eliminates the requirement for development stage entities to present inception-to-date information on the statements of operations, cash flows and stockholders’ equity (deficit). The Company early adopted this standard for the period covered by the report herein.
	In August 2014, FASB issued Accounting Standards Update No. 2014-15, Presentation of Financial Statements Going Concern – Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern. The amendments require management to assess an entity’s ability to continue as a going concern by incorporating and expanding upon certain principles that are currently in U.S. auditing standards. Specifically, the amendments: (1) provide a definition of the term substantial doubt; (2) require an evaluation every reporting period including interim periods; (3) provide principles for considering the mitigating effect of management’s plans; (4) require certain disclosures when substantial doubt is alleviated as a result of consideration of management’s plans; (5) require an express statement and other disclosures when substantial doubt is not alleviated; and (6) require an assessment for a period of one year after the date that the financial statements are issued (or available to be issued). ASU 2014-15 will be effective for annual periods ending after December 15, 2016 and interim periods within annual periods beginning after December 15, 2016 with early adoption permitted. ASU 2014-15 will be effective for the Company beginning with its annual report for fiscal 2016 and interim periods thereafter. The Company has not yet determined the effect of the adoption of this standard on the Company’s consolidated financial statements.
Organization	Organization
	Relypsa, Inc. (Relypsa or the Company) is a biopharmaceutical company dedicated to the development and commercialization of new non-absorbed polymeric drugs for important applications in renal, cardiovascular and metabolic disease. Relypsa’s lead product candidate is Patiromer for Oral Suspension, or Patiromer FOS. A new drug application has been filed with the U.S. Food and Drug Administration for Patiromer FOS for the treatment of hyperkalemia and is currently under review. The Company commenced operations on October 29, 2007. The Company’s principal operations are based in Redwood City, California and it operates in one segment. On October 28, 2014, Relypsa UK LTD was incorporated as a wholly-owned subsidiary of the Company.
Offerings	Offerings
	On November 15, 2013, the Company completed its initial public offering, or IPO, which resulted in net proceeds of $78.0 million from the issuance of 7,877,500 shares of common stock. In connection with the IPO, on an as converted basis, 18,924,883 shares of the Company’s convertible preferred stock and 2,563,076 convertible preferred stock warrants were converted into 21,487,959 shares of common stock. In addition, upon completing the IPO, the liabilities related to the convertible preferred stock warrants were re-classified to additional paid-in-capital and are no longer subject to re-measurement.
	In connection with the completion of its IPO, on November 15, 2013, the Company filed an amended and restated certificate of incorporation and bylaws, which, among other things, changed the number of authorized shares of common stock to 300,000,000 shares and preferred stock to 5,000,000 shares.
	On April 16, 2014, the Company completed an underwritten public offering of 4,130,611 shares of common stock at an offering price of $24.50 per share. The Company received aggregate net proceeds of $94.6 million, after deducting the underwriting discounts and offering related transaction costs.
	On December 4, 2014, the Company filed a shelf registration statement on Form S-3, which permitted: (a) the offering, issuance and sale by the Company of up to a maximum aggregate offering price of $250.0 million of its common stock, preferred stock, debt securities, warrants and/or units; and (b) as part of the $250.0 million, the offering, issuance and sale by the Company of up to a maximum aggregate offering price of $70.0 million of its common stock that may be issued and sold under a sales agreement with Cantor Fitzgerald & Co in one or more at-the-market offerings. As of December 31, 2014, the Company had sold 514,710 shares pursuant to its at-the-market offering program at an average price of $32.17 for an aggregate offering price of $16.6 million and the Company received aggregate net proceeds of $16.1 million. During January and February 2015, the Company sold an additional 1,549,910 shares at an average price of $34.48 for an aggregate offering price of $53.4 million. The Company received aggregate net proceeds of $51.8 million.
	On March 3, 2015, the Company completed an underwritten public offering of 4,485,000 shares of common stock at an offering price of $38.50 per share for gross proceeds of $172.3 million. The Company estimates net proceeds from the offering to be $161.9 million, after deducting the underwriting discounts and estimated expenses.

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Schedule of Shares Excluded from Computations of Diluted Net Loss per Common Share	The following outstanding shares of common stock equivalents were excluded from the computations of diluted net loss per common share attributable to common stockholders for the periods presented as the effect of including such securities would be antidilutive (in thousands):
		Year Ended December 31,
		2014		2013		2012
	Convertible preferred stock—as converted to common stock		—		—		17,281
	Warrants to purchase convertible preferred stock—as converted to common stock		—		—		2,236
	Warrants to purchase common stock		56		64		—
	Options to purchase common stock		3,813		3,545		2,163
	Restricted stock units		123		—		—
	Common stock subject to repurchase		1		5		7
			3,993		3,614		21,687

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments Measured at Fair Value on Recurring Basis	The following tables set forth the fair value of the Company’s consolidated financial instruments that were measured at fair value on a recurring basis as of December 31, 2014 and 2013 (in thousands):
			Level 1		Level 2		Level 3		Total
	31-Dec-14
	Financial assets:
	Money market funds		$	10,202	$	—	$	—	$	10,202
	Corporate bonds			—		71,763		—		71,763
	Agency bonds			—		15,162		—		15,162
	Commercial paper			—		30,846		—		30,846
	Total financial assets		$	10,202	$	117,771	$	—	$	127,973
	Liabilities:
	Contingent put option liability		$	—	$	—	$	233	$	233
	Total liabilities		$	—	$	—	$	233	$	233
			Level 1		Level 2		Level 3		Total
	31-Dec-13
	Financial assets:
	Money market funds		$	93,523	$	—	$	—	$	93,523
	Total financial assets		$	93,523	$	—	$	—	$	93,523
Summary of Changes in Estimated Fair Value of Level 3 Instruments Measured at Fair Value on Recurring Basis	As of December 31, 2014, the Company’s Level 3 liability is comprised of a contingent put option liability. The following table sets forth a summary of the changes in the estimated fair value of the Company’s level 3 instruments, which are measured at fair value on a recurring basis (in thousands):
	Balance as of December 31, 2012	$	19,529
	Issuance of preferred stock warrants		7,554
	Net increase in fair value included in other income (expense)		1,564
	Reclassification of warrant liability to equity		(28,647	)
	Balance as of December 31, 2013		—
	Issuance of contingent put option liability		110
	Increase in fair value included in other income (expense)		123
	Balance as of December 31, 2014	$	233

Cash_Cash_Equivalents_and_Shor1

Cash, Cash Equivalents, and Short-Term Investments (Tables)	12 Months Ended
	Dec. 31, 2014
Cash Cash Equivalents And Short Term Investments [Abstract]
Summary of Cash, Cash Equivalents, and Short-Term Investments	The following is a summary of cash, cash equivalents, and short-term investments (in thousands).
		Amortized		Gross		Gross			Estimated
		Cost		Unrealized		Unrealized			Fair
				Gains		Losses			Value
	31-Dec-14
	Cash and money market funds	$	17,986	$	—	$	—		$	17,986
	Corporate bonds		71,794		2		(33	)		71,763
	Agency bonds		15,163		2		(3	)		15,162
	Commercial paper		30,835		11		—			30,846
	Total financial assets	$	135,778	$	15	$	(36	)	$	135,757
	Reported as:
	Cash and cash equivalents								$	30,264
	Short-term Investments									105,493
									$	135,757
		Amortized		Gross		Gross			Estimated
		Cost		Unrealized		Unrealized			Fair
				Gains		Losses			Value
	31-Dec-13
	Cash and money market funds	$	94,759	$	—	$	—		$	94,759
	Total financial assets	$	94,759	$	—	$	—		$	94,759
	Reported as:
	Cash and cash equivalents								$	94,759
									$	94,759

