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Debt - Senior Unsecured Term Loan (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
USD ($)
loan
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Jan. 31, 2019
Oct. 18, 2018
USD ($)
Line Of Credit Facility Leverage Range [Line Items]          
Outstanding principal balance $ 983,764        
Unsecured Term Loan Due May 2021          
Line Of Credit Facility Leverage Range [Line Items]          
Interest rate at period end 3.80%        
Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Interest rate at period end 3.78%        
Less than or equal to 25% | LIBOR | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Basis spread on variable rate 1.40%        
Greater than 25% but less than or equal to 35% | LIBOR | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Basis spread on variable rate 1.45%        
Greater than 25% but less than or equal to 35% | LIBOR | $100 Million and $200 Million Term Loans          
Line Of Credit Facility Leverage Range [Line Items]          
Basis spread on variable rate 1.45%        
Greater than 35% but less than or equal to 45% | LIBOR | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Basis spread on variable rate 1.55%        
Greater than 35% but less than or equal to 45% | LIBOR | $100 Million and $200 Million Term Loans          
Line Of Credit Facility Leverage Range [Line Items]          
Basis spread on variable rate 1.60%        
Greater than 45% but less than or equal to 50% | LIBOR | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Basis spread on variable rate 1.75%        
Greater than 45% but less than or equal to 50% | LIBOR | $100 Million and $200 Million Term Loans          
Line Of Credit Facility Leverage Range [Line Items]          
Basis spread on variable rate 1.75%        
Greater than 50% but less than or equal to 55% | LIBOR | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Basis spread on variable rate 1.95%        
Greater than 50% but less than or equal to 55% | LIBOR | $100 Million and $200 Million Term Loans          
Line Of Credit Facility Leverage Range [Line Items]          
Basis spread on variable rate 1.95%        
Greater than 55% | LIBOR | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Basis spread on variable rate 2.20%        
Greater than 55% | LIBOR | $100 Million and $200 Million Term Loans          
Line Of Credit Facility Leverage Range [Line Items]          
Basis spread on variable rate 2.20%        
Line Of Credit Facility Covenant Actual Results          
Line Of Credit Facility Leverage Range [Line Items]          
Maximum leverage ratio 0.275        
Unsecured term loan | Unsecured Term Loan Due May 2021          
Line Of Credit Facility Leverage Range [Line Items]          
Debt instrument, term (in years) 5 years        
Outstanding principal balance $ 100,000 $ 100,000      
Unsecured term loan | Unsecured Term Loan Due April 2022          
Line Of Credit Facility Leverage Range [Line Items]          
Debt instrument, term (in years) 5 years        
Outstanding principal balance $ 200,000        
Number of term loans | loan 3        
Unsecured term loan | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Debt instrument, term (in years) 5 years        
Outstanding principal balance   0     $ 50,000
Unsecured term loan | LIBOR          
Line Of Credit Facility Leverage Range [Line Items]          
Interest incurred on the facility $ 10,600 $ 6,200 $ 1,300    
Unsecured term loan | LIBOR | Unsecured Term Loan Due May 2021          
Line Of Credit Facility Leverage Range [Line Items]          
Basis spread on variable rate 1.45%        
Unsecured term loan | LIBOR | Unsecured Term Loan Due April 2022          
Line Of Credit Facility Leverage Range [Line Items]          
Basis spread on variable rate 1.45%        
Unsecured term loan | LIBOR | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Basis spread on variable rate 1.45%        
Minimum | Greater than 25% but less than or equal to 35% | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Leverage Ratio 0.25        
Minimum | Greater than 35% but less than or equal to 45% | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Leverage Ratio 0.35        
Minimum | Greater than 45% but less than or equal to 50% | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Leverage Ratio 0.45        
Minimum | Greater than 50% but less than or equal to 55% | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Leverage Ratio 0.50        
Minimum | Greater than 55% | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Leverage Ratio 0.55        
Minimum | Unsecured term loan | LIBOR          
Line Of Credit Facility Leverage Range [Line Items]          
Basis spread on variable rate 1.40%        
Maximum | Less than or equal to 25% | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Leverage Ratio 0.25        
Maximum | Greater than 25% but less than or equal to 35% | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Leverage Ratio 0.35        
Maximum | Greater than 35% but less than or equal to 45% | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Leverage Ratio 0.45        
Maximum | Greater than 45% but less than or equal to 50% | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Leverage Ratio 0.50        
Maximum | Greater than 50% but less than or equal to 55% | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Leverage Ratio 0.55        
Maximum | Unsecured term loan | LIBOR          
Line Of Credit Facility Leverage Range [Line Items]          
Basis spread on variable rate 2.20%        
Subsequent Event | LIBOR | Unsecured Term Loan Due October 2023          
Line Of Credit Facility Leverage Range [Line Items]          
Interest rate at period end       2.41%