Notes Payable and Revolving Lines of Credit - Schedule of Components of Notes Payable (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Variable Rate Interest
Notes payable to Trusts	$ 3,763,474
Notes payable, net	3,738,497	$ 3,213,588
Notes Payable
Fixed Rate Interest
Notes payable at fixed rate of interest	2,095,495	2,297,968
Variable Rate Interest
Notes payable at variable rate of interest	717,979	642,970
Notes payable to Trusts	3,763,474	3,240,938
Unamortized debt issuance costs	(24,977)	(27,350)
Notes payable, net	$ 3,738,497	3,213,588
Notes Payable \| Minimum
Variable Rate Interest
Fixed Rate	2.60%
Variable Rate	3.00%
Notes Payable \| Maximum
Variable Rate Interest
Fixed Rate	6.10%
Variable Rate	3.40%
Notes Payable \| LIBOR \| Minimum
Variable Rate Interest
Basis spread on variable rate (as a percent)	1.40%
Notes Payable \| LIBOR \| Maximum
Variable Rate Interest
Basis spread on variable rate (as a percent)	1.80%
Notes Payable \| Unsecured Loan With Bank
Fixed Rate Interest
Notes payable at fixed rate of interest	$ 600,000	0
Variable Rate Interest
Notes payable at variable rate of interest	$ 350,000	$ 300,000
Notes Payable \| Unsecured Loan With Bank \| Minimum
Variable Rate Interest
Fixed Rate	3.10%
Variable Rate	2.80%
Notes Payable \| Unsecured Loan With Bank \| Maximum
Variable Rate Interest
Fixed Rate	4.00%
Variable Rate	3.20%
Notes Payable \| Unsecured Loan With Bank \| LIBOR \| Minimum
Variable Rate Interest
Basis spread on variable rate (as a percent)	1.30%
Notes Payable \| Unsecured Loan With Bank \| LIBOR \| Maximum
Variable Rate Interest
Basis spread on variable rate (as a percent)	1.70%

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- Definition

Notes Payable Fixed Rate Interest [Abstract]

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- Definition

Notes Payable Variable Rate Interest [Abstract]

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- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
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-Section S99
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-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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Contractual interest rate for funds borrowed, under the debt agreement.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

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-Name Accounting Standards Codification
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The portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a set, unchanging rate.

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The portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a rate subject to change from time to time.

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- Definition

Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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