Fair Value Measurements - Fair Value Inputs of Long-Lived Assets Held and Used (Details)
9 Months Ended 12 Months Ended
Sep. 30, 2017
ft²
$ / ft²
Dec. 31, 2016
ft²
$ / ft²
Weighted Average Sale Price Per Square Foot of Comparable Properties [Member] | Retail    
Impaired Long-Lived Assets Held and Used [Line Items]    
Property square footage (per sqft) | ft² 355,428 290,770
Weighted Average Sale Price Per Square Foot of Comparable Properties [Member] | Retail | Minimum    
Impaired Long-Lived Assets Held and Used [Line Items]    
Sales price (in USD per sqft) 13.66 17.17
Weighted Average Sale Price Per Square Foot of Comparable Properties [Member] | Retail | Maximum    
Impaired Long-Lived Assets Held and Used [Line Items]    
Sales price (in USD per sqft) 285.98 502.23
Weighted Average Sale Price Per Square Foot of Comparable Properties [Member] | Retail | Weighted Average    
Impaired Long-Lived Assets Held and Used [Line Items]    
Sales price (in USD per sqft) 53.99 58.78
Weighted Average Sale Price Per Square Foot of Comparable Properties [Member] | Industrial    
Impaired Long-Lived Assets Held and Used [Line Items]    
Property square footage (per sqft) | ft² 0 104,864
Weighted Average Sale Price Per Square Foot of Comparable Properties [Member] | Industrial | Weighted Average    
Impaired Long-Lived Assets Held and Used [Line Items]    
Sales price (in USD per sqft) 0.00 26.43
Weighted Average Sale Price Per Square Foot of Comparable Properties [Member] | Office    
Impaired Long-Lived Assets Held and Used [Line Items]    
Property square footage (per sqft) | ft² 161,346 135,675
Weighted Average Sale Price Per Square Foot of Comparable Properties [Member] | Office | Minimum    
Impaired Long-Lived Assets Held and Used [Line Items]    
Sales price (in USD per sqft) 24.82  
Weighted Average Sale Price Per Square Foot of Comparable Properties [Member] | Office | Maximum    
Impaired Long-Lived Assets Held and Used [Line Items]    
Sales price (in USD per sqft) 244.86  
Weighted Average Sale Price Per Square Foot of Comparable Properties [Member] | Office | Weighted Average    
Impaired Long-Lived Assets Held and Used [Line Items]    
Sales price (in USD per sqft) 40.14 35.00
Weighted Average Listing Price or Broker Opinion of Value Per Square Foot | Retail    
Impaired Long-Lived Assets Held and Used [Line Items]    
Property square footage (per sqft) | ft² 0 516,916
Weighted Average Listing Price or Broker Opinion of Value Per Square Foot | Retail | Minimum    
Impaired Long-Lived Assets Held and Used [Line Items]    
Listing price and broker opinion (in USD per sqft)   15.40
Weighted Average Listing Price or Broker Opinion of Value Per Square Foot | Retail | Maximum    
Impaired Long-Lived Assets Held and Used [Line Items]    
Listing price and broker opinion (in USD per sqft)   170.02
Weighted Average Listing Price or Broker Opinion of Value Per Square Foot | Retail | Weighted Average    
Impaired Long-Lived Assets Held and Used [Line Items]    
Listing price and broker opinion (in USD per sqft) 0.00 40.80
Weighted Average Listing Price or Broker Opinion of Value Per Square Foot | Industrial    
Impaired Long-Lived Assets Held and Used [Line Items]    
Property square footage (per sqft) | ft² 370,824 149,627
Weighted Average Listing Price or Broker Opinion of Value Per Square Foot | Industrial | Weighted Average    
Impaired Long-Lived Assets Held and Used [Line Items]    
Listing price and broker opinion (in USD per sqft) 5.73 9.09
Weighted Average Listing Price or Broker Opinion of Value Per Square Foot | Office    
Impaired Long-Lived Assets Held and Used [Line Items]    
Property square footage (per sqft) | ft² 0 34,992
Weighted Average Listing Price or Broker Opinion of Value Per Square Foot | Office | Weighted Average    
Impaired Long-Lived Assets Held and Used [Line Items]    
Listing price and broker opinion (in USD per sqft) 0.00 56.81