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Senior Notes payable and letter of credit facilities - Letters of credit (Details) - USD ($)
Dec. 31, 2019
Jul. 30, 2019
Letter of Credit    
Line of Credit Facility [Line Items]    
Issued $ 251,837,000  
Cash and Cash Equivalents 254,176,000  
Letter of Credit | Unsecured facility [Member]    
Line of Credit Facility [Line Items]    
Committed Capacity   $ 200,000,000.0
Uncommitted, unsecured facility [Member]    
Line of Credit Facility [Line Items]    
Issued 187,135,000  
Cash and Cash Equivalents 189,474,000  
Committed, secured facility [Member]    
Line of Credit Facility [Line Items]    
Committed Capacity 225,000,000  
Issued 64,702,000  
Cash and Cash Equivalents $ 64,702,000