Fair Value Measurements (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes receivable	$ 12,281	$ 12,132
Notes payable	1,868,700	1,946,357
Unsecured credit facilities	120,000	75,000
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities held in trust	29,530	28,134
Available-for-sale Securities	7,900
Interest rate derivative	168
Total	37,598
Derivative instruments, at fair value	(14,183)	(18,794)
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 held in trust	29,530	28,134
Available-for-sale Securities	0
Interest rate derivative	0
Total	29,530
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	1,990,000	2,116,000
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities held in trust	0	0
Available-for-sale Securities	7,900
Interest rate derivative	168
Total	8,068
Derivative instruments, at fair value	14,183	18,794
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Receivable, Fair Value	12,165	11,980
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 held in trust	0	0
Available-for-sale Securities	0
Interest rate derivative	0
Total	0
Derivative instruments, at fair value	$ 0	$ 0
Unsecured Credit Facilities [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Free Interest Rate	1.10%	1.30%
Unsecured Credit Facilities [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Free Interest Rate	1.20%	1.30%
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	$ 120,000	$ 75,000
Notes Payable, Other Payables [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Free Interest Rate	1.60%	0.90%
Notes Payable, Other Payables [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Free Interest Rate	3.80%	3.40%
Notes Receivable [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Free Interest Rate	7.30%	7.40%
Notes Receivable [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Free Interest Rate	7.30%	7.40%