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Fair Value Measurements Fair Value Measurements (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds $ 276,392 $ 283,322
Municipal bonds 1,950 1,851
Total 278,342 285,173
Fair Value, Inputs, Level 1 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 276,392 283,322
Municipal bonds 0 0
Total 276,392 283,322
Fair Value, Inputs, Level 2 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 0 0
Municipal bonds 1,950 1,851
Total 1,950 1,851
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Money market funds 0 0
Municipal bonds 0 0
Total $ 0 $ 0