Revenue from Contracts with Customers - Contract Balances (Details)	3 Months Ended	9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($) item	Dec. 31, 2018 USD ($)
Revenue from Contracts with Customers
Signing bonuses received from vendors | item		2
Contract liabilities	$ 2,000,000.0	$ 2,000,000.0	$ 2,600,000
Decrease in recognized revenues and contract liabilities	200,000	600,000
Change in contract liabilities due to changes in transaction price estimates		0
Receivables from contracts with customers or contract assets	$ 0	$ 0	$ 0