Quarterly report pursuant to Section 13 or 15(d)

Long-Term Debt (Narrative) (Details)

v3.8.0.1
Long-Term Debt (Narrative) (Details) - USD ($)
3 Months Ended
Mar. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Senior Notes $ 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
Debt and Capital Lease Obligations 45,000,000 95,000,000
Letters of Credit Outstanding, Amount $ 68,200,000.0 $ 71,400,000.0
Senior Notes [Member]    
Debt Instrument [Line Items]    
Effective average interest rate 4.79% 4.37%
Tranche A [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Periodic Payment, Principal $ 50,000,000  
Fixed interest rate 4.00%  
Tranche B [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Periodic Payment, Principal $ 45,000,000  
Fixed interest rate 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  
Sweep Program [Member]    
Debt Instrument [Line Items]    
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases 30,000,000  
2015 Credit Agreement [Member]    
Debt Instrument [Line Items]    
Line of Credit Facility, Maximum Borrowing Capacity 350,000,000  
Original borrowing capacity 250,000,000  
Line Of Credit Facility Accordion 100,000,000  
Accordion Feature Exercised 50,000,000  
Line of Credit Facility, Current Borrowing Capacity $ 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%