Summary of Significant Accounting Policies - AOCI and Concentrations of Credit Risk (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) club	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Accounting Policies [Line Items]
Number of clubs | club	165
Foreign currency translation adjustments, net of tax of $0 for the years ended in December 31, 2017, 2016 and 2015	$ 42,000	$ (176,000)	$ (165,000)
Foreign currency translation adjustment, tax	0	0	0
Cash and cash equivalents held at one financial institution	19,932,000
Cash and cash equivalents	$ 30,321,000	$ 45,596,000	$ 76,217,000	$ 93,452,000
Switzerland
Accounting Policies [Line Items]
Number of clubs | club	3