v3.10.0.1
Interest Rate Swap (Details) - Designated as Hedging Instrument - Interest Rate Swap - Cash Flow Hedge - USD ($)
1 Months Ended
Apr. 30, 2017
Jun. 30, 2018
Dec. 31, 2017
Derivative [Line Items]      
Fixed interest rate 1.78%    
Notional amount $ 500,000,000 $ 400,000,000  
Reduction in notional amount per year $ 100,000,000    
Term of contract 5 years    
Effective fixed interest rate on long-term debt   4.03%  
Other Assets      
Derivative [Line Items]      
Fair value of derivative   $ 8,000,000 $ 2,900,000
LIBOR      
Derivative [Line Items]      
Basis spread on variable rate 0.75%