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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities $ 348,222 $ 308,589
Long-term AFS securities 240,055 241,411
Total long term debt 246,000 243,000
Long-term debt, fair value 266,000 279,000
Level 1 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Deferred compensation 25,817 29,526
Level 2 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Deferred compensation 0 0
Level 3 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Deferred compensation 0 0
Fair Value | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Deferred compensation 25,817 29,526
Debt Securities | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 348,222 308,589
Long-term AFS securities 240,055 241,411
Debt Securities | Level 1 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 0 0
Long-term AFS securities 0 0
Debt Securities | Level 2 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 348,222 308,589
Long-term AFS securities 240,055 241,411
Debt Securities | Level 3 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 0 0
Long-term AFS securities 0 0
Corporate debt securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 138,635 150,107
Long-term AFS securities 138,234 92,004
Corporate debt securities | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 138,635 150,107
Long-term AFS securities 138,234 92,004
Corporate debt securities | Level 1 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 0 0
Long-term AFS securities 0 0
Corporate debt securities | Level 2 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 138,635 150,107
Long-term AFS securities 138,234 92,004
Corporate debt securities | Level 3 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 0 0
Long-term AFS securities 0 0
U.S. Treasury securities    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 112,784 70,951
Long-term AFS securities 53,932 52,628
U.S. Treasury securities | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 112,784 70,951
Long-term AFS securities 53,932 52,628
U.S. Treasury securities | Level 1 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 0 0
Long-term AFS securities 0 0
U.S. Treasury securities | Level 2 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 112,784 70,951
Long-term AFS securities 53,932 52,628
U.S. Treasury securities | Level 3 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 0 0
Long-term AFS securities 0 0
U.S. governmental agency securities | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 77,155 45,640
Long-term AFS securities 47,889 96,779
U.S. governmental agency securities | Level 1 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 0 0
Long-term AFS securities 0 0
U.S. governmental agency securities | Level 2 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 77,155 45,640
Long-term AFS securities 47,889 96,779
U.S. governmental agency securities | Level 3 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 0 0
Long-term AFS securities 0 0
Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 11,800 32,591
Commercial paper | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 11,800 32,591
Commercial paper | Level 1 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 0 0
Commercial paper | Level 2 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 11,800 32,591
Commercial paper | Level 3 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 0 0
Certificates of deposit    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 7,848 9,300
Certificates of deposit | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 7,848 9,300
Certificates of deposit | Level 1 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 0 0
Certificates of deposit | Level 2 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities 7,848 9,300
Certificates of deposit | Level 3 | Recurring    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Short-term AFS securities $ 0 $ 0