Property_and_Equipment_Tables

Property and Equipment (Tables)	12 Months Ended
	Dec. 31, 2014
Property Plant And Equipment [Abstract]
Summary of Property and Equipment	The following table is a summary of property and equipment (in thousands):
		December 31,
		2014			2013
	Machinery and equipment	$	2,829		$	2,315
	Computers and software		657			126
	Leasehold improvements		6,135			4,998
	Furniture and fixtures		1,009			596
			10,630			8,035
	Less accumulated depreciation and amortization		(7,163	)		(2,411	)
		$	3,467		$	5,624

Accrued_Liabilities_Tables

Accrued Liabilities (Tables)	12 Months Ended
	Dec. 31, 2014
Payables And Accruals [Abstract]
Schedule of Accrued Liabilities	Accrued liabilities consisted of the following (in thousands):
		December 31,
		2014		2013
	Accrued contract manufacturing	$	6,655	$	3,617
	Accrued accounts payable		795		771
	Accrued professional and consulting services		184		85
	Other		103		104
	Accrued clinical trial expenses		41		1,370
		$	7,778	$	5,947

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Payments Under Supply Agreement	The following are the combined future minimum payments under the terms of the Lanxess Supply Agreement, the DPx Supply Agreement, and Patheon Supply Agreement as of December 31, 2014.  These amounts are based on forecasts that include estimates of future market demand, quantity discounts and manufacturing efficiencies (in thousands):
	2015	$	45,266
	2016		41,174
	2017		37,800
	2018		42,400
	2019		42,201
	Thereafter		22,000
	Total future minimum payments	$	230,841
Schedule of Future Minimum Payments Under Operating Lease	Future minimum operating lease payments under the Lease at December 31, 2014, assuming the lease commences February 1, 2015 are as follows (in thousands):
	2015	$	2,178
	2016		3,178
	2017		3,340
	2018		3,440
	2019		3,543
	Thereafter		20,064
	Total future minimum payments	$	35,743

Convertible_Preferred_Stock_an1

Convertible Preferred Stock and Stockholders' Equity (Deficit) (Tables)	12 Months Ended
	Dec. 31, 2014
Stockholders Equity Note [Abstract]
Summary of Unvested Common Stock	The following table summarizes information about unvested common stock (in thousands except per share amounts):
		Number of			Weighted-
		Shares			Average
					Purchase Price
					($)
	Unvested shares at December 31, 2011		11		$	3.96
	Shares sold		36			4.13
	Shares vested		(40	)		4.13
	Shares repurchased		—			—
	Unvested shares at December 31, 2012		7			4.13
	Shares sold		23			4.12
	Shares vested		(25	)		4.01
	Shares repurchased		—			—
	Unvested shares at December 31, 2013		5			3.78
	Shares sold		—			—
	Shares vested		(3	)		4.49
	Shares repurchased		-1			5.27
	Unvested shares at December 31, 2014		1		$	4.49

Equity_Compensation_Plans_and_1

Equity Compensation Plans and Stock Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Summary of Option Activity Under 2007 Plan 2013 Plan and Inducement Plan and Related Information	The following table summarizes option activity under the 2007 Plan, the 2013 Plan, and the Inducement Plan and related information (in thousands, except per share and contractual term amounts):
		Shares			Shares			Weighted-			Weighted-		Aggregate
		Available			Subject to			Average			Average		Intrinsic
		for Grant			Outstanding			Exercise			Remaining		Value
					Options			Price Per			Contractual
								Share			Term
											(in years)
	Options outstanding at December 31, 2011		57			1,491			4.13
	Additional shares reserved under plan		718			—			—
	Options granted		(921	)		921			3.96
	Options exercised		—			(36	)		4.13
	Options forfeited		213			(213	)		4.3
	Options outstanding at December 31, 2012		67			2,163			4.13
	Additional shares reserved under plan		2,571			—			—
	Options granted		(1,444	)		1,444			7.91
	Options exercised		—			(23	)		4.12
	Options forfeited		39			(39	)		4.12
	Options outstanding at December 31, 2013		1,233			3,545			5.63
	Additional shares reserved under the Inducement plan		1,000			—			—
	Options granted		(1,004	)		1,004			25.63
	Restricted stock units granted		-130			—			—
	Options exercised		—			(675	)		4.31
	Options forfeited		61			(61	)		10.95
	Restricted stock units cancelled		7			—			—
	Balances outstanding at December 31, 2014		1,167			3,813		$	11.05			8.3	$	75,837
	Vested and expected to vest at December 31, 2014		—			3,658		$	10.79			8.3	$	73,701
	Vested at December 31, 2014		—			1,427		$	5.5			7.5	$	36,148
Stock Options Outstanding and Exercisable by Exercise Price	The following table summarizes information about stock options outstanding at December 31, 2014:
		Options Outstanding									Options Exercisable
	Exercise Price	Number			Weighted			Weighted			Number		Weighted
		Outstanding			Average			Average			Exercisable		Average
					Remaining			Exercise					Exercise
					Contractual			Price					Price
					Life (Years)
	$2.76 - $4.30		1,285,437			7.3		$	3.94			1,285,437	$	3.94
	$4.65 - $11.00		1,503,836			8.3			7.24			1,447,892		7.09
	$19.41 - $22.52		336,750			9.7			20.69			—		—
	$23.24 - $23.96		94,153			9.3			23.63			6,072		23.29
	$25.70 - $41.40		592,535			9.6			28.65			12,499		33.76
	Total		3,812,711			8.3		$	11.05			2,751,900	$	5.78
Schedule of Stock-Based Compensation Expense	Stock-based compensation expense included in the Company’s consolidated statements of operations is as follows (in thousands):
		Year Ended December 31,
		2014			2013			2012
	Research and development:
	Employees	$	1,657		$	776		$	368
	Non-employee consultants		4,045			1,659			117
	General and administrative:
	Employees		3,980			1,474			692
	Non-employee consultants		13			—			—
	Total stock-based compensation expense	$	9,695		$	3,909		$	1,177
Summary of Restricted Stock Unit Activity	The following table summarizes RSU activity for the twelve months ended December 31, 2014 (in thousands, except per share amounts):
		Restricted Stock Units
		Number of Shares			Weighted Average
					Grant Date Fair
					Value Per Share
	Nonvested at December 31, 2013		—		$	—
	Granted		130			27.07
	Vested		—			—
	Canceled		-7			30.96
	Nonvested at December 31, 2014		123		$	26.85
Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Schedule of Valuation Assumptions Used to Calculate Fair Value of Stock Options	The following table illustrates the assumptions for the Black-Scholes option-pricing model used in determining the fair value of options granted to employees:
		Year Ended December 31,
		2014			2013			2012
	Expected volatility		87-109	%		95-110	%		87-95	%
	Expected dividends		0	%		0	%		0	%
	Expected term (in years)		6			6			6
	Risk-free rate		1.6-2.0	%		1.1-1.8	%		0.8-1.1	%
Non Employee Stock Option
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Schedule of Valuation Assumptions Used to Calculate Fair Value of Stock Options	The fair value of non-employee stock options was calculated using the Black-Scholes valuation model, based on the following assumptions:
		Year Ended December 31,
		2014			2013			2012
	Expected volatility		92	%		91%-122	%		83	%
	Expected dividends		0	%		0	%		0	%
	Expected term (in years)		9-Jul			10-May			10-Sep
	Risk-free rate		1.8-2.0	%		1.3-2.7	%		1.6-1.7	%

