Convertible Preferred Stock and Stockholders' Equity (Deficit) (Details 2) (USD $)	12 Months Ended			80 Months Ended	12 Months Ended			80 Months Ended	12 Months Ended			80 Months Ended				0 Months Ended	12 Months Ended												12 Months Ended				12 Months Ended								12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013	Dec. 31, 2013 Research and development	Dec. 31, 2012 Research and development	Dec. 31, 2011 Research and development	Dec. 31, 2013 Research and development	Dec. 31, 2013 General and administrative	Dec. 31, 2012 General and administrative	Dec. 31, 2011 General and administrative	Dec. 31, 2013 General and administrative	Dec. 31, 2012 Conversion of convertible preferred stock	Dec. 31, 2013 Stock warrants	Dec. 31, 2012 Stock warrants	Sep. 13, 2013 ESPP	Dec. 31, 2013 ESPP	Dec. 31, 2013 Stock option	Dec. 31, 2012 Stock option	Dec. 31, 2011 Stock option	Dec. 31, 2013 Employee stock option	Dec. 31, 2012 Employee stock option	Dec. 31, 2011 Employee stock option	Dec. 31, 2012 Restricted stock awards	Dec. 31, 2013 Awards available under the 2013 Plan	Dec. 31, 2012 Awards available under the 2013 Plan	Dec. 31, 2013 Exchangeable shares	Dec. 31, 2012 Exchangeable shares	Dec. 31, 2013 Non-employee stock option	Dec. 31, 2012 Non-employee stock option	Dec. 31, 2011 Non-employee stock option	Dec. 31, 2012 Combined plans Performance-based milestones and market conditions	Dec. 31, 2013 Combined plans Stock option	Dec. 31, 2012 Combined plans Stock option	Dec. 31, 2013 Combined plans Stock option Performance-based	Dec. 31, 2012 Combined plans Stock option Performance-based	Dec. 31, 2012 Combined plans Stock option Performance-based milestones and market conditions	Dec. 31, 2013 Combined plans Stock option Maximum	Dec. 31, 2013 Combined plans Restricted stock awards	Aug. 31, 2013 2007 Plan	Dec. 31, 2013 2007 Plan Restricted stock awards Cliff vest	Dec. 31, 2013 2007 Plan Restricted stock awards inside the plans	Dec. 31, 2012 2007 Plan Restricted stock awards inside the plans	Dec. 31, 2013 2007 Plan Restricted stock awards outside the plans	Dec. 31, 2012 2007 Plan Restricted stock awards outside the plans	Aug. 28, 2013 2013 Plan
Convertible Preferred Stock and Stockholders' Equity (Deficit)
Shares authorized																																								2,423,072
Stock options generally vested on first anniversary (as a percent)																																	25.00%
General vesting period after first year																																	3 years						4 years
Term of stock awards																																						10 years
Shares reserved for issuance	3,899,433	9,737,086		3,899,433									7,229,590	36,074	36,074	729,000	729,000				1,726,991	1,432,369			1,003,526	154,448	403,842	884,605																		1,020,000
Additional shares authorized (as a percent)																2.00%																														4.00%
Monthly vesting period																																	4 years
Additional shares authorized																450,000
Stock purchases under the plan (in shares)																	0
Options
Outstanding at the beginning of the period (in shares)																																163,573	1,432,369		272,001
Granted (in shares)																																	355,780
Exercised (in shares)																																	(35,853)
Canceled (in shares)																																	(25,305)
Outstanding at the end of the period (in shares)																																163,573	1,726,991	1,432,369	174,730	272,001
Vested and expected to vest at the end of the period (in shares)																																	1,555,091
Exercisable at the end of the period (in shares)																																	668,667
Weighted Average Exercise Price Per Share
Balance at the beginning of the period (in dollars per share)																																	$ 1.43
Granted (in dollars per share)																																	$ 5.60
Exercised (in dollars per share)																																	$ 0.65
Canceled (in dollars per share)																																	$ 2.24
Balance at the end of the period (in dollars per share)																																	$ 2.30	$ 1.43
Vested and expected to vest at the end of the period (in dollars per share)																																	$ 1.92
Exercisable at the end of the period (in dollars per share)																																	$ 1.52
Weighted Average Remaining Contractual Term
Outstanding at the end of the period																																	8 years 5 months 19 days	9 years 1 month 20 days
Vested and expected to vest at the end of the period																																	8 years 4 months 13 days
Exercisable at the end of the period																																	7 years 10 months 20 days
Aggregate Intrinsic Value
Outstanding at the end of the period (in dollars)																																	$ 7,203,000	$ 51,000
Vested and expected to vest at the end of the period (in dollars)																																	6,935,000
Exercisable at the end of the period (in dollars)																																	3,190,000
Aggregate grant date fair value																																			800,000	200,000	200,000
Unrecognized compensation cost related to outstanding options																																	1,900,000	800,000
Expected recognition period of unrecognized compensation cost																																	2 years 10 months 24 days	3 years 4 months 24 days
Weighted-average grant date fair value per share of employee options granted (in dollars per share)																		$ 4.40	$ 1.11	$ 1.24
Cash received upon exercise of options (in dollars per share)																		23,000	15,000	14,000
Proceeds received from issuance of common stock	23,000	207,000	14,000	294,000																				200,000
Number of Shares
Balance at the beginning of the period (in shares)																																										434,884		564,904	564,904
Balance at the end of the period (in shares)																																										434,884	434,884	564,904	564,904
Weighted-Average Price
Balance at the beginning of the period (in dollars per share)																																										$ 0.468		$ 0.007	$ 0.007
Balance at the end of the period (in dollars per share)																																										$ 0.468	$ 0.468	$ 0.007	$ 0.007
Unvested shares issued (in shares)																																										91,740	121,332
Number of shares vested																																							56,713		35,027
Weighted-average assumptions to determine fair value of stock options
Risk-free interest rate (as a percent)																					1.60%	1.00%	1.10%						2.10%	1.20%	1.10%
Expected volatility (as a percent)																					90.00%	94.00%	90.00%						91.00%	94.00%	90.00%
Expected term																					5 years 10 months 24 days	6 years 1 month 6 days	6 years 1 month 6 days						7 years 3 months 18 days	7 years 6 months	6 years 1 month 6 days
Expected dividend yield (as a percent)																					0.00%	0.00%	0.00%						0.00%	0.00%	0.00%
Total stock-based compensation expense	$ 1,554,000	$ 155,000	$ 172,000	$ 2,323,000	$ 912,000	$ 97,000	$ 156,000	$ 1,569,000	$ 642,000	$ 58,000	$ 16,000	$ 754,000
Common stock reserved for future issuance
Shares reserved for issuance	3,899,433	9,737,086		3,899,433									7,229,590	36,074	36,074	729,000	729,000				1,726,991	1,432,369			1,003,526	154,448	403,842	884,605																		1,020,000