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Stock-Based Compensation - Option Activity (Details) - Common Class B - Marketwise, LLC - shares
6 Months Ended 12 Months Ended
Jun. 30, 2021
Dec. 31, 2020
Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward]      
Unvested, beginning balance (in shares) 75,044 77,981 55,000
Granted (in shares) 17,690 62,676 59,099
Vested (in shares) (29,848) (65,613) (36,118)
Unvested, ending balance (in shares) 62,886 75,044 77,981