Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Accrued expenses	$ 2,371	$ 3,037
Long-lived intangible assets and fixed assets — basis difference	19,884	22,146
Net operating loss carryforwards	80,615	68,356
Stock-based compensation	16,886	19,515
Deferred revenue	2,739	4,060
Convertible note hedge	1,467	6,387
Other	2,721	2,276
Total deferred tax assets	126,683	125,777
Valuation allowance	(118,606)	(111,775)
Deferred tax assets, net of valuation allowance	8,077	14,002
Deferred tax liabilities:
Prepaid expenses and deferred commissions	(5,672)	(7,718)
Convertible note discount	(1,074)	(4,721)
Other	(410)	(591)
Total deferred tax liabilities	(7,156)	(13,030)
Net deferred tax assets (liabilities)	$ 921	$ 972