Deferred Commissions - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Deferred Costs [Abstract]
Deferred commissions	$ 18,034,000		$ 18,246,000
Amortization expense for deferred commissions	2,078,000	$ 1,534,000
Deferred commissions impairment charges	$ 0	$ 0