Revenue from Contracts with Customers - Contract Balances (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) item	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Revenue from Contracts with Customers
Signing bonuses received from vendors | item	2
Contract liabilities	$ 1,000,000.0	$ 1,800,000
Decrease in recognized revenues and contract liabilities	800,000	800,000	$ 800,000
Change in contract liabilities due to changes in transaction price estimates	0
Receivables from contracts with customers or contract assets	$ 0	$ 0