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Debt - Credit Agreement - (Narrative) (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Feb. 14, 2018
Dec. 31, 2014
Debt Instrument [Line Items]      
Unused Capacity, Commitment Fee Percentage 0.20%    
Federal Fund Rate [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Basis Spread on Variable Rate 0.50%    
London Interbank Offered Rate (LIBOR) [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Basis Spread on Variable Rate 1.00%    
Additional Applicable Rate Added to Base Rate, Minimum 1.30%    
Base Rate [Member]      
Debt Instrument [Line Items]      
Additional Applicable Rate Added to Base Rate, Minimum 0.30%    
Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Basis Spread on Variable Rate 0.875%    
Minimum [Member] | Base Rate [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Basis Spread on Variable Rate 0.00%    
Maximum [Member] | London Interbank Offered Rate (LIBOR) [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Basis Spread on Variable Rate 1.70%    
Maximum [Member] | Base Rate [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Basis Spread on Variable Rate 0.70%    
Revolving Credit Facility [Member]      
Debt Instrument [Line Items]      
Maximum Borrowing Capacity $ 380,000,000 $ 380,000,000  
Amount available to increase borrowing capacity   420,000,000  
Revolving line of credit borrowings 0    
Line of credit facility, amount outstanding 100,000,000    
Letter of Credit [Member]      
Debt Instrument [Line Items]      
Maximum Borrowing Capacity   $ 75,000,000  
Line of credit facility, amount outstanding $ 1,000,000    
Swing Line Loan [Member]      
Debt Instrument [Line Items]      
Maximum Borrowing Capacity     $ 25,000,000