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Long-Term Debt Senior Credit Facility (Details) - USD ($)
$ in Thousands
Apr. 01, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Permitted indebtedness $ 300,000  
Line of Credit Facility, Current Borrowing Capacity 73,000  
Line of Credit Facility, Potential Incremental Increases 25,000  
Senior Credit Facility - Revolving credit borrowings 4,500 $ 0
Letters of Credit Outstanding, Amount 12,000  
Line of Credit Facility, Remaining Borrowing Capacity 56,500  
Letter of Credit [Member]    
Debt Instrument [Line Items]    
Line of Credit Facility, Current Borrowing Capacity $ 20,000  
Alternative Base Rate [Member]    
Debt Instrument [Line Items]    
Line of Credit Facility, Interest Rate 1.75%  
Alternative Base Rate [Member] | Minimum [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 1.75%  
Alternative Base Rate [Member] | Maximum [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 2.75%  
London Interbank Offered Rate (LIBOR) [Member]    
Debt Instrument [Line Items]    
Line of Credit Facility, Interest Rate 2.75%  
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 2.75%  
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Interest Rate, Stated Percentage 3.75%