Fair Value Measurements - Schedule of Information About Fair Value Inputs Used to Calculate Fair Value (Details)
12 Months Ended
Dec. 31, 2017
ft²
Property
$ / ft²
Dec. 31, 2016
ft²
Property
$ / ft²
Dec. 31, 2015
Property
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Number of Properties | Property 2,480 2,615 2,629
Fair Value Estimated Using Comparable Properties      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Number of Properties | Property 17 16  
Fair Value Estimated Using Listing Price Or Broker Opinion Of Value      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Number of Properties | Property 1 17  
Fair Value Estimated Using Signed Purchase And Sale Agreements      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Number of Properties | Property 8 9  
Retail | Fair Value Estimated Using Comparable Properties      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Square Footage | ft² 364,940 290,770  
Retail | Fair Value Estimated Using Comparable Properties | Weighted Average      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 55.68 58.78  
Retail | Fair Value Estimated Using Comparable Properties | Minimum      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 13.66 17.17  
Retail | Fair Value Estimated Using Comparable Properties | Maximum      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 305.05 502.23  
Retail | Fair Value Estimated Using Listing Price Or Broker Opinion Of Value      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Square Footage | ft² 22,500 516,916  
Retail | Fair Value Estimated Using Listing Price Or Broker Opinion Of Value | Weighted Average      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 88.89 40.80  
Retail | Fair Value Estimated Using Listing Price Or Broker Opinion Of Value | Minimum      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 88.89 15.40  
Retail | Fair Value Estimated Using Listing Price Or Broker Opinion Of Value | Maximum      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 88.89 170.72  
Retail | Fair Value Estimated Using Signed Purchase And Sale Agreements      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Square Footage | ft² 150,376 265,610  
Retail | Fair Value Estimated Using Signed Purchase And Sale Agreements | Weighted Average      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 230.52 95.11  
Retail | Fair Value Estimated Using Signed Purchase And Sale Agreements | Minimum      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 55.30 19.66  
Retail | Fair Value Estimated Using Signed Purchase And Sale Agreements | Maximum      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 346.23 393.02  
Industrial | Fair Value Estimated Using Comparable Properties      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Square Footage | ft² 370,824 104,864  
Industrial | Fair Value Estimated Using Comparable Properties | Weighted Average      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 5.35 26.43  
Industrial | Fair Value Estimated Using Comparable Properties | Minimum      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 3.30    
Industrial | Fair Value Estimated Using Comparable Properties | Maximum      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 8.56    
Industrial | Fair Value Estimated Using Listing Price Or Broker Opinion Of Value      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Square Footage | ft²   149,627  
Industrial | Fair Value Estimated Using Listing Price Or Broker Opinion Of Value | Weighted Average      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft)   9.09  
Industrial | Fair Value Estimated Using Signed Purchase And Sale Agreements      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Square Footage | ft² 223,747    
Industrial | Fair Value Estimated Using Signed Purchase And Sale Agreements | Weighted Average      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 37.09    
Industrial | Fair Value Estimated Using Signed Purchase And Sale Agreements | Minimum      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 24.02    
Industrial | Fair Value Estimated Using Signed Purchase And Sale Agreements | Maximum      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 54.21    
Office | Fair Value Estimated Using Comparable Properties      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Square Footage | ft² 161,346 135,675  
Office | Fair Value Estimated Using Comparable Properties | Weighted Average      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 40.14 35.00  
Office | Fair Value Estimated Using Comparable Properties | Minimum      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 24.82    
Office | Fair Value Estimated Using Comparable Properties | Maximum      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft) 244.86    
Office | Fair Value Estimated Using Listing Price Or Broker Opinion Of Value      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Square Footage | ft²   34,992  
Office | Fair Value Estimated Using Listing Price Or Broker Opinion Of Value | Weighted Average      
Fair Value Inputs, Assets, Quantitative Information [Line Items]      
Weighted Average Price (usd/sq ft)   56.81