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Fair Value Measurements Fair Value Measurements (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Assets, Fair Value Disclosure [Abstract]    
Money Market Funds $ 283,322 $ 273,768
Municipal bonds 1,851 9,875
Total 285,173 283,643
Available-for-sale Securities [Abstract]    
Available-for-sale Securities, Amortized Cost Basis 1,854 9,875
Available-for-sale Securities, Gross Unrealized Gain 0 3
Available-for-sale Securities, Gross Unrealized Loss (3) (3)
Fair Value 1,851 9,875
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]    
Less than one year 1,299  
One to five years 552  
Fair Value 1,851 9,875
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]    
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis 1,299  
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis 555  
Amortized cost 1,854  
Level 1 [Member]    
Assets, Fair Value Disclosure [Abstract]    
Money Market Funds 283,322 273,768
Municipal bonds 0 0
Total 283,322 273,768
Level 2 [Member]    
Assets, Fair Value Disclosure [Abstract]    
Money Market Funds 0 0
Municipal bonds 1,851 9,875
Total 1,851 9,875
Level 3 [Member]    
Assets, Fair Value Disclosure [Abstract]    
Money Market Funds 0 0
Municipal bonds 0 0
Total $ 0 $ 0