Quarterly report pursuant to Section 13 or 15(d)

Long-Term Debt - Additional Information (Details)

v3.19.2
Long-Term Debt - Additional Information (Details) - USD ($)
6 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Debt Instrument [Line Items]    
Senior notes $ 45,000,000 $ 45,000,000
Debt instrument, due date Jan. 03, 2021  
Fixed interest rate 4.79%  
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.125%  
Letter Of Credit Fee In Percentage 1.00%  
Letters of Credit Outstanding, Amount $ 49,000,000.0 $ 61,500,000
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 $ 250,000,000.0  
Line Of Credit Facility Accordion 100,000,000.0  
Line of Credit Facility, Maximum Borrowing Capacity 350,000,000.0  
Accordion Feature Exercised 50,000,000.0  
Line of Credit Facility, Current Borrowing Capacity 300,000,000.0  
Letter of Credit [Member]    
Debt Instrument [Line Items]    
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases 100,000,000.0  
Unsecured Senior Note Due January 3, 2021 [Member]    
Debt Instrument [Line Items]    
Scheduled principal payment $ 45,000,000.0