Summary of Significant Accounting Policies - Initial Public Offering (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Concentration of Credit Risk
Amount in excess of FDIC limits	$ 227,900	$ 119,000
Goodwill and Other Intangible Assets
Useful lives (in years)	12 years 4 months 24 days
Self-Insurance Program
Self insured claims liability	$ 16,100	11,300
Letters of credit	$ 12,200	10,800
Revenue Recognition
Lifetime memberships period	18 years
Advertising Expense
Advertising costs capitalized	$ 6,500	6,900
Advertising expenses	86,600	76,000	$ 76,200
Shipping and Handling Fees and Costs
Shipping, Handling and Transportation Costs	4,100	2,300	5,600
New Accounting Pronouncements
Deferred tax asset, noncurrent	155,551	24,433	$ 6,234
Assets.
Advertising Expense
Advertising costs capitalized	5,200	5,500
Deferred revenue
Advertising Expense
Advertising costs capitalized	$ 1,200	$ 1,400
Building and improvements
Property and Equipment, net
Estimated useful lives	P40Y
Minimum | Leasehold improvements
Property and Equipment, net
Estimated useful lives	P3Y
Minimum | Furniture and equipment
Property and Equipment, net
Estimated useful lives	P3Y
Minimum | Software
Property and Equipment, net
Estimated useful lives	P3Y
Maximum | Leasehold improvements
Property and Equipment, net
Estimated useful lives	P40Y
Maximum | Furniture and equipment
Property and Equipment, net
Estimated useful lives	P12Y
Maximum | Software
Property and Equipment, net
Estimated useful lives	P5Y
Membership and customer lists
Goodwill and Other Intangible Assets
Useful lives (in years)	11 years 2 months 12 days
Trademarks and trade names
Goodwill and Other Intangible Assets
Useful lives (in years)	15 years
Websites
Goodwill and Other Intangible Assets
Useful lives (in years)	10 years