Fair Value Measurements (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impairment of Real Estate	$ (1,672,000)	$ (374,000)
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total			$ 42,760,000
Impairment of Real Estate			(6,579,000)
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Trading, and Equity Securities, FV-NI	36,318,000		33,354,000
Available-for-sale Securities	8,082,000		7,933,000
Interest Rate Derivative Assets, at Fair Value	11,909,000		17,482,000
Total	56,309,000		58,769,000
Derivative instruments, at fair value	(224,000)		(5,491,000)
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total			0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Trading, and Equity Securities, FV-NI	36,318,000		33,354,000
Available-for-sale Securities	0		0
Interest Rate Derivative Assets, at Fair Value	0		0
Total	36,318,000		33,354,000
Derivative instruments, at fair value	0		0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Payable, Fair Value	3,066,580,000		2,961,769,000
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total			42,760,000
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Trading, and Equity Securities, FV-NI	0		0
Available-for-sale Securities	8,082,000		7,933,000
Interest Rate Derivative Assets, at Fair Value	11,909,000		17,482,000
Total	19,991,000		25,415,000
Derivative instruments, at fair value	(224,000)		(5,491,000)
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total			0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Trading, and Equity Securities, FV-NI	0		0
Available-for-sale Securities	0		0
Interest Rate Derivative Assets, at Fair Value	0		0
Total	0		0
Derivative instruments, at fair value	0		0
Unsecured Credit Facilities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unsecured credit facilities, Fair Value	675,769,000		710,902,000
Parent Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities, Change in Unrealized Holding Gain (Loss)	2,200,000	(384,000)
Notes payable	3,009,886,000		3,006,478,000
Unsecured credit facilities	673,852,000		$ 708,734,000
Impairment of Real Estate	$ (1,672,000)	$ (16,054,000)