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Reconciliation of Statutory Federal Income Tax Rate To Company Effective Tax Rate	The reconciliation of the statutory federal income tax rate to the Company’s effective tax rate is as follows:
		Year Ended December 31,
		2014			2013			2012
	Federal statutory rate	34	%		34	%		34	%
	State statutory rate	5.8			5.8			5.8
	Research and development tax credit	2.3			1.7			1.4
	Permanent item	(1.0	)		(1.0	)		(0.8	)
	Valuation allowance	(41.1	)		(40.5	)		(40.4	)
	Provision for income taxes	—	%		—	%		—	%
Tax Effects of Temporary Differences and Carry Forwards	The tax effects of temporary differences and carryforwards that give rise to significant portions of the deferred tax assets are as follows (in thousands):
		December 31,
		2014				2013				2012
	Federal and state net operating loss carryforwards	$	91,604			$	64,768			$	40,323
	Federal and state research and development credits		6,253				4,703				3,550
	Depreciation		3,036				1,538				1,421
	Intangible assets		—				(6	)			(12	)
	Accruals and reserves		7,900				4,942				2,718
			108,793				75,945				48,000
	Valuation allowance		(108,793	)			(75,945	)			(48,000	)
	Net deferred tax assets	$	—			$	—			$	—
Schedule of Unrecognized Tax Benefits	The Company recognizes uncertain tax positions when it is more-likely-than-not, based on the technical merits, that the position will not be sustained upon examination. A reconciliation of the beginning and ending amounts of unrecognized tax benefits is as follows (in thousands):
		2014				2013				2012
	Balance at January 1	$	2,476			$	1,836			$	1,455
	Increases related to current period tax positions		713				588				350
	Increases related to prior period tax positions		—				52			31
	Balance as of December 31	$	3,189			$	2,476			$	1,836

Subsequent_Events_Tables

Subsequent Events (Tables)	12 Months Ended
	Dec. 31, 2014
Subsequent Events [Abstract]
Schedule of Adjusted Condensed Balance Sheet Data	The table below shows, on a pro forma basis, the impact of the Company’s offering on certain condensed balance sheet items. The as adjusted condensed balance sheet data below gives effect to the sale of 6,034,910 shares of securities.
		As of December 31, 2014
		Actual			As Adjusted
			(unaudited)
			(in thousands)
	Balance Sheet Data:
	Cash, cash equivalents and short-term investments	$	135,757		$	349,509
	Working capital		122,291			336,043
	Total assets		151,839			365,591
	Accumulated deficit		(305,728	)		(305,728	)
	Total stockholders’ equity		118,461			332,213

Selected_Quarterly_Financial_D1

Selected Quarterly Financial Data (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Summary of Quarterly Financial Information	The following table contains quarterly financial information for 2014 and 2013. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period.
			December 31,			September 30,			June 30,			March 31,			December 31,			September 30,			June 30,			March 31,
			2014			2014			2014			2014			2013			2013			2013			2013
			(In thousands, except share and per share data)
	Operating expenses:
	Research and		$	16,596		$	11,647		$	11,075		$	10,909		$	10,914		$	12,158		$	13,295		$	22,604
	development(1)
	General and			10,486			7,311			5,322			4,795			3,732			2,685			2,988			2,535
	administrative
	Total operating			27,082			18,958			16,397			15,704			14,646			14,843			16,283			25,139
	expenses
	Loss from			(27,082	)		(18,958	)		(16,397	)		(15,704	)		(14,646	)		(14,843	)		(16,283	)		(25,139	)
	operations
	Interest and other			69			17			36			27			13,314			(10,382	)		(3,549	)		(864	)
	income (expense),
	net(2)
	Interest expense			(635	)		(494	)		(376	)		(391	)		(407	)		(410	)		(388	)		(248	)
	Net loss			(27,648	)		(19,435	)		(16,737	)		(16,068	)		(1,739	)		(25,635	)		(20,220	)		(26,251	)
	Deemed dividend to			—			—			—			—			(7,336	)		—			—			—
	preferred
	stockholders(3)
	Net loss attributable		$	(27,648	)	$	(19,435	)	$	(16,737	)	$	(16,068	)	$	(9,075	)	$	(25,635	)	$	(20,220	)	$	(26,251	)
	to common
	stockholders
	Net loss per share		$	(0.80	)	$	(0.57	)	$	(0.51	)	$	(0.54	)	$	(0.68	)	$	(79.91	)	$	(64.87	)	$	(84.61	)
	attributable to
	common
	stockholders basic
	and diluted
	Weighted average common			34,368,776			34,064,780			33,141,384			29,710,415			13,430,244			320,802			311,703			310,275
	shares used to compute
	net loss per share
	attributable to common
	stockholders, basic and
	diluted(4)
	-1	During the quarter ended March 31, 2013, a $12.5 million milestone payment was made to Amgen pursuant to the amended and restated IP license and assignment agreement, resulting from the initiation of dosing in the pivotal Phase 3 trial for patiromer.
	-2	During 2013 interest and other income (expense), net is primarily related to the change in value of warrants to acquire convertible preferred stock that were issued in connection with preferred stock financings, capital loan and equipment line of credit.
	-3	During 2013, the Company recognized deemed dividends to preferred stockholders as a result of the issuance of warrants as part of the C-2 financing.
	-4	During the quarter ended December 31, 2013, shares outstanding increased as a result of the initial public offering of the Company’s common stock and the conversion of convertible preferred stock and warrants into common stock.

Organization_Additional_Inform

Organization - Additional Information (Details) (USD $)	0 Months Ended	12 Months Ended		0 Months Ended	2 Months Ended		0 Months Ended
	Apr. 16, 2014	Dec. 31, 2014	Dec. 31, 2013	Mar. 03, 2015	Mar. 03, 2015	Feb. 28, 2015	Nov. 15, 2013	Dec. 15, 2014
		Segment
Description Of Business And Significant Accounting Policies [Line Items]
Commencement date of operations		29-Oct-07
Number of business segments		1
Aggregate net proceeds from issuance of common stock	$94,600,000	$94,607,000	$77,964,000
Common stock, shares issued		35,045,000	29,710,000
Common stock, shares authorized		300,000,000	300,000,000
Preferred stock, shares authorized		5,000,000	5,000,000
Subsequent Event
Description Of Business And Significant Accounting Policies [Line Items]
Aggregate net proceeds from issuance of common stock				161,900,000		51,800,000
Issuance of common stock, shares					6,034,910	1,549,910
Shares issued, price per share						$34.48
Aggregate offering price						53,400,000
Gross proceeds from issuance of common stock				172,300,000
Common Stock
Description Of Business And Significant Accounting Policies [Line Items]
Issuance of common stock, shares	4,130,611	4,131,000	7,878,000
Shares issued, price per share	$24.50
Common Stock | Subsequent Event
Description Of Business And Significant Accounting Policies [Line Items]
Issuance of common stock, shares				4,485,000
Shares issued, price per share				$38.50	38.5
Initial Public Offering
Description Of Business And Significant Accounting Policies [Line Items]
Aggregate net proceeds from issuance of common stock							78,000,000
Common stock, shares issued							7,877,500
Conversion of preferred stock into common stock, shares							21,487,959
Common stock, shares authorized							300,000,000
Preferred stock, shares authorized							5,000,000
Initial Public Offering | Convertible Preferred Stock
Description Of Business And Significant Accounting Policies [Line Items]
Common stock, shares issued							18,924,883
Initial Public Offering | Convertible Preferred Stock Warrants
Description Of Business And Significant Accounting Policies [Line Items]
Common stock, shares issued							2,563,076
Shelf Registration | Maximum
Description Of Business And Significant Accounting Policies [Line Items]
Aggregate offering price								250,000,000
Shelf Registration | Common Stock
Description Of Business And Significant Accounting Policies [Line Items]
Issuance of common stock, shares		515,000
Shelf Registration | Common Stock | Maximum
Description Of Business And Significant Accounting Policies [Line Items]
Aggregate offering price								70,000,000
Shelf Registration
Description Of Business And Significant Accounting Policies [Line Items]
Aggregate net proceeds from issuance of common stock		16,061,000
Issuance of common stock, shares		514,710
Shares issued, price per share		$32.17
Aggregate offering price		$16,600,000
Relypsa UK LTD
Description Of Business And Significant Accounting Policies [Line Items]
Date of entity incorporation		28-Oct-14

