Debt - Fair Value (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Principal outstanding	$ 1,147,929	$ 1,036,139
Unsecured Debt \| Line of Credit
Debt Instrument [Line Items]
Principal outstanding	245,000	28,000
Unsecured Debt \| Term Loan
Debt Instrument [Line Items]
Long-term debt, gross	450,000	450,000
Unamortized debt issuance costs	(3,934)	(3,392)
Principal outstanding	446,066	446,608
Unsecured Debt \| Unsecured Notes
Debt Instrument [Line Items]
Long-term debt, gross	400,000	400,000
Unamortized debt issuance costs	(1,832)	(2,034)
Principal outstanding	398,168	397,966
Secured Debt
Debt Instrument [Line Items]
Long-term debt, gross	59,299	164,326
Unamortized fair market value premium	63	112
Unamortized debt issuance costs	(667)	(873)
Principal outstanding	58,695	163,565
Reported Value Measurement [Member]
Debt Instrument [Line Items]
Long-term debt, gross	1,154,299	1,042,326
Unamortized fair market value premium	63	112
Unamortized debt issuance costs	(6,433)	(6,299)
Principal outstanding	1,147,929	1,036,139
Reported Value Measurement [Member] \| Unsecured Debt \| Line of Credit
Debt Instrument [Line Items]
Long-term debt, gross	245,000	28,000
Reported Value Measurement [Member] \| Unsecured Debt \| Term Loan
Debt Instrument [Line Items]
Long-term debt, gross	450,000	450,000
Reported Value Measurement [Member] \| Unsecured Debt \| Unsecured Notes
Debt Instrument [Line Items]
Long-term debt, gross	400,000	400,000
Reported Value Measurement [Member] \| Secured Debt \| Mortgages [Member]
Debt Instrument [Line Items]
Long-term debt, gross	59,299	164,326
Fair Value, Inputs, Level 3 \| Fair Value
Debt Instrument [Line Items]
Long-term debt, fair value	1,172,998	1,043,190
Fair Value, Inputs, Level 3 \| Fair Value \| Unsecured Debt \| Line of Credit
Debt Instrument [Line Items]
Long-term debt, fair value	245,000	28,000
Fair Value, Inputs, Level 3 \| Fair Value \| Unsecured Debt \| Term Loan
Debt Instrument [Line Items]
Long-term debt, fair value	450,000	450,000
Fair Value, Inputs, Level 3 \| Fair Value \| Unsecured Debt \| Unsecured Notes
Debt Instrument [Line Items]
Long-term debt, fair value	417,210	399,091
Fair Value, Inputs, Level 3 \| Fair Value \| Secured Debt \| Mortgages [Member]
Debt Instrument [Line Items]
Long-term debt, fair value	$ 60,788	$ 166,099
Minimum \| Fair Value, Inputs, Level 3 \| Long-term Debt \| Income Approach Valuation Technique
Debt Instrument [Line Items]
Fair Value Inputs, Discount Rate	2.38%	1.92%
Maximum \| Fair Value, Inputs, Level 3 \| Long-term Debt \| Income Approach Valuation Technique
Debt Instrument [Line Items]
Fair Value Inputs, Discount Rate	4.25%	4.85%

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.

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

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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.

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- Definition

Amount, after accumulated amortization, of debt premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
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-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

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- Definition

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
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-Subparagraph (bbb)
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- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: 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(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
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-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Name Accounting Standards Codification
-Topic 210
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

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