Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue:
Products									$ 310,716	$ 256,263	$ 258,968
Services									6,382	6,569	10,998
Total revenues	$ 72,509	$ 79,458	$ 85,177	$ 79,954	$ 70,604	$ 64,337	$ 64,079	$ 63,812	317,098	262,832	269,966
Cost of revenue (excluding depreciation and amortization):
Products									210,281	162,488	164,837
Services									4,848	7,767	7,196
Total cost of revenue									$ 215,129	$ 170,255	$ 172,033