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Cash (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Mar. 31, 2018
Dec. 31, 2017
Cash and Cash Equivalents [Abstract]        
Cash and cash equivalents $ 771,003 $ 729,547    
Cash restricted or segregated under regulations and other 17,324 6,500    
Total cash, cash equivalents and restricted cash shown in the statement of cash flows $ 788,327 $ 736,047 $ 637,308 $ 532,887