Revenue from Contracts with Customers - Contract Balances (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) item	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Revenue from Contracts with Customers
Signing bonuses received from vendors | item			2
Contract liabilities	$ 2,200,000		$ 2,200,000		$ 2,600,000
Decrease in recognized revenues and contract liabilities	200,000	$ 200,000	400,000	$ 400,000
Change in contract liabilities due to changes in transaction price estimates			0
Receivables from contracts with customers or contract assets	$ 0		$ 0		$ 0