Fair Value Measurements - Summary of Fair Value Measurements of Company's Financial Assets and Liabilities (Details) (Fair Value, Recurring, USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Assets:
Assets at fair value	$ 105,510	$ 32,652
Liabilities:
Liabilities at fair value		6,456
Convertible preferred stock warrant liability
Liabilities:
Liabilities at fair value		6,456
Level 1
Assets:
Assets at fair value	105,410	32,472
Liabilities:
Liabilities at fair value		0
Level 1 | Convertible preferred stock warrant liability
Liabilities:
Liabilities at fair value		0
Level 2
Assets:
Assets at fair value	100	180
Liabilities:
Liabilities at fair value		0
Level 2 | Convertible preferred stock warrant liability
Liabilities:
Liabilities at fair value		0
Level 3
Assets:
Assets at fair value	0	0
Liabilities:
Liabilities at fair value		6,456
Level 3 | Convertible preferred stock warrant liability
Liabilities:
Liabilities at fair value		6,456
Money Market Funds
Assets:
Assets at fair value	105,410	32,472
Money Market Funds | Level 1
Assets:
Assets at fair value	105,410	32,472
Money Market Funds | Level 2
Assets:
Assets at fair value	0	0
Money Market Funds | Level 3
Assets:
Assets at fair value	0	0
Certificate of Deposit
Assets:
Assets at fair value	100	180
Certificate of Deposit | Level 1
Assets:
Assets at fair value	0	0
Certificate of Deposit | Level 2
Assets:
Assets at fair value	100	180
Certificate of Deposit | Level 3
Assets:
Assets at fair value	$ 0	$ 0