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FINANCING ARRANGEMENTS - Revolving Credit Facility (Details) - USD ($)
1 Months Ended
Apr. 30, 2018
Mar. 31, 2018
Jun. 30, 2018
Feb. 28, 2018
Jun. 30, 2017
Debt Instrument [Line Items]          
Long-term debt     $ 90,000,000   $ 120,599,000
Revolving Credit Facility, March 2018 | Line of Credit | Revolving Credit Facility          
Debt Instrument [Line Items]          
Maximum borrowing capacity $ 295,000,000.0 $ 260,000,000.0      
Debt term   5 years      
Increase limit for revolving credit commitments   $ 150,000,000.0      
Increase limit credit facility $ 35,000,000.0        
Long-term debt     90,000,000   0
Revolving credit facility remaining borrowing capacity     203,500,000    
Revolving Credit Facility, March 2018 | Line of Credit | Letter of Credit          
Debt Instrument [Line Items]          
Maximum borrowing capacity   30,000,000.0      
Long-term debt     1,500,000    
Revolving Credit Facility, March 2018 | Line of Credit | Swingline Loan          
Debt Instrument [Line Items]          
Maximum borrowing capacity   30,000,000.0      
Revolving Credit Facility, June 2011 | Line of Credit | Revolving Credit Facility          
Debt Instrument [Line Items]          
Long-term debt     $ 0   0
Revolving credit facility remaining borrowing capacity         $ 198,500,000
Debt instrument face amount       $ 200,000,000.0  
Interest expense   $ 100,000      
Long-term line of credit       0  
Revolving Credit Facility, June 2011 | Line of Credit | Letter of Credit          
Debt Instrument [Line Items]          
Long-term line of credit       $ 1,500,000  
Minimum | Revolving Credit Facility, March 2018 | Line of Credit | Revolving Credit Facility          
Debt Instrument [Line Items]          
Facility fee (as a percent)   0.25%      
Maximum | Revolving Credit Facility, March 2018 | Line of Credit | Revolving Credit Facility          
Debt Instrument [Line Items]          
Facility fee (as a percent)   0.40%      
London Interbank Offered Rate (LIBOR) | Minimum | Revolving Credit Facility, March 2018 | Line of Credit | Revolving Credit Facility          
Debt Instrument [Line Items]          
Basis spread (as a percent)   1.25%      
London Interbank Offered Rate (LIBOR) | Maximum | Revolving Credit Facility, March 2018 | Line of Credit | Revolving Credit Facility          
Debt Instrument [Line Items]          
Basis spread (as a percent)   1.85%