Summary of Significant Accounting Policies - Concentration of Revenue and Accounts Receivable (Details) - Customer concentration	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Revenue | Customer A
Concentration of Revenue and Accounts Receivable
Concentration risk percentage	49.00%
Revenue | Customer B
Concentration of Revenue and Accounts Receivable
Concentration risk percentage	18.00%	58.00%
Revenue | Customer C
Concentration of Revenue and Accounts Receivable
Concentration risk percentage	13.00%	28.00%
Revenue | Customer D
Concentration of Revenue and Accounts Receivable
Concentration risk percentage	12.00%
Accounts Receivable | Customer A
Concentration of Revenue and Accounts Receivable
Concentration risk percentage	37.00%		26.00%
Accounts Receivable | Customer C
Concentration of Revenue and Accounts Receivable
Concentration risk percentage	12.00%		10.00%
Accounts Receivable | Customer D
Concentration of Revenue and Accounts Receivable
Concentration risk percentage	16.00%		26.00%
Accounts Receivable | Customer E
Concentration of Revenue and Accounts Receivable
Concentration risk percentage	11.00%		12.00%
Accounts Receivable | Customer F
Concentration of Revenue and Accounts Receivable
Concentration risk percentage			17.00%