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Summary of Significant Accounting Policies - Debt Issuance Costs (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Deb issuance costs, gross   $ 1,906
Amortization of deferred financing fees    
Amortization expense in 2019 $ 4,801  
Amortization expense in 2020 3,909  
Amortization expense in 2021 3,795  
Amortization expense in 2022 1,952  
Amortization expense in 2023 1,635  
Amortization expense thereafter 16,166  
Unsecured Term Loans, Senior Notes, and Mortgage Notes Payable    
Debt Instrument [Line Items]    
Deb issuance costs, gross 44,117 39,821
Accumulated amortization, debt issuance costs gross 16,665 13,085
Revolving Credit Facility    
Debt Instrument [Line Items]    
Deb issuance costs, gross 17,170 17,170
Accumulated amortization, debt issuance costs gross $ 12,364 $ 10,784