Performance-Based Common Stock Warrants - Narrative (Details)		9 Months Ended	24 Months Ended
	Mar. 09, 2016 USD ($) vesting_period $ / shares shares	Sep. 30, 2021 USD ($) shares	Dec. 31, 2019 shares	Dec. 31, 2017 shares
Class of Warrant or Right [Line Items]
Unrecognized estimated fair value of unvested warrants | $		$ 200,000
Common stock purchase warrants
Class of Warrant or Right [Line Items]
Number of shares called by warrants (in shares)	725,000
Exercise price of warrants (in dollars per share) | $ / shares	$ 54.55
Number of vesting periods | vesting_period	3
Term of successive vesting periods	2 years	2 years	2 years	2 years
Vesting period one	2 years
Vested warrants outstanding (in shares)		275,000
Unearned underlying shares (in shares)		225,000
Number of warrants vested (in shares)		0
Common stock purchase warrants | Comcast Corporation
Class of Warrant or Right [Line Items]
Number of warrants vested in period (in shares)			100,000	175,000
Maximum warrants available to vest (in shares)			250,000	250,000
Common stock purchase warrants | Supply threshold
Class of Warrant or Right [Line Items]
Aggregate level of purchases, $260 million threshold | $	$ 260,000,000
Aggregate level of purchases, $340 million threshold | $	$ 340,000,000
Supply threshold amount | $		$ 340,000,000