Summary_of_Significant_Account3

Summary of Significant Accounting Policies - Additional Information (Details) (USD $)

3 Months Ended

12 Months Ended

In Thousands, except Share data, unless otherwise specified

Dec. 31, 2014

Sep. 30, 2014

Jun. 30, 2014

Mar. 31, 2014

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Oct. 31, 2013

Significant Accounting Policies [Line Items]

Accumulated deficit

($305,728)

($225,840)

($305,728)

($225,840)

Net loss

$27,648

$19,435

$16,737

$16,068

$1,739

$25,635

$20,220

$26,251

$79,888

$73,845

$43,725

Convertible preferred stock converted into common stock

18,924,883

18,924,883

472,187

Employee Stock Purchase Plan

Significant Accounting Policies [Line Items]

Purchase price per share on fair market value on purchase date

85.00%

Patents

Significant Accounting Policies [Line Items]

Estimated useful life of acquired patent rights

7 years

Maximum

Significant Accounting Policies [Line Items]

Estimated useful life of plant and equipment

7 years

Maximum | Employee Stock Purchase Plan

Significant Accounting Policies [Line Items]

Purchase price per share of fair market value on first day of offering period

85.00%

Minimum

Significant Accounting Policies [Line Items]

Estimated useful life of plant and equipment

3 years

Credit Concentration Risk | Investments | Maximum

Significant Accounting Policies [Line Items]

Concentration Risk, Percentage

5.00%

Summary_of_Significant_Account4

Summary of Significant Accounting Policies - Schedule of Shares Excluded from Computations of Diluted Net Loss per Common Share (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computations of diluted net loss per common share	3,993,000	3,614,000	21,687,000
Convertible Preferred Stock - As Converted to Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computations of diluted net loss per common share			17,281,000
Warrants to Purchase Convertible Preferred Stock - As Converted to Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computations of diluted net loss per common share			2,236,000
Warrants To Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computations of diluted net loss per common share	56,000	64,000
Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computations of diluted net loss per common share	3,813,000	3,545,000	2,163,000
Restricted Stock Units
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computations of diluted net loss per common share	123,000
Common Stock Subject to Repurchase
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Common stock equivalents excluded from computations of diluted net loss per common share	1,000	5,000	7,000

Fair_Value_Measurements_Fair_V

Fair Value Measurements - Fair Value of Financial Instruments Measured at Fair Value on Recurring Basis (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Financial assets:
Total financial assets	$127,973	$93,523
Liabilities:
Total liabilities	233
Contingent Put Option Liability
Liabilities:
Total liabilities	233
Corporate Bonds
Financial assets:
Total financial assets	71,763
Agency Bonds
Financial assets:
Total financial assets	15,162
Level 1
Financial assets:
Total financial assets	10,202	93,523
Level 2
Financial assets:
Total financial assets	117,771
Level 2 | Corporate Bonds
Financial assets:
Total financial assets	71,763
Level 2 | Agency Bonds
Financial assets:
Total financial assets	15,162
Level 3
Liabilities:
Total liabilities	233
Level 3 | Contingent Put Option Liability
Liabilities:
Total liabilities	233
Money Market Funds
Financial assets:
Total financial assets	10,202	93,523
Money Market Funds | Level 1
Financial assets:
Total financial assets	10,202	93,523
Commercial Paper
Financial assets:
Total financial assets	30,846
Commercial Paper | Level 2
Financial assets:
Total financial assets	$30,846

Fair_Value_Measurements_Summar

Fair Value Measurements - Summary of Changes in Estimated Fair Value of Level 3 Instruments Measured at Fair Value on Recurring Basis (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning Balance	$0	$19,529
Net increase in fair value included in other income (expense)	123	1,564
Reclassification of warrant liability to equity		-28,647
Ending Balance	233	0
Preferred Stock Warrants
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement with unobservable inputs reconciliation, issuances of instruments		7,554
Contingent Put Option Liability
Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Fair value measurement with unobservable inputs reconciliation, issuances of instruments	$110

Fair_Value_Measurements_Additi

Fair Value Measurements - Additional Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Oct. 31, 2013
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Exercise price of warrants		$15.31
PWERM
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Liquidity event scenarios	The future liquidity event scenarios and the probability of each were as follows: 1) initial public offering 65%; 2) remain private 10%; 3) strategic merger or sale 20%; or 4) dissolution or liquidation 5%.
Convertible Preferred Stock
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Exercise price of warrants	0.17

Cash_Cash_Equivalents_and_Shor2

Cash, Cash Equivalents, and Short-Term Investments - Summary of Cash, Cash Equivalents, and Short-Term Investments (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
In Thousands, unless otherwise specified
Cash And Cash Equivalents [Line Items]
Amortized Cost	$135,778	$94,759
Gross Unrealized Gains	15	0
Gross Unrealized Losses	-36	0
Estimated Fair Value	135,757	94,759
Cash and cash equivalents	30,264	94,759	15,221	15,249
Short-term investments	105,493
Total financial assets	135,757	94,759
Cash and Money Market Funds
Cash And Cash Equivalents [Line Items]
Amortized Cost	17,986	94,759
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	17,986	94,759
Commercial Paper
Cash And Cash Equivalents [Line Items]
Amortized Cost	30,835
Gross Unrealized Gains	11
Gross Unrealized Losses	0
Estimated Fair Value	30,846
Corporate Bonds
Cash And Cash Equivalents [Line Items]
Amortized Cost	71,794
Gross Unrealized Gains	2
Gross Unrealized Losses	-33
Estimated Fair Value	71,763
Agency Bonds
Cash And Cash Equivalents [Line Items]
Amortized Cost	15,163
Gross Unrealized Gains	2
Gross Unrealized Losses	-3
Estimated Fair Value	$15,162

Cash_Cash_Equivalents_and_Shor3

Cash, Cash Equivalents, and Short-Term Investments - Additional Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Cash Cash Equivalents And Short Term Investments [Abstract]
Realized gain or losses on available-for-sale securities	$0	$0
Available for sale securities, continuous unrealized loss position for more than one year	$0

Property_and_Equipment_Summary

Property and Equipment - Summary of Property and Equipment (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Property Plant And Equipment [Line Items]
Property and equipment, gross	$10,630	$8,035
Less accumulated depreciation and amortization	-7,163	-2,411
Property and equipment, net	3,467	5,624
Machinery and Equipment
Property Plant And Equipment [Line Items]
Property and equipment, gross	2,829	2,315
Computers and Software
Property Plant And Equipment [Line Items]
Property and equipment, gross	657	126
Leasehold Improvements
Property Plant And Equipment [Line Items]
Property and equipment, gross	6,135	4,998
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Property and equipment, gross	$1,009	$596

