Fair Value Measurements - Fair Value of Warrants (Details)	3 Months Ended
	Mar. 31, 2019 $ / shares
Tranche one of warrants issued with the Amazon.com, Inc transaction agreement
Fair Value
Fair value per warrant	$ 1.15
Tranche two of warrants issued with the Amazon.com, Inc. Transaction Agreement
Fair Value
As of vesting date, first installment - second tranche	2.16
As of vesting date, second installment - second tranche	1.54
As of period end - second tranche	2.05
As of prior period end - second tranche	1.57
Tranche one of warrants issued with the Walmart Stores Inc transaction agreement
Fair Value
Fair value per warrant	1.88
Tranche two of warrants issued with the Walmart Stores, Inc. Transaction Agreement
Fair Value
As of period end - second tranche	1.94
As of prior period end - second tranche	$ 1.71