Annual report pursuant to Section 13 and 15(d)

Long-Term Debt (Narrative) (Details)

v3.6.0.2
Long-Term Debt (Narrative) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Debt Instrument [Line Items]    
Senior Notes $ 95,000 $ 120,000
Letters of Credit 9,975 $ 12,317
Tranche A [Member]    
Debt Instrument [Line Items]    
Periodic principal payments $ 50,000  
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate 4.00%  
Tranche B [Member]    
Debt Instrument [Line Items]    
Periodic principal payments $ 45,000  
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate 4.79%  
Senior Notes [Member]    
Debt Instrument [Line Items]    
Effective average interest rate 4.37% 4.68%
Minimum [Member]    
Debt Instrument [Line Items]    
Interest Rate Spread added to variable rate 0.00%  
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.125%  
Maximum [Member]    
Debt Instrument [Line Items]    
Interest Rate Spread added to variable rate 0.50%  
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.20%  
Sweep Program [Member]    
Debt Instrument [Line Items]    
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases $ 30,000  
Letter of Credit [Member]    
Debt Instrument [Line Items]    
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases 100,000  
Letters of Credit 74,600 $ 67,700
2015 Credit Agreement [Member]    
Debt Instrument [Line Items]    
LineOfCreditFacilityOriginalBorrowingCapacity 250,000  
Line Of Credit Facility Accordion 100,000  
Maximum borrowing capacity $ 350,000  
Interest Rate Spread added to variable rate 1.00% 1.00%
Letter Of Credit Fee In Percentage 1.00% 1.00%
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.125%
Accordion Feature Exercised During the Period Amount $ 50,000  
Line of Credit Facility, Current Borrowing Capacity $ 300,000  
2015 Credit Agreement [Member] | Minimum [Member]    
Debt Instrument [Line Items]    
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage   0.125%
2015 Credit Agreement [Member] | Maximum [Member]    
Debt Instrument [Line Items]    
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage   0.175%
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]    
Debt Instrument [Line Items]    
Interest Rate Spread added to variable rate 1.00%  
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]    
Debt Instrument [Line Items]    
Interest Rate Spread added to variable rate 1.50%