Intangible Assets, net - Components of gross and net intangible asset balances (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization of intangible assets	$ 148	$ 123	$ 297	$ 247
Finite-lived intangible assets
Net carrying amount	2,327		$ 2,327
Finite-lived intangible assets acquired				$ 500
Finite-lived intangible assets, remaining amortization period			3 years 11 months 6 days
Intellectual Property
Finite-lived intangible assets
Gross carrying amount	4,750		$ 4,750		$ 4,750
Accumulated amortization	(2,423)		(2,423)		(2,126)
Net carrying amount	$ 2,327		$ 2,327		$ 2,624