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Debt - Summary of Unsecured Note Balances, Interest Rate and Maturity Date Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Net Principal Balance	$ 5,933,286	$ 5,038,812
Weighted average interest rate	4.25%	4.35%
Fixed Rate Public Notes [Member]
Debt Instrument [Line Items]
Net Principal Balance	$ 5,485,884	$ 4,591,373
Weighted average interest rate	4.37%	4.61%
Debt instrument maturity year, start	2020	2020
Debt instrument maturity year, end	2047	2047
Fixed Rate Public Notes [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest Rate Ranges	2.85%	2.85%
Fixed Rate Public Notes [Member] | Maximum [Member]
Debt Instrument [Line Items]
Interest Rate Ranges	7.57%	7.57%
Floating Rate Public Notes [Member]
Debt Instrument [Line Items]
Net Principal Balance	$ 447,402	$ 447,439
Weighted average interest rate	2.84%	1.82%
Debt instrument, maturity year	2019	2019

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- Definition

Debt instrument maturity year.

+ References

No definition available.

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- Definition

Debt instrument maturity year, end.

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- Definition

Debt instrument maturity year, start.

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Weighted average interest rate of debt outstanding.

+ References

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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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-Name Accounting Standards Codification
-Topic 210
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-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 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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