Property_and_Equipment_Additio

Property and Equipment - Additional Information (Detail) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Property Plant And Equipment [Abstract]
Depreciation and amortization	$5,492	$1,088	$394

Accrued_Liabilities_Schedule_o

Accrued Liabilities - Schedule of Accrued Liabilities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Payables And Accruals [Abstract]
Accrued contract manufacturing	$6,655	$3,617
Accrued accounts payable	795	771
Accrued professional and consulting services	184	85
Other	103	104
Accrued clinical trial expenses	41	1,370
Total accrued liabilities	$7,778	$5,947

Borrowings_Additional_Informat

Borrowings - Additional Information (Details) (USD $)

0 Months Ended

12 Months Ended

0 Months Ended

12 Months Ended

1 Months Ended

3 Months Ended

Jun. 30, 2008

Dec. 31, 2014

Dec. 31, 2013

Apr. 27, 2010

30-May-14

Dec. 31, 2010

Sep. 30, 2010

Dec. 31, 2014

Oct. 31, 2013

31-May-13

Dec. 31, 2008

Nov. 30, 2013

Dec. 31, 2012

Jan. 31, 2013

Payment

Payment

Debt Instrument [Line Items]

Line of credit amount

$1,500,000

$1,600,000

Line of credit, interest rate

8.00%

5.75%

Exercise price of warrants

$15.31

Line of credit principal amount outstanding

600,000

600,000

Line of credit, terms of repayment

36 equal monthly installments

Line of credit, remaining and expired amount

300,000

300,000

Line of credit, expiry date

30-Jun-13

Proceeds from advance

3,549,000

12,404,000

Convertible Promissory notes

Debt Instrument [Line Items]

Loan amount

6,000,000

Debt instrument, interest rate

8.00%

Debt instrument maturity date

30-Sep-10

Capital Loan

Debt Instrument [Line Items]

Number of debt repayments

38

38

30

Exercise price of warrants

$23.69

Fair value of warrants

200,000

200,000

Loan amount

9,000,000

Debt instrument, interest rate

9.85%

Prepayment charges of outstanding loan

11.85%

Line of credit principal amount outstanding

100,000

Line of credit increased

300,000

Warrant issued to purchase common stock

7,808

12,661

7,808

Final payment as a percentage of amounts borrowed

8.50%

Warrants, fair value assumptions, interest rate

2.06%

Warrants, fair value assumptions, expected life

7 years

Warrants, fair value assumptions, volatility factor

91.67%

Warrants, fair value assumptions, dividend yield

0.00%

Total interest expense

1,700,000

1,300,000

Proceeds from advance

15,000,000

Repayment of outstanding debt

11,000,000

Final payment fee

400,000

400,000

Advance not yet received

20,000,000

20,000,000

Debt instrument, payment terms

Payments on the new $15.0 million loan will be interest only through January 2016, followed by 30 months of equal monthly payments of interest and principal.

Warrants exercised

13,609

Estimated fair value of contingent put option liability

200,000

200,000

Principal amount outstanding on loan

15,000,000

15,000,000

Capital Loan | Initial advance

Debt Instrument [Line Items]

Debt instrument, interest rate

7.40%

7.40%

Capital Loan | Three Month LIBOR Rate | Second Tranche

Debt Instrument [Line Items]

Debt instrument, basis spread on variable rate

7.17%

Capital Loan | Interest Only First Twelve Months

Debt Instrument [Line Items]

Number of debt repayments

42

Debt instrument, interest rate

10.00%

Series A Convertible Preferred Stock | Capital Loan

Debt Instrument [Line Items]

Warrants issued to purchase preferred stock

26,162

Fair value of warrants

200,000

Warrant issued to purchase common stock

9,800

Series B-1 Convertible Preferred Stock | Convertible Promissory notes

Debt Instrument [Line Items]

Warrants issued to purchase preferred stock

88,868

453,953

Exercise price of warrants

$13.50

Fair value of warrants

300,000

Warrant issued to purchase common stock

44,713

Aggregate principal amount converted to stock

6,100,000

Conversion price per share

$13.50

Series C-1 Convertible Preferred Stock

Debt Instrument [Line Items]

Warrants issued to purchase preferred stock

2,076,643

Exercise price of warrants

$0.17

Series C-1 Convertible Preferred Stock | Capital Loan

Debt Instrument [Line Items]

Warrants issued to purchase preferred stock

54,437

Exercise price of warrants

$9.18

Line of Credit

Debt Instrument [Line Items]

Number of debt repayments

36

36

Fair value of warrants

25,000

25,000

Warrant issued to purchase common stock

3,998

3,998

Final payment as a percentage of amounts borrowed

9.25%

Warrants, fair value assumptions, interest rate

1.07%

Warrants, fair value assumptions, expected life

7 years

Warrants, fair value assumptions, volatility factor

91.60%

Warrants, fair value assumptions, dividend yield

0.00%

Total interest expense

100,000

100,000

Line of credit, utilized amount

1,300,000

Line of Credit | Series A-1 Convertible Preferred Stock

Debt Instrument [Line Items]

Warrants issued to purchase preferred stock

2,616

Exercise price of warrants

$17.20

Warrants expiration term

10 years

Fair value of warrants

$17,000

Line of Credit | Series C-1 Convertible Preferred Stock

Debt Instrument [Line Items]

Warrants issued to purchase preferred stock

6,968

Exercise price of warrants

$9.18

Minimum

Debt Instrument [Line Items]

Number of debt repayments

36

Minimum | Capital Loan

Debt Instrument [Line Items]

Prepayment charges of outstanding loan

1.00%

Maximum

Debt Instrument [Line Items]

Number of debt repayments

42

Maximum | Capital Loan

Debt Instrument [Line Items]

Prepayment charges of outstanding loan

3.00%

Commitments_and_Contingencies_1

Commitments and Contingencies - Additional Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Commitments And Contingencies [Line Items]
Supply agreement term	The Lanxess Supply Agreement terminates on December 31, 2020 unless terminated earlier. The Company may extend the Lanxess Supply Agreement for an additional five years with notice.
Agreement termination date	31-Dec-20
Extended agreement period	5 years
Net value of the leasehold improvements and other assets	$1,500,000
Deferred rent liability	1,156,000	640,000
Lease commencement date	1-Feb-15
Lease termination period	60 days
Operating lease, term of contract	10 years
Operating lease, renewal term	5 years
Deferred rent, long-term	983,000	4,394,000
Additional lease allowance	2,200,000
One-time lease improvement allowance	600,000
Rent expense	-2,100,000	1,100,000	1,500,000
Indemnification agreements	In the normal course of business, the Company enters into contracts and agreements that contain a variety of representations and warranties and provide for general indemnifications. The Company’s exposure under these agreements is unknown because it involves claims that may be made against the Company in the future, but have not yet been made. To date, the Company has not paid any claims or been required to defend any action related to its indemnification obligations. However, the Company may record charges in the future as a result of these indemnification obligations. The Company also has indemnification obligations to its officers and directors for specified events or occurrences, subject to some limits, while they are serving at the Company’s request in such capacities. There have been no claims to date and the Company has director and officer insurance that may enable the Company to recover a portion of any amounts paid for future potential claims. The Company believes the fair value of these indemnification agreements is minimal. Accordingly, the Company has not recorded any liabilities for these agreements as of December 31, 2014.
Irrevocable Letter of Credit
Commitments And Contingencies [Line Items]
Security deposit	$700,000
DPx Fine Chemicals
Commitments And Contingencies [Line Items]
Supply agreement term	The Agreement terminates on May 14, 2021 unless earlier terminated.
Agreement termination date	14-May-21
Patheon Inc
Commitments And Contingencies [Line Items]
Supply agreement term	The Patheon Supply Agreement may be terminated by the Company with written notice under certain provisions.

