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Marketable Securities and Certificates of Deposit - Amortized Cost and Estimated Fair Value of Marketable Securities (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Marketable Securities, Amortized Cost Basis $ 203,635 $ 63,454
Marketable Securities, Accumulated Gross Unrealized Gain, before Tax 7,875 5,765
Marketable Securities, Accumulated Gross Unrealized Loss, before Tax 382 4,187
Marketable Securities 211,128 65,032
Securities Investments - Debt Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Available-for-sale Securities, Amortized Cost Basis 1,298 1,806
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax 0 0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax 5 11
Available-for-sale Securities 1,293 1,795
Securities Investments - Equity Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Trading Securities, Cost 666 5,388
Trading Securities, Accumulated Gross Unrealized Gain, before Tax 0 0
Trading Securities, Accumulated Gross Unrealized Loss, before Tax 310 4,144
Trading Securities 356 1,244
Debt Securities Carried by EGL [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Trading Securities, Cost 145,731 34,233
Trading Securities, Accumulated Gross Unrealized Gain, before Tax 159 87
Trading Securities, Accumulated Gross Unrealized Loss, before Tax 63 26
Trading Securities 145,827 34,294
investment Funds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Trading Securities, Cost 55,940 22,027
Trading Securities, Accumulated Gross Unrealized Gain, before Tax 7,716 5,678
Trading Securities, Accumulated Gross Unrealized Loss, before Tax 4 6
Trading Securities $ 63,652 $ 27,699