Assets and Liabilities Measured at Fair Value on a Recurring Basis - Schedule of unobservable inputs related to Level 3 fair value measurements (Details)	6 Months Ended
	Jun. 30, 2017
Office | Real Estate Properties and Joint Ventures | Minimum | Discounted Cash Flow Analysis Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount Rate	5.50%
Terminal Capitalization Rate	4.30%
Office | Real Estate Properties and Joint Ventures | Minimum | Direct Capitalization Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Overall Capitalization Rate	3.80%
Office | Real Estate Properties and Joint Ventures | Maximum | Discounted Cash Flow Analysis Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount Rate	8.00%
Terminal Capitalization Rate	7.30%
Office | Real Estate Properties and Joint Ventures | Maximum | Direct Capitalization Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Overall Capitalization Rate	7.00%
Office | Real Estate Properties and Joint Ventures | Weighted Average | Discounted Cash Flow Analysis Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount Rate	6.50%
Terminal Capitalization Rate	5.50%
Office | Real Estate Properties and Joint Ventures | Weighted Average | Direct Capitalization Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Overall Capitalization Rate	4.80%
Industrial | Real Estate Properties and Joint Ventures | Minimum | Discounted Cash Flow Analysis Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount Rate	5.70%
Terminal Capitalization Rate	4.80%
Industrial | Real Estate Properties and Joint Ventures | Minimum | Direct Capitalization Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Overall Capitalization Rate	4.00%
Industrial | Real Estate Properties and Joint Ventures | Maximum | Discounted Cash Flow Analysis Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount Rate	8.60%
Terminal Capitalization Rate	8.00%
Industrial | Real Estate Properties and Joint Ventures | Maximum | Direct Capitalization Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Overall Capitalization Rate	7.50%
Industrial | Real Estate Properties and Joint Ventures | Weighted Average | Discounted Cash Flow Analysis Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount Rate	6.60%
Terminal Capitalization Rate	5.50%
Industrial | Real Estate Properties and Joint Ventures | Weighted Average | Direct Capitalization Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Overall Capitalization Rate	4.90%
Apartment | Real Estate Properties and Joint Ventures | Minimum | Discounted Cash Flow Analysis Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount Rate	5.00%
Terminal Capitalization Rate	4.00%
Apartment | Real Estate Properties and Joint Ventures | Minimum | Direct Capitalization Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Overall Capitalization Rate	3.30%
Apartment | Real Estate Properties and Joint Ventures | Maximum | Discounted Cash Flow Analysis Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount Rate	7.30%
Terminal Capitalization Rate	6.00%
Apartment | Real Estate Properties and Joint Ventures | Maximum | Direct Capitalization Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Overall Capitalization Rate	5.50%
Apartment | Real Estate Properties and Joint Ventures | Weighted Average | Discounted Cash Flow Analysis Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount Rate	6.10%
Terminal Capitalization Rate	4.80%
Apartment | Real Estate Properties and Joint Ventures | Weighted Average | Direct Capitalization Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Overall Capitalization Rate	4.20%
Apartment | Mortgage Loans Payable | Minimum | Discounted Cash Flow Analysis Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	28.50%
Equivalency Rate	2.70%
Apartment | Mortgage Loans Payable | Minimum | Net Present Value Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	28.50%
Weighted Average Cost of Capital Risk Premium Multiple	1.20%
Apartment | Mortgage Loans Payable | Maximum | Discounted Cash Flow Analysis Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	65.80%
Equivalency Rate	3.60%
Apartment | Mortgage Loans Payable | Maximum | Net Present Value Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	65.80%
Weighted Average Cost of Capital Risk Premium Multiple	1.50%
Apartment | Mortgage Loans Payable | Weighted Average | Discounted Cash Flow Analysis Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	41.40%
Equivalency Rate	3.10%
Apartment | Mortgage Loans Payable | Weighted Average | Net Present Value Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	41.40%
Weighted Average Cost of Capital Risk Premium Multiple	1.30%
Retail | Real Estate Properties and Joint Ventures | Minimum | Discounted Cash Flow Analysis Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount Rate	5.00%
Terminal Capitalization Rate	4.30%
Retail | Real Estate Properties and Joint Ventures | Minimum | Direct Capitalization Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Overall Capitalization Rate	3.80%
Retail | Real Estate Properties and Joint Ventures | Maximum | Discounted Cash Flow Analysis Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount Rate	10.40%
Terminal Capitalization Rate	8.80%
Retail | Real Estate Properties and Joint Ventures | Maximum | Direct Capitalization Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Overall Capitalization Rate	8.50%
Retail | Real Estate Properties and Joint Ventures | Weighted Average | Discounted Cash Flow Analysis Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount Rate	6.40%
Terminal Capitalization Rate	5.20%
Retail | Real Estate Properties and Joint Ventures | Weighted Average | Direct Capitalization Valuation Technique | Income Approach Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Overall Capitalization Rate	4.60%
Retail | Mortgage Loans Payable | Minimum | Discounted Cash Flow Analysis Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	18.10%
Equivalency Rate	2.70%
Retail | Mortgage Loans Payable | Minimum | Net Present Value Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	18.10%
Weighted Average Cost of Capital Risk Premium Multiple	1.10%
Retail | Mortgage Loans Payable | Maximum | Discounted Cash Flow Analysis Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	50.60%
Equivalency Rate	4.40%
Retail | Mortgage Loans Payable | Maximum | Net Present Value Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	50.60%
Weighted Average Cost of Capital Risk Premium Multiple	1.30%
Retail | Mortgage Loans Payable | Weighted Average | Discounted Cash Flow Analysis Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	31.80%
Equivalency Rate	3.50%
Retail | Mortgage Loans Payable | Weighted Average | Net Present Value Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	31.80%
Weighted Average Cost of Capital Risk Premium Multiple	1.20%
Office and Industrial | Mortgage Loans Payable | Minimum | Discounted Cash Flow Analysis Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	34.80%
Equivalency Rate	3.40%
Office and Industrial | Mortgage Loans Payable | Minimum | Net Present Value Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	34.80%
Weighted Average Cost of Capital Risk Premium Multiple	1.20%
Office and Industrial | Mortgage Loans Payable | Maximum | Discounted Cash Flow Analysis Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	69.40%
Equivalency Rate	4.90%
Office and Industrial | Mortgage Loans Payable | Maximum | Net Present Value Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	69.40%
Weighted Average Cost of Capital Risk Premium Multiple	1.60%
Office and Industrial | Mortgage Loans Payable | Weighted Average | Discounted Cash Flow Analysis Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	43.30%
Equivalency Rate	3.70%
Office and Industrial | Mortgage Loans Payable | Weighted Average | Net Present Value Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	43.30%
Weighted Average Cost of Capital Risk Premium Multiple	1.30%
Office, Retail and Storage | Loans Receivable | Minimum | Discounted Cash Flow Analysis Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	58.90%
Equivalency Rate	4.20%
Office, Retail and Storage | Loans Receivable | Maximum | Discounted Cash Flow Analysis Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	79.20%
Equivalency Rate	8.30%
Office, Retail and Storage | Loans Receivable | Weighted Average | Discounted Cash Flow Analysis Valuation Technique
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loan to Value Ratio	75.90%
Equivalency Rate	6.30%