Condensed Quarterly Financial Data (unaudited) - Schedule of Condensed Quarterly Financial Data (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 15,618us-gaap_Revenues	$ 12,343us-gaap_Revenues	$ 10,881us-gaap_Revenues	$ 8,751us-gaap_Revenues	$ 10,120us-gaap_Revenues	$ 8,389us-gaap_Revenues	$ 7,218us-gaap_Revenues	$ 5,676us-gaap_Revenues	$ 47,593us-gaap_Revenues	$ 31,403us-gaap_Revenues	$ 22,973us-gaap_Revenues
Net loss attributable to common stockholders	$ (12,380)	$ (12,148)	$ (14,088)	$ (11,422)	$ (8,827)	$ (7,700)	$ (7,806)	$ (9,601)	$ (50,038)	$ (33,934)	$ (25,241)
Net loss per share-basic and diluted	$ (0.68)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.67)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.78)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.68)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.60)us-gaap_EarningsPerShareBasicAndDiluted	$ (0.53)us-gaap_EarningsPerShareBasicAndDiluted	$ (13.69)us-gaap_EarningsPerShareBasicAndDiluted	$ (17.88)us-gaap_EarningsPerShareBasicAndDiluted	$ (2.80)us-gaap_EarningsPerShareBasicAndDiluted	$ (4.44)us-gaap_EarningsPerShareBasicAndDiluted	$ (71.10)us-gaap_EarningsPerShareBasicAndDiluted

The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

No definition available.

Name:	us-gaap_EarningsPerShareBasicAndDiluted
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration