Net Loss Per Share - Basic and Diluted Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net loss									$ (61,335)	$ (66,837)	$ (85,516)
Weighted-average shares of common stock outstanding (in shares)	57,826	57,627	56,935	56,642	56,300	55,964	55,278	54,827	57,262	55,595	54,171
Net loss per share — basic and diluted (usd per share)	$ (0.16)	$ (0.26)	$ (0.37)	$ (0.29)	$ (0.33)	$ (0.22)	$ (0.32)	$ (0.33)	$ (1.07)	$ (1.20)	$ (1.58)

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.

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