Assets and Liabilities Measured at Fair Value on a Recurring Basis - Schedule of unobservable inputs related to Level 3 fair value measurements (Details) - Level 3: Significant Unobservable Inputs	Mar. 31, 2020	Mar. 31, 2019
Discount Rate | Real Estate Properties and Joint Ventures | Office | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.055	0.055
Discount Rate | Real Estate Properties and Joint Ventures | Office | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.085	0.086
Discount Rate | Real Estate Properties and Joint Ventures | Office | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.066	0.065
Discount Rate | Real Estate Properties and Joint Ventures | Industrial | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.052	0.053
Discount Rate | Real Estate Properties and Joint Ventures | Industrial | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.093	0.093
Discount Rate | Real Estate Properties and Joint Ventures | Industrial | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.067	0.068
Discount Rate | Real Estate Properties and Joint Ventures | Apartment | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.053	0.055
Discount Rate | Real Estate Properties and Joint Ventures | Apartment | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.078	0.078
Discount Rate | Real Estate Properties and Joint Ventures | Apartment | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.064	0.065
Discount Rate | Real Estate Properties and Joint Ventures | Retail | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.053	0.050
Discount Rate | Real Estate Properties and Joint Ventures | Retail | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.117	0.107
Discount Rate | Real Estate Properties and Joint Ventures | Retail | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.067	0.064
Discount Rate | Real Estate Properties and Joint Ventures | Hotel | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.103
Discount Rate | Real Estate Properties and Joint Ventures | Hotel | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.103
Capitalization Rate | Real Estate Properties and Joint Ventures | Office | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.040	0.040
Capitalization Rate | Real Estate Properties and Joint Ventures | Office | Minimum | Direct Capitalization
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.039	0.038
Capitalization Rate | Real Estate Properties and Joint Ventures | Office | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.075	0.075
Capitalization Rate | Real Estate Properties and Joint Ventures | Office | Maximum | Direct Capitalization
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.070	0.070
Capitalization Rate | Real Estate Properties and Joint Ventures | Office | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.055	0.055
Capitalization Rate | Real Estate Properties and Joint Ventures | Office | Weighted Average | Direct Capitalization
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.049	0.048
Capitalization Rate | Real Estate Properties and Joint Ventures | Industrial | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.043	0.044
Capitalization Rate | Real Estate Properties and Joint Ventures | Industrial | Minimum | Direct Capitalization
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.035	0.040
Capitalization Rate | Real Estate Properties and Joint Ventures | Industrial | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.083	0.083
Capitalization Rate | Real Estate Properties and Joint Ventures | Industrial | Maximum | Direct Capitalization
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.077	0.078
Capitalization Rate | Real Estate Properties and Joint Ventures | Industrial | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.054	0.055
Capitalization Rate | Real Estate Properties and Joint Ventures | Industrial | Weighted Average | Direct Capitalization
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.048	0.049
Capitalization Rate | Real Estate Properties and Joint Ventures | Apartment | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.043	0.038
Capitalization Rate | Real Estate Properties and Joint Ventures | Apartment | Minimum | Direct Capitalization
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.038	0.033
Capitalization Rate | Real Estate Properties and Joint Ventures | Apartment | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.068	0.068
Capitalization Rate | Real Estate Properties and Joint Ventures | Apartment | Maximum | Direct Capitalization
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.060	0.060
Capitalization Rate | Real Estate Properties and Joint Ventures | Apartment | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.051	0.050
Capitalization Rate | Real Estate Properties and Joint Ventures | Apartment | Weighted Average | Direct Capitalization
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.046	0.045
Capitalization Rate | Real Estate Properties and Joint Ventures | Retail | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.050	0.043
Capitalization Rate | Real Estate Properties and Joint Ventures | Retail | Minimum | Direct Capitalization
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.040	0.033
Capitalization Rate | Real Estate Properties and Joint Ventures | Retail | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.094	0.088
Capitalization Rate | Real Estate Properties and Joint Ventures | Retail | Maximum | Direct Capitalization
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.110	0.105
Capitalization Rate | Real Estate Properties and Joint Ventures | Retail | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.056	0.053
Capitalization Rate | Real Estate Properties and Joint Ventures | Retail | Weighted Average | Direct Capitalization
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.051	0.047
Capitalization Rate | Real Estate Properties and Joint Ventures | Hotel | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.078
Capitalization Rate | Real Estate Properties and Joint Ventures | Hotel | Direct Capitalization
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.078
Capitalization Rate | Real Estate Properties and Joint Ventures | Hotel | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.078
Capitalization Rate | Real Estate Properties and Joint Ventures | Hotel | Weighted Average | Direct Capitalization
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Real Estate Properties and Joint Ventures	0.078
Loan to Value Ratio | Loans Payable | Apartment | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.300	0.311
Loan to Value Ratio | Loans Payable | Apartment | Minimum | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.300	0.311