Commitments_and_Contingencies_2

Commitments and Contingencies - Schedule of Future Minimum Payments Under Supply Agreement(Details) (DPx Fine Chemicals, Lanxess and Patheon, USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
DPx Fine Chemicals, Lanxess and Patheon
Long Term Purchase Commitment [Line Items]
2015	$45,266
2016	41,174
2017	37,800
2018	42,400
2019	42,201
Thereafter	22,000
Total future minimum payments	$230,841

Commitments_and_Contingencies_3

Commitments and Contingencies - Schedule of Future Minimum Payments Under Operating Lease (Details) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Commitments And Contingencies Disclosure [Abstract]
2015	$2,178
2016	3,178
2017	3,340
2018	3,440
2019	3,543
Thereafter	20,064
Total future minimum payments	$35,743

Technology_Agreement_Additiona

Technology Agreement - Additional Information (Details) (USD $)	1 Months Ended	3 Months Ended
In Millions, unless otherwise specified	Feb. 28, 2013	Mar. 31, 2013
Finite Lived Intangible Assets [Line Items]
Research and development expense	$12.50	$12.50
Acquisition expenses	$30
Minimum
Finite Lived Intangible Assets [Line Items]
Transaction cost, additional payment percentage on purchase price	6.70%
Maximum
Finite Lived Intangible Assets [Line Items]
Transaction cost, additional payment percentage on purchase price	10.00%

Convertible_Preferred_Stock_an2

Convertible Preferred Stock and Stockholders' Equity (Deficit) - Additional Information (Details) (USD $)	0 Months Ended	1 Months Ended	12 Months Ended			0 Months Ended
	Apr. 16, 2014	Oct. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Nov. 15, 2013	Nov. 30, 2013	Dec. 15, 2014
Convertible Preferred Stock And Stockholders Equity [Line Items]
Net proceeds from issuance of convertible preferred stock				$14,983,000	$64,571,000
Exercise price of warrants		$15.31
Increase to stockholders’ equity (deficit)		7,300,000
Convertible preferred stock converted into common stock		472,187	18,924,883
Preferred stock, shares outstanding			0	0
Preferred stock, dividend rate			8.00%
Dividends declared or paid			0
Net proceeds from public offerings	94,600,000		94,607,000	77,964,000
Unvested common stock, shares			1,276	5,125	6,627
Unvested common stock, value			6,000	23,000	27,000
Initial Public Offering
Convertible Preferred Stock And Stockholders Equity [Line Items]
Convertible preferred stock converted into common stock							18,924,883
Net proceeds from public offerings						78,000,000
Shelf Registration
Convertible Preferred Stock And Stockholders Equity [Line Items]
Issuance of common stock, shares			514,710
Shares issued, price per share			$32.17
Aggregate offering price			16,600,000
Net proceeds from public offerings			16,061,000
Minimum
Convertible Preferred Stock And Stockholders Equity [Line Items]
Purchase price per share			$3.96
Maximum
Convertible Preferred Stock And Stockholders Equity [Line Items]
Purchase price per share			$7.40
Maximum | Shelf Registration
Convertible Preferred Stock And Stockholders Equity [Line Items]
Aggregate offering price								250,000,000
Common Stock
Convertible Preferred Stock And Stockholders Equity [Line Items]
Issuance of common stock, shares	4,130,611		4,131,000	7,878,000
Shares issued, price per share	$24.50
Common Stock | Shelf Registration
Convertible Preferred Stock And Stockholders Equity [Line Items]
Issuance of common stock, shares			515,000
Common Stock | Minimum
Convertible Preferred Stock And Stockholders Equity [Line Items]
Preferred stock holders, ownership percentage			55.00%
Common Stock | Maximum | Shelf Registration
Convertible Preferred Stock And Stockholders Equity [Line Items]
Aggregate offering price								70,000,000
Series C-1 Convertible Preferred Stock
Convertible Preferred Stock And Stockholders Equity [Line Items]
Issuance of common stock, shares					7,076,901
Shares issued, price per share					$9.18
Net proceeds from issuance of convertible preferred stock					65,000,000
Number of securities issued on purchase of excess than pro-rata shares					2
Warrants issued to purchase preferred stock					2,076,643
Exercise price of warrants					$0.17
Warrant exercisable contractual life					5 years
Series C-1 Convertible Preferred Stock | Minimum
Convertible Preferred Stock And Stockholders Equity [Line Items]
Preferred stock holders, ownership percentage			50.00%
Series C-2 Convertible Preferred Stock
Convertible Preferred Stock And Stockholders Equity [Line Items]
Issuance of common stock, shares		1,633,126
Shares issued, price per share		$9.18
Net proceeds from issuance of convertible preferred stock		$15,000,000
Number of securities issued on purchase of excess than pro-rata shares		2
Warrants issued to purchase preferred stock		479,221
Exercise price of warrants		$0.17
Warrant exercisable contractual life		5 years
Series A Series A-1 And B-2 Convertible Preferred Stock | Initial Public Offering
Convertible Preferred Stock And Stockholders Equity [Line Items]
Preferred stock, conversion basis			one-for-one
Series C-1 And C-2 Convertible Preferred Stock | Initial Public Offering
Convertible Preferred Stock And Stockholders Equity [Line Items]
Preferred stock, conversion basis			one-for-one
Series B-1 Convertible Preferred Stock | Initial Public Offering
Convertible Preferred Stock And Stockholders Equity [Line Items]
Preferred stock, conversion basis			one-for-1.47

Convertible_Preferred_Stock_an3

Convertible Preferred Stock and Stockholders' Equity (Deficit) - Summary Of Unvested Common Stock (Details) (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stockholders Equity Note [Abstract]
Number of shares, Unvested at beginning	5	7	11
Number of shares, Shares sold		23	36
Number of shares, Shares vested	-3	-25	-40
Number of shares, Shares repurchased	-1
Number of shares, Unvested at ending	1	5	7
Weighted Average Purchase Price, Unvested shares, Beginning balance	$3.78	$4.13	$3.96
Weighted Average Purchase Price, Shares sold		$4.12	$4.13
Weighted Average Purchase Price, Shares vested	$4.49	$4.01	$4.13
Weighted Average Purchase Price, Shares repurchased	$5.27
Weighted Average Purchase Price, Unvested shares, Ending balance	$4.49	$3.78	$4.13

Equity_Compensation_Plans_and_2

Equity Compensation Plans and Stock Based Compensation - Additional Information (Details) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Nov. 30, 2013	Jun. 30, 2014
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$9,695,000	$3,909,000	$1,177,000
Weighted-average grant-date fair value of options grant	$19.77	$6.23	$3.10
Total fair value of options vested	9,100,000	5,000,000	1,100,000
Intrinsic value of options exercised	14,900,000	158,000	8,000
Unrecognized compensation cost	29,700,000
Expected period of compensation expense recognized	2 years 10 months 24 days
Shares Subject to Outstanding Options, Options granted		23,000	36,000
Common stock options, exercise price	$11.05	$7.40	$3.96
Restricted Stock Units
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected period of compensation expense recognized	3 years 6 months
Restricted stock units, vested	0
Unrecognized stock-based compensation expense	2,900,000
Non Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Subject to Outstanding Options, Options granted	0	290	29,651
ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for issuance	255,317
Stock-based compensation expense	$100,000
2007 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option granted period	10 years
Option vest over period	4 years
2013 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option granted period				10 years
Share issued or transferred under the Plan				1,276,587
Common stock reserved for issuance	531,213
2014 Employment Commencement Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Share issued or transferred under the Plan	363,800
Common stock reserved for inducement options					1,000,000

