Summary of Significant Accounting Policies - Schedule of Annual Dues Deferred Revenue (Details) - Annual Dues - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Revenue Arrangement [Line Items]
Deferred revenue recognition period	12 months
Balance at beginning of period	$ 14,227	$ 13,106	$ 12,912
New billings	34,837	33,774	31,952
Revenue recognized	(33,767)	(32,653)	(31,758)
Balance at end of period	$ 15,297	$ 14,227	$ 13,106