Notes Payable, net - Unsecured Revolving Credit Facility (Details) - USD ($)	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Outstanding borrowings	$ 2,380,652,000	$ 2,439,311,000
Unsecured Debt
Line of Credit Facility [Line Items]
Outstanding borrowings	2,015,000,000	1,975,000,000
Unsecured Debt \| Revolving Credit Facility
Line of Credit Facility [Line Items]
Outstanding borrowings	140,000,000	100,000,000
Remaining borrowing capacity	460,000,000	300,000,000
Total borrowing capacity	$ 600,000,000	$ 400,000,000
Unsecured Debt \| Revolving Credit Facility \| Minimum
Line of Credit Facility [Line Items]
Facility fee-annual rate	0.15%	0.20%
Unsecured Debt \| Revolving Credit Facility \| Maximum
Line of Credit Facility [Line Items]
Facility fee-annual rate	0.30%	0.35%
Unsecured Debt \| Revolving Credit Facility \| LIBOR \| Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.05%	1.15%
Unsecured Debt \| Revolving Credit Facility \| LIBOR \| Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.50%	1.85%

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

No definition available.

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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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-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

+ Details

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Period Type:	instant

- Definition

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

+ References

No definition available.

+ Details

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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.

+ References

No definition available.

+ Details

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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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- Details

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