Annual report pursuant to Section 13 and 15(d)

Long-Term Debt (Narrative) (Details)

v3.8.0.1
Long-Term Debt (Narrative) (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Senior Notes $ 95,000,000 $ 95,000,000
Letters of Credit 0 9,975,000
Letters of Credit Outstanding, Amount 71.4 $ 74.6
Tranche A [Member]    
Debt Instrument [Line Items]    
Periodic principal payments $ 50,000,000  
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate 4.00%  
Tranche B [Member]    
Debt Instrument [Line Items]    
Periodic principal payments $ 45,000,000  
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate 4.79%  
Senior Notes [Member]    
Debt Instrument [Line Items]    
Effective average interest rate 4.37%  
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 $ 30,000,000  
Letter of Credit [Member]    
Debt Instrument [Line Items]    
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases 100,000,000  
2015 Credit Agreement [Member]    
Debt Instrument [Line Items]    
LineOfCreditFacilityOriginalBorrowingCapacity 250,000,000  
Line Of Credit Facility Accordion 100,000,000  
Maximum borrowing capacity $ 350,000,000  
Letter Of Credit Fee In Percentage 1.00% 1.00%
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage  
Accordion Feature Exercised $ 50,000,000  
Line of Credit Facility, Current Borrowing Capacity $ 300,000,000  
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%