Fair Value Measurements (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes receivable	$ 0		$ 0		$ 15,803
Provision for impairment, net of tax	(2,974)	$ 0	(3,348)	$ 0
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	35,079		35,079		31,662
Available-for-sale Securities	7,593		7,593		9,974
Interest Rate Derivative Assets, at Fair Value	25,472		25,472		14,515
Total	68,144		68,144		56,151
Derivative instruments, at fair value					(177)
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	22,054		22,054
Provision for impairment, net of tax			(26,200)
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	35,079		35,079		31,662
Available-for-sale Securities	0		0		0
Interest Rate Derivative Assets, at Fair Value	0		0		0
Total	35,079		35,079		31,662
Derivative instruments, at fair value					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]
Total	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	3,081,052		3,081,052		3,058,044
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		0
Available-for-sale Securities	7,593		7,593		9,974
Interest Rate Derivative Assets, at Fair Value	25,472		25,472		14,515
Total	33,065		33,065		24,489
Derivative instruments, at fair value					(177)
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	48,254		48,254
Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes Receivable, Fair Value	0		0		15,660
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		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
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	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	715,000		715,000		625,000
Parent Company [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes payable	3,118,002		3,118,002		2,971,715
Unsecured credit facilities	713,498		713,498		$ 623,262
Provision for impairment, net of tax	$ (12,533)	$ 0	$ (28,587)	$ 0