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

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
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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
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
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-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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Balance Type:	credit
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	instant

- Definition

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

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after accumulated amortization, of debt discount (premium).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28567-108399

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-Publisher FASB
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Balance Type:	debit
Period Type:	instant

- 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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Balance Type:	debit
Period Type:	instant

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

+ Details

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

- 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

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

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-Publisher FASB
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Balance Type:	credit
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- Definition

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.

+ References

No definition available.

+ Details

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Debt - Long-Term Debt (Details) - USD ($)	3 Months Ended
Debt - Long-Term Debt (Details) - USD ($)	Mar. 31, 2018	Jan. 31, 2018	Dec. 31, 2017	Aug. 31, 2014
Debt Instrument [Line Items]
Unsecured revolving credit facility	$ 0		$ 0
Deferred financing costs, net	(18,223,000)		(17,407,000)
Total	610,826,000		717,164,000
Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	617,361,000		721,224,000
Total principal	1,932,361,000		1,701,224,000
Unamortized discount, net of unamortized premium	(3,651,000)		(3,370,000)
Deferred financing costs, net	(10,430,000)		(9,133,000)
Total	1,918,280,000		1,688,721,000
Metro Center \| Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 93,428,000		93,948,000
Stated Rate (as a percent)	3.59%
Effective Rate (as a percent)	3.68%
10 Union Square \| Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 50,000,000		50,000,000
Stated Rate (as a percent)	3.70%
Effective Rate (as a percent)	3.97%
1542 Third Avenue \| Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 30,000,000		30,000,000
Stated Rate (as a percent)	4.29%
Effective Rate (as a percent)	4.54%
First Stamford Place \| Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 180,000,000		180,000,000
Stated Rate (as a percent)	4.28%
Effective Rate (as a percent)	4.39%
1010 Third Avenue and 77 West 55th Street \| Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 39,534,000		39,710,000
Stated Rate (as a percent)	4.01%
Effective Rate (as a percent)	4.22%
10 Bank Street \| Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 34,399,000		34,602,000
Stated Rate (as a percent)	4.23%
Effective Rate (as a percent)	4.35%
383 Main Avenue \| Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 30,000,000		30,000,000
Stated Rate (as a percent)	4.44%
Effective Rate (as a percent)	4.55%
1333 Broadway \| Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 160,000,000		66,602,000
Stated Rate (as a percent)	4.21%	4.21%
Effective Rate (as a percent)	4.21%
Face amount		$ 160,000,000.0
1400 Broadway - First Lien \| Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 0		66,632,000
Stated Rate (as a percent)	0.00%
Effective Rate (as a percent)	0.00%
1400 Broadway - Second Lien \| Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 0		9,172,000
Stated Rate (as a percent)	0.00%
Effective Rate (as a percent)	0.00%
111 West 33rd Street - First Lien \| Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 0		74,045,000
Stated Rate (as a percent)	0.00%
Effective Rate (as a percent)	0.00%
111 West 33rd Street - Second Lien \| Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 0		9,369,000
Stated Rate (as a percent)	0.00%
Effective Rate (as a percent)	0.00%
1350 Broadway \| Fixed rate mortgage debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 0		37,144,000
Stated Rate (as a percent)	0.00%
Effective Rate (as a percent)	0.00%
Senior unsecured notes - exchangeable \| Convertible Debt
Debt Instrument [Line Items]
Fixed rate mortgage debt	$ 250,000,000		250,000,000
Stated Rate (as a percent)	2.63%			2.625%
Effective Rate (as a percent)	3.93%
Face amount				$ 250,000,000.0
Senior unsecured notes, Series A \| Senior notes
Debt Instrument [Line Items]
Total	$ 100,000,000		100,000,000
Stated Rate (as a percent)	3.93%
Effective Rate (as a percent)	3.96%
Senior unsecured notes, Series B \| Senior notes
Debt Instrument [Line Items]
Total	$ 125,000,000		125,000,000
Stated Rate (as a percent)	4.09%
Effective Rate (as a percent)	4.12%
Senior unsecured notes, Series C \| Senior notes
Debt Instrument [Line Items]
Total	$ 125,000,000		125,000,000
Stated Rate (as a percent)	4.18%
Effective Rate (as a percent)	4.21%
Senior unsecured notes, Series D \| Senior notes
Debt Instrument [Line Items]
Total	$ 115,000,000		$ 115,000,000
Stated Rate (as a percent)	4.08%		4.08%
Effective Rate (as a percent)	4.11%
Face amount			$ 115,000,000.0
Senior unsecured notes, Series E \| Senior notes
Debt Instrument [Line Items]
Total	$ 160,000,000		$ 0
Stated Rate (as a percent)	4.26%		4.26%
Effective Rate (as a percent)	4.26%
Face amount			$ 160,000,000.0
Senior unsecured notes, Series F \| Senior notes
Debt Instrument [Line Items]
Total	$ 175,000,000		$ 0
Stated Rate (as a percent)	4.44%		4.44%
Effective Rate (as a percent)	4.44%
Face amount			$ 175,000,000.0
Unsecured revolving credit facility \| Revolving credit facility
Debt Instrument [Line Items]
Unsecured revolving credit facility	$ 0		0
Effective Rate (as a percent)	2.98%
Unsecured revolving credit facility \| Revolving credit facility \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.10%
Unsecured term loan facility \| Revolving credit facility
Debt Instrument [Line Items]
Unsecured revolving credit facility	$ 265,000,000		$ 265,000,000
Stated Rate (as a percent)	3.35%
Fixed pay rate (as a percent)	2.1485%
Unsecured term loan facility \| Revolving credit facility \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)	1.20%
First Stamford Place - First Lien \| Fixed rate mortgage debt
Debt Instrument [Line Items]
Stated Rate (as a percent)	4.09%
Face amount	$ 164,000,000
First Stamford Place - Second Lien \| Fixed rate mortgage debt
Debt Instrument [Line Items]
Stated Rate (as a percent)	6.25%
Face amount	$ 16,000,000