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Cash, Cash Equivalents, And Marketable Securities Cash, Cash Equivalents, And Marketable Securities narrative (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Cash, Cash Equivalents And Marketable Securities [Abstract]    
Financial Instruments, Owned, US Government and Agency Obligations, at Fair Value $ 1,561,918 $ 1,626,764
Debt securities gross unrealized gains $ 500