Fair Value Measurements (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016 USD ($) property	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes receivable	$ 10,487		$ 10,480
Notes payable	1,670,750		1,699,771
Unsecured credit facilities	164,550		164,514
Available-for-sale Securities	$ 7,890
Number of impaired land parcels | property	1
Provision for impairment	$ 866	$ 0
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities	29,615		29,093
Available-for-sale Securities	7,890		7,922
Total	37,505		37,015
Derivative instruments, at fair value	(34,765)		(20,539)
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities held in trust	1,596		0
Provision for impairment	800
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities	29,615		29,093
Available-for-sale Securities	0		0
Total	29,615		29,093
Derivative instruments, at fair value	0		0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities held in trust	0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Payable, Fair Value	1,783,000		1,793,200
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities	0		0
Available-for-sale Securities	7,890		7,922
Total	7,890		7,922
Derivative instruments, at fair value	34,765		20,539
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities held in trust	1,596
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Receivable, Fair Value	10,461		10,620
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities	0		0
Available-for-sale Securities	0		0
Total	0		0
Derivative instruments, at fair value	0		$ 0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities held in trust	$ 0
Unsecured Credit Facilities [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Free Interest Rate	1.40%		1.10%
Unsecured Credit Facilities [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Free Interest Rate	1.40%		1.10%
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	$ 164,800		$ 165,300
Notes Payable, Other Payables [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Free Interest Rate	2.60%		2.80%
Notes Payable, Other Payables [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Free Interest Rate	4.10%		4.20%
Notes Receivable [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Free Interest Rate	6.90%		6.30%
Notes Receivable [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Free Interest Rate	6.90%		6.30%