Fair Value Measurements (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes receivable	$ 13,984		$ 13,984		$ 10,481
Notes payable	2,943,986		2,943,986		1,363,925
Unsecured credit facilities	578,144		578,144		278,495
Provision for impairment	0	$ 0	0	$ (1,666)
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading Securities	30,720		30,720		28,588
Available-for-sale Securities	10,054		10,054		7,420
Interest Rate Derivative Assets, at Fair Value	10,201		10,201		11,622
Total	50,975		50,975		47,630
Derivative instruments, at fair value	(1,182)		(1,182)		(580)
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	30,720		30,720		28,588
Available-for-sale Securities	0		0		0
Interest Rate Derivative Assets, at Fair Value	0		0		0
Total	30,720		30,720		28,588
Derivative instruments, at fair value	0		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	3,027,557		3,027,557		1,435,000
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	0		0		0
Available-for-sale Securities	10,054		10,054		7,420
Interest Rate Derivative Assets, at Fair Value	10,201		10,201		11,622
Total	20,255		20,255		19,042
Derivative instruments, at fair value	(1,182)		(1,182)		(580)
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Receivable, Fair Value	13,869		13,869		10,380
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		0
Available-for-sale Securities	0		0		0
Interest Rate Derivative Assets, at Fair Value	0		0		0
Total	0		0		0
Derivative instruments, at fair value	0		$ 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			1.70%		1.50%
Unsecured Credit Facilities [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Free Interest Rate			2.20%		1.60%
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	$ 580,000		$ 580,000		$ 279,700
Notes Payable, Other Payables [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Free Interest Rate			3.10%		2.90%
Notes Payable, Other Payables [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Free Interest Rate			3.70%		3.90%
Notes Receivable [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Free Interest Rate			3.50%		7.20%
Notes Receivable [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk Free Interest Rate			7.40%		7.20%