XML 51 R40.htm IDEA: XBRL DOCUMENT v3.21.1
FAIR VALUE MEASUREMENTS - Gross holding gains and fair value of held-to-maturity securities (Details)
Dec. 31, 2020
USD ($)
Fair Value, Recurring | Level 1  
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Fair Value $ 517,500,000