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Marketable Securities - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Marketable Securities [Abstract]      
Fair value of investments owned $ 96,100   $ 208,200
Marketable securities 96,061   208,176
Realized and unrealized net gains on marketable securities 11,400    
Trading securities with fair value     198,300
Realized and unrealized net gain from trading securities   $ 5,900  
Investments classified as available-for-sale, fair value     $ 9,900
Gains (loss) recognized related to marketable securities classified as available-for-sale   300  
Marketable securities sold at fair value 123,541 $ 7,829  
Purchase of marketable securities $ 0