Annual report pursuant to Section 13 and 15(d)

Long-Term Debt - Additional Information (Details)

v3.10.0.1
Long-Term Debt - Additional Information (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Senior notes $ 45,000,000 $ 95,000,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.125% 0.125%
Letter Of Credit Fee In Percentage 1.00% 1.00%
Letters of Credit Outstanding, Amount $ 61,500,000 $ 71,400,000
Long-term Line of Credit 0 $ 0
Long-term debt $ 45,000,000  
Long-term debt, maturity 2021  
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%  
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%  
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]    
Debt Instrument [Line Items]    
Interest Rate Spread added to Rate 1.00%  
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]    
Debt Instrument [Line Items]    
Interest Rate Spread added to Rate 1.50%  
2015 Credit Agreement [Member]    
Debt Instrument [Line Items]    
Original borrowing capacity $ 250,000,000  
Line of Credit Facility, Maximum Borrowing Capacity 350,000,000  
Line Of Credit Facility Accordion 100,000,000  
Accordion Feature Exercised 50,000,000  
Line of Credit Facility, Current Borrowing Capacity 300,000,000  
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  
Tranche A [Member]    
Debt Instrument [Line Items]    
Debt instrument, periodic payment $ 50,000,000  
Fixed interest rate 4.00%  
Tranche B [Member]    
Debt Instrument [Line Items]    
Debt instrument, periodic payment $ 45,000,000  
Fixed interest rate 4.79%  
Senior Notes [Member]    
Debt Instrument [Line Items]    
Effective average interest rate 4.79% 4.37%