Commitments and Contingencies - Concentrations of Credit Risk (Details) - item	6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Accounts receivable. | Credit risk
Customer Concentration
Concentration Risk Number of Customers	2		2
Concentration risk (as a percent)	78.80%		59.90%
Accounts receivable. | Credit risk | Customer one
Customer Concentration
Concentration risk (as a percent)	56.20%		40.00%
Accounts receivable. | Credit risk | Customer two
Customer Concentration
Concentration risk (as a percent)	22.60%		19.90%
Revenues. | Customer concentration
Customer Concentration
Concentration risk (as a percent)	71.30%	49.40%
Revenues. | Customer concentration | Customer one
Customer Concentration
Concentration risk (as a percent)		10.80%
Revenues. | Customer concentration | Walmart
Customer Concentration
Concentration risk (as a percent)	40.20%	38.60%
Revenues. | Customer concentration | Amazon
Customer Concentration
Concentration risk (as a percent)	31.10%