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Short and Long-Term Debt and Commitments Related to Letters of Credit Narrative (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Jul. 22, 2010
USD ($)
Debt instrument [Line Items]      
Senior unsecured notes, interest rate 5.09%    
Revolving credit facility, current borrowing capacity $ 175,000    
Revolving credit facility, noncurrent 67,000 $ 54,500  
Swing line commitment 15,000    
Revolving credit facility, maximum borrowing capacity 210,000    
Senior unsecured notes $ 24,000 30,000 $ 30,000
Number of principal payments 5    
Anniversary when principal payments begin 6    
Unused commitments under credit facility $ 103,929    
Committed under credit facility 7,110    
Revolving credit facility borrowings $ 67,000 $ 54,500  
Long-term debt, weighted average interest rate 3.56% 3.55%  
Long-term debt, fiscal year maturity [Abstract]      
2017 $ 16,701    
2018 73,829    
2019 6,160    
2020 6,169    
2021 $ 4,292    
Minimum [Member]      
Debt instrument [Line Items]      
Line of credit facility, commitment fee percentage 0.10%    
Maximum [Member]      
Debt instrument [Line Items]      
Line of credit facility, commitment fee percentage 0.25%    
Line of credit [Member]      
Debt instrument [Line Items]      
Revolving credit facility, noncurrent $ 71,071    
Revolving credit facility borrowings 71,071    
Letter of credit [Member]      
Debt instrument [Line Items]      
Letters of credit outstanding $ 64,225 $ 59,350  
Base rate [Member] | Line of credit [Member]      
Debt instrument [Line Items]      
Outstanding borrowings interest rate margin adjustment range 0.50%    
London Interbank Offered Rate (LIBOR) [Member] | Line of credit [Member]      
Debt instrument [Line Items]      
Outstanding borrowings interest rate margin adjustment range 1.50%    
London Interbank Offered Rate (LIBOR) [Member] | Line of credit [Member] | Minimum [Member]      
Debt instrument [Line Items]      
Outstanding borrowings interest rate margin adjustment range 0.75%    
London Interbank Offered Rate (LIBOR) [Member] | Line of credit [Member] | Maximum [Member]      
Debt instrument [Line Items]      
Outstanding borrowings interest rate margin adjustment range 1.50%    
Revolving credit facility [Member] | Letter of credit [Member]      
Debt instrument [Line Items]      
Letters of credit outstanding $ 4,071 57,347  
Accounts receivable securitization [Member]      
Debt instrument [Line Items]      
Senior unsecured notes, interest rate 0.90%    
Long-term debt, fiscal year maturity [Abstract]      
Unused commitment fee threshold percent 0.50    
Debt instrument variable rate base rate calculation, default rate 2.00%    
Accounts receivable securitization [Member] | Letter of credit [Member]      
Debt instrument [Line Items]      
Letters of credit outstanding $ 58,150    
Long-term debt, fiscal year maturity [Abstract]      
Debt Instrument, Term 1 year    
Accounts receivable securitization [Member] | Letter of credit [Member] | Minimum [Member]      
Long-term debt, fiscal year maturity [Abstract]      
Letter of credit, unused capacity, commitment fee percentage 0.0035    
Accounts receivable securitization [Member] | Letter of credit [Member] | Maximum [Member]      
Long-term debt, fiscal year maturity [Abstract]      
Letter of credit, unused capacity, commitment fee percentage 0.0045    
Accounts receivable securitization [Member] | Base rate, federal funds [Member]      
Long-term debt, fiscal year maturity [Abstract]      
Debt instrument, variable rate, base rate calculation 0.50%    
Line of credit [Member] | Letter of credit [Member]      
Debt instrument [Line Items]      
Letters of credit outstanding $ 2,004 $ 2,003  
Line of credit [Member] | London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]      
Debt instrument [Line Items]      
Outstanding borrowings interest rate margin adjustment range 1.50%    
Letter of credit [Member]      
Debt instrument [Line Items]      
Revolving credit facility, maximum borrowing capacity $ 100,000