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Shareholders' Equity / Partners' Capital - Conversion of Series E Convertible Unit into Series D Convertible Units (Details)
$ in Thousands
 12 Months Ended
Feb. 29, 2016
USD ($)
shares
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Class of Stock [Line Items]        
Gain on conversion of Series E units | $ $ 11,500 $ 0 $ 0 $ 11,463
Series E Convertible Units        
Class of Stock [Line Items]        
Contingent beneficial conversion, Series E | $ $ 2,800      
Series E Convertible Units | Operating Partnership        
Class of Stock [Line Items]        
Units owned (in shares) | shares 4,375,000      
Series E Convertible Units | AH LLC | Operating Partnership        
Class of Stock [Line Items]        
Units owned (in shares) | shares 4,375,000      
Business acquisition, equity interest issued or issuable, number of shares, multiplier 1