Equity_Compensation_Plans_and_3

Equity Compensation Plans and Stock Based Compensation - Summary of Option Activity Under 2007 Plan 2013 Plan and Inducement Plan and Related Information (Details) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Available for Grant, Restricted stock units granted	-130,000
Shares Subject to Outstanding Options, Options granted		23,000	36,000
Shares Subject to Outstanding Options, Vested	2,751,900
Weighted-Average Exercise Price, Options Outstanding, beginning balance	$7.40	$3.96
Weighted-Average Exercise Price, Options Outstanding, ending balance	$11.05	$7.40	$3.96
Weighted-Average Exercise Price, Vested	$5.78
2007 Plan, 2013 Plan and Inducement Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Available for Grant, Options Outstanding, beginning balance	1,233,000	67,000	57,000
Shares Available for Grant, Additional shares reserved	1,000,000	2,571,000	718,000
Shares Available for Grant, Options granted	-1,004,000	-1,444,000	-921,000
Shares Available for Grant, Restricted stock units granted	-130,000
Shares Available for Grant, Options forfeited	61,000	39,000	213,000
Shares Available for Grant, Restricted stock units cancelled	7,000
Shares Available for Grant, Options Outstanding, ending balance	1,167,000	1,233,000	67,000
Shares Subject to Outstanding Options, Options Outstanding, beginning balance	3,545,000	2,163,000	1,491,000
Shares Subject to Outstanding Options, Options granted	1,004,000	1,444,000	921,000
Shares Subject to Outstanding Options, Options exercised	-675,000	-23,000	-36,000
Shares subject to Outstanding Options, Options forfeited	-61,000	-39,000	-213,000
Shares Subject to Outstanding Options, Options Outstanding, ending balance	3,813,000	3,545,000	2,163,000
Shares Subject to Outstanding Options, Vested and expected to vest	3,658,000
Shares Subject to Outstanding Options, Vested	1,427,000
Weighted-Average Exercise Price, Options Outstanding, beginning balance	$5.63	$4.13	$4.13
Weighted-Average Exercise Price, Options granted	$25.63	$7.91	$3.96
Weighted-Average Exercise Price, Options exercised	$4.31	$4.12	$4.13
Weighted-Average Exercise Price, Options forfeited	$10.95	$4.12	$4.30
Weighted-Average Exercise Price, Options Outstanding, ending balance	$11.05	$5.63	$4.13
Weighted-Average Exercise Price, Vested and expected to vest	$10.79
Weighted-Average Exercise Price, Vested	$5.50
Weighted-Average Remaining Contractual Term, Options Outstanding	8 years 3 months 18 days
Weighted-Average Remaining Contractual Term, Vested and expected to vest	8 years 3 months 18 days
Weighted-Average Remaining Contractual Term, Vested	7 years 6 months
Aggregate Intrinsic Value, Options Outstanding	$75,837
Aggregate Intrinsic Value, Vested and expected to vest	73,701
Aggregate intrinsic value, Vested	$36,148

Equity_Compensation_Plans_and_4

Equity Compensation Plans and Stock Based Compensation - Stock Options Outstanding and Exercisable by Exercise Price (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock Options Outstanding , Number Outstanding	3,812,711
Stock Options Outstanding , Weighted-Average Contractual Life (Years)	8 years 3 months 18 days
Stock Options Outstanding, Weighted Average Exercise Price	$11.05	$7.40	$3.96
Stock Options Exercisable, Number Exercisable	2,751,900
Options Exercisable, Weighted Average Exercise Price	$5.78
Exercise Price 2.76 - 4.30
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock Options Outstanding , Number Outstanding	1,285,437
Stock Options Outstanding , Weighted-Average Contractual Life (Years)	7 years 3 months 18 days
Stock Options Outstanding, Weighted Average Exercise Price	$3.94
Stock Options Exercisable, Number Exercisable	1,285,437
Options Exercisable, Weighted Average Exercise Price	$3.94
Exercise Price 4.65 - 11.00
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock Options Outstanding , Number Outstanding	1,503,836
Stock Options Outstanding , Weighted-Average Contractual Life (Years)	8 years 3 months 18 days
Stock Options Outstanding, Weighted Average Exercise Price	$7.24
Stock Options Exercisable, Number Exercisable	1,447,892
Options Exercisable, Weighted Average Exercise Price	$7.09
Exercise Price 19.41- 22.52
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock Options Outstanding , Number Outstanding	336,750
Stock Options Outstanding , Weighted-Average Contractual Life (Years)	9 years 8 months 12 days
Stock Options Outstanding, Weighted Average Exercise Price	$20.69
Exercise Price 23.24 - 23.96
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock Options Outstanding , Number Outstanding	94,153
Stock Options Outstanding , Weighted-Average Contractual Life (Years)	9 years 3 months 18 days
Stock Options Outstanding, Weighted Average Exercise Price	$23.63
Stock Options Exercisable, Number Exercisable	6,072
Options Exercisable, Weighted Average Exercise Price	$23.29
Exercise Price 25.70 - 41.40
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock Options Outstanding , Number Outstanding	592,535
Stock Options Outstanding , Weighted-Average Contractual Life (Years)	9 years 7 months 6 days
Stock Options Outstanding, Weighted Average Exercise Price	$28.65
Stock Options Exercisable, Number Exercisable	12,499
Options Exercisable, Weighted Average Exercise Price	$33.76

Equity_Compensation_Plans_and_5

Equity Compensation Plans and Stock Based Compensation - Schedule of Stock-Based Compensation Expense (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation	$9,695	$3,909	$1,177
Research and Development | Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation	1,657	776	368
Research and Development | Non-employee Consultants
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation	4,045	1,659	117
General and Administrative | Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation	3,980	1,474	692
General and Administrative | Non-employee Consultants
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation	$13

Equity_Compensation_Plans_and_6

Equity Compensation Plans and Stock Based Compensation - Schedule of Valuation Assumptions Used to Calculate Fair Value of Employee Stock Options (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected dividends	0.00%	0.00%	0.00%
Expected term (in years)	6 years	6 years
Risk-free rate, minimum	1.60%	1.10%	0.80%
Risk-free rate, maximum	2.00%	1.80%	1.10%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	87.00%	95.00%	87.00%
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	109.00%	110.00%	95.00%

Equity_Compensation_Plans_and_7

Equity Compensation Plans and Stock Based Compensation - Schedule of Valuation Assumptions Used to Calculate Fair Value of Nonemployee Stock Options (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility	92.00%		83.00%
Expected dividends	0.00%	0.00%	0.00%
Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility		91.00%
Expected term (in years)	7 years	5 years	9 years
Risk-free rate	1.80%	1.30%	1.60%
Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility		122.00%
Expected term (in years)	9 years	10 years	10 years
Risk-free rate	2.00%	2.70%	1.70%

