Related Party Transactions - Additional Information (Details) (USD $)	1 Months Ended		3 Months Ended		9 Months Ended		1 Months Ended
	May 31, 2014	Dec. 31, 2011 Founders	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Feb. 28, 2014	Dec. 31, 2013		Jan. 31, 2012	Dec. 31, 2012
Related Party Transaction [Line Items]
Number of founders entered into notes receivable agreement		4
Notes receivable from related parties		$ 133,000
Note receivable interest rate		0.20%
Imputed interest rate in relation to the notes		4.00%
Notes receivable from related parties								107,000	[1]
Outstanding balance of notes receivable	111,000
Accrued interest amount forgiven					1,000
Convertible preferred stock, shares issued			21,131,217		21,131,217			9,153,906
Collaboration and license revenue			16,052,000	506,000	24,613,000	1,519,000
Deferred revenue under collaboration license agreement, current			25,176,000		25,176,000			14,283,000	[1]
Deferred revenue under collaboration license agreement, non-current			43,526,000		43,526,000			28,567,000	[1]
Reduction of research and development expense			1,200,000	700,000	3,600,000	1,200,000
Prepaid assets			17,950,000		17,950,000			5,688,000	[1]
Receivables from related parties								278,000	[1]
Research and development expense			18,496,000	4,934,000	51,357,000	22,057,000
Accounts payable - related parties			4,963,000		4,963,000			383,000	[1]
Accrued other			3,154,000		3,154,000			1,922,000
Purchase commitment amount of services performed					17,100,000
General and administration expense			362,000	0	507,000	0
Interest expense from transactions with related party			0	1,595,000	2,687,000	1,595,000
InteKrin Acquisition
Related Party Transaction [Line Items]
Business Combination Consideration Transferred1							5,000,000
Securities held by Mr.Lanfear in InteKrin Russia							10.00%
Bridge Loans
Related Party Transaction [Line Items]
Aggregate principle amount				10,000,000		10,000,000
Share price				$ 0.0167		$ 0.0167
Total aggregate amount of the bridge loans from related parties				7,100,000		7,100,000
Carrying value of the bridge loans net of discount								3,100,000
Interest expense from transactions with related party			0	1,600,000	2,700,000	1,600,000
Prepaid Clinical, Material, Manufacturing and Other - Related Parties
Related Party Transaction [Line Items]
Prepaid assets			5,542,000		5,542,000			3,177,000
Daiichi Sankyo - Related Party
Related Party Transaction [Line Items]
Collaboration and license revenue			432,000	506,000	1,445,000	1,519,000
Deferred revenue under collaboration license agreement			4,600,000		4,600,000			6,100,000		10,000,000
Deferred revenue under collaboration license agreement, current			1,700,000		1,700,000			2,000,000
Deferred revenue under collaboration license agreement, non-current			2,900,000		2,900,000			4,100,000
Reduction of research and development expense			1,200,000	700,000	3,600,000	1,200,000
Daiichi Sankyo - Related Party | Series B
Related Party Transaction [Line Items]
Convertible preferred stock, shares issued			2,867,426		2,867,426
Daiichi Sankyo - Related Party | Minimum
Related Party Transaction [Line Items]
Related party transaction ownership percentage			10.00%		10.00%
Cook
Related Party Transaction [Line Items]
Receivables from related parties								278,000
Research and development expense			100,000	700,000	4,100,000	6,100,000
Cook | Prepaid Clinical, Material, Manufacturing and Other - Related Parties
Related Party Transaction [Line Items]
Prepaid assets								3,000,000
Cook | Series B
Related Party Transaction [Line Items]
Convertible preferred stock, shares issued										2,150,569	2,150,569
Cook | Minimum | Series B
Related Party Transaction [Line Items]
Related party transaction ownership percentage										10.00%	10.00%
Medpace Inc
Related Party Transaction [Line Items]
Research and development expense			6,200,000	500,000	14,900,000	3,300,000
Accounts payable - related parties			4,900,000		4,900,000			383,000
Minimum fee commitment			35,000,000		35,000,000
Purchase commitment amount of services performed					17,100,000
Medpace Inc | Prepaid Clinical, Material, Manufacturing and Other - Related Parties
Related Party Transaction [Line Items]
Prepaid assets			5,500,000		5,500,000			198,000
Medpace Inc | Accrued Clinical and Manufacturing Related Parties
Related Party Transaction [Line Items]
Accrued other			3,200,000		3,200,000			2,800,000
Recruiting Services
Related Party Transaction [Line Items]
Research and development expense			0	0	241,000	63,000
Accounts payable - related parties			86,000		86,000			0
Recruiting Services | Accrued Clinical and Manufacturing Related Parties
Related Party Transaction [Line Items]
Accrued other			60,000		60,000
Recruiting Services | Recruiting Services
Related Party Transaction [Line Items]
Prepaid assets								0
General and administration expense			$ 362,000	$ 0	$ 507,000	$ 0
Executive Team
Related Party Transaction [Line Items]
Number of founders entered into notes receivable agreement		3

[1] Amounts have been derived from the December 31, 2013 audited financial statements included in the Company’s registration statement on Form S-1 (No. 333-198906) filed with the Securities and Exchange Commission on November 6, 2014