Loan to Value Ratio | Loans Payable | Apartment | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.690	0.623
Loan to Value Ratio | Loans Payable | Apartment | Maximum | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.690	0.623
Loan to Value Ratio | Loans Payable | Apartment | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.466	0.480
Loan to Value Ratio | Loans Payable | Apartment | Weighted Average | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.466	0.480
Loan to Value Ratio | Loans Payable | Retail | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.371	0.319
Loan to Value Ratio | Loans Payable | Retail | Minimum | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.371	0.319
Loan to Value Ratio | Loans Payable | Retail | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.633	0.559
Loan to Value Ratio | Loans Payable | Retail | Maximum | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.638	0.559
Loan to Value Ratio | Loans Payable | Retail | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.442	0.395
Loan to Value Ratio | Loans Payable | Retail | Weighted Average | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.442	0.395
Loan to Value Ratio | Loans Payable | Office and Industrial | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.311	0.360
Loan to Value Ratio | Loans Payable | Office and Industrial | Minimum | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.311	0.360
Loan to Value Ratio | Loans Payable | Office and Industrial | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.593	0.624
Loan to Value Ratio | Loans Payable | Office and Industrial | Maximum | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.593	0.624
Loan to Value Ratio | Loans Payable | Office and Industrial | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.458	0.471
Loan to Value Ratio | Loans Payable | Office and Industrial | Weighted Average | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.458	0.471
Loan to Value Ratio | Loans Receivable, including those with related parties | Apartment, Hotel, Industrial, Office, Retail and Storage | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Receivable	0.309
Loan to Value Ratio | Loans Receivable, including those with related parties | Apartment, Hotel, Industrial, Office, Retail and Storage | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Receivable	0.902
Loan to Value Ratio | Loans Receivable, including those with related parties | Apartment, Hotel, Industrial, Office, Retail and Storage | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Receivable	0.730
Loan to Value Ratio | Loans Receivable | Office, Retail and Storage | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Receivable		0.708
Loan to Value Ratio | Loans Receivable | Office, Retail and Storage | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Receivable		0.792
Loan to Value Ratio | Loans Receivable | Office, Retail and Storage | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Receivable		0.756
Equivalency Rate | Loans Payable | Apartment | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.031	0.035
Equivalency Rate | Loans Payable | Apartment | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.041	0.042
Equivalency Rate | Loans Payable | Apartment | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.037	0.040
Equivalency Rate | Loans Payable | Retail | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.031	0.040
Equivalency Rate | Loans Payable | Retail | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.045	0.049
Equivalency Rate | Loans Payable | Retail | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.039	0.042
Equivalency Rate | Loans Payable | Office and Industrial | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.032	0.037
Equivalency Rate | Loans Payable | Office and Industrial | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.043	0.055
Equivalency Rate | Loans Payable | Office and Industrial | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	0.038	0.042
Equivalency Rate | Loans Receivable, including those with related parties | Apartment, Hotel, Industrial, Office, Retail and Storage | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Receivable	0.031
Equivalency Rate | Loans Receivable, including those with related parties | Apartment, Hotel, Industrial, Office, Retail and Storage | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Receivable	0.103
Equivalency Rate | Loans Receivable, including those with related parties | Apartment, Hotel, Industrial, Office, Retail and Storage | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Receivable	0.060
Equivalency Rate | Loans Receivable | Office, Retail and Storage | Minimum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Receivable		0.060
Equivalency Rate | Loans Receivable | Office, Retail and Storage | Maximum | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Receivable		0.083
Equivalency Rate | Loans Receivable | Office, Retail and Storage | Weighted Average | Discounted Cash Flow
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Receivable		0.071
Weighted Average Cost of Capital Risk Premium Multiple | Loans Payable | Apartment | Minimum | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	1.20	1.20
Weighted Average Cost of Capital Risk Premium Multiple | Loans Payable | Apartment | Maximum | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	1.60	1.40
Weighted Average Cost of Capital Risk Premium Multiple | Loans Payable | Apartment | Weighted Average | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	1.30	1.30
Weighted Average Cost of Capital Risk Premium Multiple | Loans Payable | Retail | Minimum | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	1.20	1.20
Weighted Average Cost of Capital Risk Premium Multiple | Loans Payable | Retail | Maximum | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	1.50	1.30
Weighted Average Cost of Capital Risk Premium Multiple | Loans Payable | Retail | Weighted Average | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	1.30	1.20
Weighted Average Cost of Capital Risk Premium Multiple | Loans Payable | Office and Industrial | Minimum | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	1.20	1.20
Weighted Average Cost of Capital Risk Premium Multiple | Loans Payable | Office and Industrial | Maximum | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	1.50	1.40
Weighted Average Cost of Capital Risk Premium Multiple | Loans Payable | Office and Industrial | Weighted Average | Net Present Value
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Loans Payable	1.30	1.30