Revenue Recognition - Receivables, Assets and Liabilities From Contracts With Customers (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Short-term contract assets (sales commissions)	$ 97	$ 79
Long-term contract assets (sales commissions)	1,700	1,400
Short-term contract liabilities (deferred revenue)	42,510	42,494
Long-term contract liabilities (deferred revenue)	$ 27,398	$ 24,387