Goodwill and Intangible Assets - Finite-lived Intangible Assets and Related Accumulated Amortization (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Intangible Assets
Cost or Fair Value	$ 47,349	$ 9,485
Accumulated Amortization	(8,642)	(6,099)
Net	38,707	3,386
Amortization expense	$ 2,600	1,000	$ 800
Useful lives (in years)	12 years 4 months 24 days
Trademarks and trade names
Intangible Assets
Cost or Fair Value	$ 14,187
Accumulated Amortization	(312)
Net	$ 13,875
Useful lives (in years)	15 years
Membership and customer lists
Intangible Assets
Cost or Fair Value	$ 28,988	9,485
Accumulated Amortization	(8,194)	(6,099)
Net	$ 20,794	$ 3,386
Useful lives (in years)	11 years 2 months 12 days
Websites
Intangible Assets
Cost or Fair Value	$ 4,174
Accumulated Amortization	(136)
Net	$ 4,038
Useful lives (in years)	10 years