Debt - Term Loan - Narrative (Details)
Nov. 03, 2015
USD ($)
extension_option
Jun. 30, 2018
USD ($)
Dec. 31, 2017
USD ($)
Debt Instrument [Line Items]      
Amount outstanding   $ 346,500,000 $ 112,000,000
Term Loan      
Debt Instrument [Line Items]      
Number of one-year extension options | extension_option 2    
Extension option, term 1 year    
Accordion feature, maximum borrowing capacity $ 600,000,000    
Reborrow period after repayment 30 days    
Period subject to occurrence limitation 12 months    
Amount outstanding   0  
Available borrowing capacity   420,000,000  
Origination costs $ 2,400,000    
Unamortized deferred financing costs   $ 300,000 $ 700,000
Term Loan | Achievement of Two of Three Credit Ratings, S&P at least BBB-, Fitch at least BBB-, Moody's at least Baa3 | Minimum | LIBOR      
Debt Instrument [Line Items]      
Basis spread on variable rate 0.90%    
Term Loan | Achievement of Two of Three Credit Ratings, S&P at least BBB-, Fitch at least BBB-, Moody's at least Baa3 | Maximum | LIBOR      
Debt Instrument [Line Items]      
Basis spread on variable rate 1.75%