Equity_Compensation_Plans_and_8

Equity Compensation Plans and Stock Based Compensation - Summary of Restricted Stock Unit Activity (Details) (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014
Compensation Related Costs [Abstract]
Number of Shares, Granted	130
Number of Shares, Canceled	-7
Number of Shares, Nonvested at Ending	123
Weighted Average Grant Date Fair Value Per Share, Granted	$27.07
Weighted Average Grant Date Fair Value Per Share, Canceled	$30.96
Weighted Average Grant Date Fair Value Per Share, Nonvested at Ending	$26.85

Income_Taxes_Reconciliation_of

Income Taxes - Reconciliation of Statutory Federal Income Tax Rate To Company Effective Tax Rate (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Federal statutory rate	34.00%	34.00%	34.00%
State statutory rate	5.80%	5.80%	5.80%
Research and development tax credit	2.30%	1.70%	1.40%
Permanent item	-1.00%	-1.00%	-0.80%
Valuation allowance	-41.10%	-40.50%	-40.40%
Provision for income taxes	0.00%	0.00%	0.00%

Income_Taxes_Additional_Inform

Income Taxes - Additional Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Line Items]
Deferred Tax Assets, Gross	$108,793,000	$75,945,000	$48,000,000
Tax benefit	0	0	0
Valuation allowance	32,800,000	27,900,000	7,500,000
Excess stock option deductions	9,300,000
Deferred income tax asset for net operating loss	28,600,000
Research and development expense carry forwards	5,200,000
Future tax expenses	5,600,000
Federal Income Tax [Member]
Income Tax Disclosure [Line Items]
Operating loss carryforwards	240,100,000
Operating loss expiration year	2027
California State Income Tax [Member]
Income Tax Disclosure [Line Items]
Operating loss carryforwards	$234,300,000
Operating loss expiration year	2017

Income_Taxes_Tax_Effects_of_Te

Income Taxes - Tax Effects of Temporary Differences and Carry Forwards (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Income Tax Disclosure [Abstract]
Federal and state net operating loss carryforwards	$91,604	$64,768	$40,323
Federal and state research and development credits	6,253	4,703	3,550
Depreciation	3,036	1,538	1,421
Intangible assets		-6	-12
Accruals and reserves	7,900	4,942	2,718
Deferred Tax Assets, Gross	108,793	75,945	48,000
Valuation allowance	-108,793	-75,945	-48,000
Net deferred tax assets	$0	$0	$0

Income_Taxes_Schedule_of_Unrec

Income Taxes - Schedule of Unrecognized Tax Benefits (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Beginning balance	$2,476	$1,836	$1,455
Increases related to current period tax positions	713	588	350
Increases related to prior period tax positions		52	31
Ending balance	$3,189	$2,476	$1,836

401k_Savings_Plan_Additional_I

401(k) Savings Plan - Additional Information (Details) (401(k) Savings Plan)	12 Months Ended
	Dec. 31, 2014
401(k) Savings Plan
Defined Contribution Plan Disclosure [Line Items]
Employee minimum percent contribution for their annual compensation	1.00%
Employee maximum percent contribution for their annual compensation	90.00%

Related_Party_Transaction_Addi

Related Party Transaction - Additional Information (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Related Party Transactions [Abstract]
Description of related party transaction	In October 2010, the Company entered into a Consulting and Independent Contractor Agreement with an entity controlled by Dr. Klaus Veitinger, a member of its board of directors, under which Dr. Veitinger provides certain consulting services to the Company in connection with patiromer.
Terms and manner of related party settlement	This agreement was terminated in conjunction with the Company’s IPO that was completed in November 2013.
Amount paid to related party		$0.10	$0.20

Subsequent_Events_Additional_I

Subsequent Events - Additional Information (Details) (USD $)	0 Months Ended	12 Months Ended		0 Months Ended	2 Months Ended
	Apr. 16, 2014	Dec. 31, 2014	Dec. 31, 2013	Mar. 03, 2015	Mar. 03, 2015	Feb. 28, 2015
Subsequent Event [Line Items]
Net proceeds from public offerings	$94,600,000	$94,607,000	$77,964,000
Common Stock
Subsequent Event [Line Items]
Shares issued, price per share	$24.50
Issuance of common stock, shares	4,130,611	4,131,000	7,878,000
Subsequent Event
Subsequent Event [Line Items]
Number of sold shares pursuant to its at-the -market offering program						1,549,910
Net proceeds from public offerings				161,900,000		51,800,000
Shares issued, price per share						$34.48
Issuance of common stock, shares					6,034,910	1,549,910
Gross proceeds from issuance of common stock				$172,300,000
Subsequent Event | Common Stock
Subsequent Event [Line Items]
Shares issued, price per share				$38.50	38.5
Issuance of common stock, shares				4,485,000

Subsequent_Events_Schedule_of_

Subsequent Events - Schedule of Adjusted Condensed Balance Sheet Data (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
In Thousands, unless otherwise specified
Balance Sheet Data:
Cash, cash equivalents and short-term investments	$135,757	$94,759
Working capital	122,291
Total assets	151,839	106,031
Accumulated deficit	-305,728	-225,840
Total stockholders’ equity	118,461	74,850	-147,361	-86,275
As Adjusted
Balance Sheet Data:
Cash, cash equivalents and short-term investments	349,509
Working capital	336,043
Total assets	365,591
Accumulated deficit	-305,728
Total stockholders’ equity	$332,213

Selected_Quarterly_Financial_D2

Selected Quarterly Financial Data (unaudited) - Summary of Quarterly Financial Information (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Operating expenses:
Research and development	$16,596	$11,647	$11,075	$10,909	$10,914	$12,158	$13,295	$22,604	$50,227	$58,971	$36,052
General and administrative	10,486	7,311	5,322	4,795	3,732	2,685	2,988	2,535	27,914	11,940	7,285
Total operating expenses	27,082	18,958	16,397	15,704	14,646	14,843	16,283	25,139	78,141	70,911	43,337
Loss from operations	-27,082	-18,958	-16,397	-15,704	-14,646	-14,843	-16,283	-25,139	-78,141	-70,911	-43,337
Interest and other income (expense), net	69	17	36	27	13,314	-10,382	-3,549	-864	149	-1,481	-382
Interest expense	-635	-494	-376	-391	-407	-410	-388	-248	-1,896	-1,453	-6
Net loss	-27,648	-19,435	-16,737	-16,068	-1,739	-25,635	-20,220	-26,251	-79,888	-73,845	-43,725
Deemed dividend to preferred stockholders					-7,336					-7,336	-18,716
Net loss attributable to common stockholders	($27,648)	($19,435)	($16,737)	($16,068)	($9,075)	($25,635)	($20,220)	($26,251)	($79,888)	($81,181)	($62,441)
Net loss per share attributable to common stockholders, basic and diluted	($0.80)	($0.57)	($0.51)	($0.54)	($0.68)	($79.91)	($64.87)	($84.61)	($2.43)	($22.42)	($205.45)
Weighted average common shares used to compute net loss per share attributable to common stockholders, basic and diluted	34,368,776	34,064,780	33,141,384	29,710,415	13,430,244	320,802	311,703	310,275	32,837,508	3,620,235	303,927

Selected_Quarterly_Financial_D3

Selected Quarterly Financial Data (unaudited) - Summary of Quarterly Financial Information (Parenthetical) (Details) (USD $)	1 Months Ended	3 Months Ended
In Millions, unless otherwise specified	Feb. 28, 2013	Mar. 31, 2013
Selected Quarterly Financial Information [Abstract]
Research and development expense	$12.50	$12.50