Fair Value Measurements - Fair value measurement inputs (Details)	Mar. 31, 2021 $ / shares	Dec. 31, 2020 $ / shares	Dec. 31, 2019 $ / shares
Stock price
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Promissory note, fair value measurement inputs	3.75	3.00	3.01
Time
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Promissory note, expected term	1 year 6 months	1 year 9 months	3 months
Risk-free rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Promissory note, fair value measurement inputs	0.0012	0.0012	0.0155
Volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Promissory note, fair value measurement inputs	0.506	0.623	0.382