Summary of Significant Accounting Policies - Summary of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Measurements, Recurring [Member]
Assets:
Cash Equivalents	$ 45,513
Investments:
Total assets	195,154
Liabilities:
Liability for preferred stock warrant		$ 1,582
Total liabilities		1,582
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets:
Cash Equivalents	45,513
Investments:
Total assets	195,154
Fair Value, Inputs, Level 3 [Member]
Liabilities:
Liability for preferred stock warrant		1,582
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Liabilities:
Liability for preferred stock warrant		1,582
Total liabilities		$ 1,582
Corporate Bonds [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	44,206
Corporate Bonds [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	44,206
Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	83,327
Commercial Paper [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	83,327
Government Securities [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	22,108
Government Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Investments:
Investments	$ 22,108