Revenue from Contracts with Customers - Contract Balances (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) item	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Revenue from Contracts with Customers
Signing bonuses received from vendors | item			2
Contract liabilities	$ 1,200,000		$ 1,200,000		$ 1,800,000
Decrease in recognized revenues and contract liabilities	200,000	$ 200,000	600,000	$ 600,000
Change in contract liabilities due to changes in transaction price estimates	0	$ 0	0	$ 0
Receivables from contracts with customers or contract assets	$ 0		$ 0		$ 0