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Marketable Securities - Net gains and losses (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Marketable Securities    
Net gains (losses) recognized on equity securities $ 12,981 $ (2,336)
Less: net gains recognized on equity securities redeemed (1)  
Unrealized gains (losses) recognized on equity securities $ 12,980 $ (2,336)