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Marketable Securities (Fair Value and Gross Unrealized Losses for AFS) (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Sep. 30, 2017
Schedule of Available-for-sale Securities [Line Items]    
Fair value of available-for-sale securities in continuous unrealized loss position for less than twelve months $ 29,945 $ 29,947
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss (42) (24)
Fair value of available-for-sale securities in continuous unrealized loss position for twelve months or longer 0 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0 0
Corporate Bonds    
Schedule of Available-for-sale Securities [Line Items]    
Fair value of available-for-sale securities in continuous unrealized loss position for less than twelve months 24,191 26,196
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss (26) (20)
Fair value of available-for-sale securities in continuous unrealized loss position for twelve months or longer 0 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0 0
Certificates of Deposit    
Schedule of Available-for-sale Securities [Line Items]    
Fair value of available-for-sale securities in continuous unrealized loss position for less than twelve months 5,754 3,751
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss (16) (4)
Fair value of available-for-sale securities in continuous unrealized loss position for twelve months or longer 0 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss $ 0 $ 0