Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Assets:
Assets fair value		$ 210,616	$ 160,054
Liabilities:
Liabilities fair value		24,500	18,500
Contingent Consideration | Crisp Acquisition
Liabilities:
Liabilities fair value	[1]	24,500	18,500
Cash and Cash Equivalents | Money Market Funds
Assets:
Assets fair value		150,526	100,152
Cash and Cash Equivalents | Certificates of Deposit
Assets:
Assets fair value		9,216
Short-Term Investments | Certificates of Deposit
Assets:
Assets fair value		50,874	59,902
Level 1
Assets:
Assets fair value		150,526	100,152
Level 1 | Cash and Cash Equivalents | Money Market Funds
Assets:
Assets fair value		150,526	100,152
Level 2
Assets:
Assets fair value		60,090	59,902
Level 2 | Cash and Cash Equivalents | Certificates of Deposit
Assets:
Assets fair value		9,216
Level 2 | Short-Term Investments | Certificates of Deposit
Assets:
Assets fair value		50,874	59,902
Level 3
Liabilities:
Liabilities fair value		24,500	18,500
Level 3 | Contingent Consideration | Crisp Acquisition
Liabilities:
Liabilities fair value	[1]	$ 24,500	$ 18,500

[1] Included in contingent consideration related to acquisitions