Quarterly report pursuant to Section 13 or 15(d)

Long-Term Debt (Narrative) (Details)

v3.10.0.1
Long-Term Debt (Narrative) (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2018
Dec. 31, 2017
Debt Instrument [Line Items]      
Senior Notes $ 45,000,000 $ 45,000,000 $ 95,000,000
Interest Rate Spread added to Rate 1.00% 1.00%  
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.125% 0.125%  
Letter Of Credit Fee In Percentage 1.00% 1.00%  
Long-term Line of Credit $ 0 $ 0 0
Debt and Capital Lease Obligations 45,000,000 45,000,000 95,000,000
Letters of Credit Outstanding, Amount $ 61,500,000 $ 61,500,000 $ 71,400,000
Senior Notes [Member]      
Debt Instrument [Line Items]      
Effective average interest rate 4.79% 4.79% 4.37%
Tranche A [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Periodic Payment, Principal   $ 50,000,000  
Fixed interest rate 4.00% 4.00%  
Tranche B [Member]      
Debt Instrument [Line Items]      
Debt Instrument, Periodic Payment, Principal   $ 45,000,000  
Fixed interest rate 4.79% 4.79%  
Maximum [Member]      
Debt Instrument [Line Items]      
Interest Rate Spread added to Rate   0.50%  
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage   0.20%  
Minimum [Member]      
Debt Instrument [Line Items]      
Interest Rate Spread added to Rate   0.00%  
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage   0.125%  
Letter of Credit [Member]      
Debt Instrument [Line Items]      
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases $ 100,000,000 $ 100,000,000  
Sweep Program [Member]      
Debt Instrument [Line Items]      
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases 30,000,000 30,000,000  
2015 Credit Agreement [Member]      
Debt Instrument [Line Items]      
Line of Credit Facility, Maximum Borrowing Capacity 350,000,000 350,000,000  
Original borrowing capacity 250,000,000 250,000,000  
Line Of Credit Facility Accordion 100,000,000 100,000,000  
Accordion Feature Exercised 50,000,000 50,000,000  
Line of Credit Facility, Current Borrowing Capacity $ 300,000,000 $ 300,000,000  
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]      
Debt Instrument [Line Items]      
Interest Rate Spread added to Rate   1.50%  
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]      
Debt Instrument [Line Items]      
Interest Rate Spread added to Rate   1.00%