Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Fair Value Measurements Recurring - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financial Assets:
Financial Assets, fair value	$ 442,441
Financial Liabilities:
Financial Liabilities, fair value		$ 989
Convertible Preferred Stock Warrant Liability
Financial Liabilities:
Financial Liabilities, fair value		100
Common Stock Warrant Liability
Financial Liabilities:
Financial Liabilities, fair value		889
Level 1
Financial Assets:
Financial Assets, fair value	116,642
Level 2
Financial Assets:
Financial Assets, fair value	325,799
Level 3
Financial Liabilities:
Financial Liabilities, fair value		989
Level 3 | Convertible Preferred Stock Warrant Liability
Financial Liabilities:
Financial Liabilities, fair value		100
Level 3 | Common Stock Warrant Liability
Financial Liabilities:
Financial Liabilities, fair value		889
Money Market Funds
Financial Assets:
Financial Assets, fair value	116,642	110,001
Money Market Funds | Level 1
Financial Assets:
Financial Assets, fair value	116,642	$ 110,001
U S Government Agencies Debt Securities
Financial Assets:
Financial Assets, fair value	204,068
U S Government Agencies Debt Securities | Level 2
Financial Assets:
Financial Assets, fair value	204,068
Corporate Debt Securities
Financial Assets:
Financial Assets, fair value	83,267
Corporate Debt Securities | Level 2
Financial Assets:
Financial Assets, fair value	83,267
Commercial Paper
Financial Assets:
Financial Assets, fair value	38,464
Commercial Paper | Level 2
Financial Assets:
Financial Assets, fair value	$ 38,464