Summary of Significant Accounting Policies - Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2017
Equipment | Minimum
Property, Plant and Equipment
Useful life (in years)	3 years
Equipment | Maximum
Property, Plant and Equipment
Useful life (in years)	7 years
Rental equipment | Minimum
Property, Plant and Equipment
Useful life (in years)	5 years
Rental equipment | Maximum
Property, Plant and Equipment
Useful life (in years)	6 years
Leasehold improvements | Minimum
Property, Plant and Equipment
Useful life (in years)	3 years
Leasehold improvements | Maximum
Property, Plant and Equipment
Useful life (in years)	10 years
Capitalized software and website development costs
Property, Plant and Equipment
Useful life (in years)	3 years