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

No definition available.

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- 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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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

Name:	us-gaap_DebtInstrumentInterestRateEffectivePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
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
-Name Accounting Standards Codification
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-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

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-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

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-Publisher FASB
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-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

+ Details

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Data Type:	xbrli:monetaryItemType
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
-SubTopic 10
-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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Balance Type:	na
Period Type:	instant

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

+ Details

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Balance Type:	credit
Period Type:	instant

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

Debt - Schedule of Long term Debt (Details) - USD ($)	12 Months Ended
Debt - Schedule of Long term Debt (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Aug. 31, 2015	Mar. 31, 2015	Aug. 31, 2014
Debt Instrument [Line Items]
Unsecured revolving credit facility	$ 0	$ 35,192,000
Deferred financing costs, net	(10,904,000)	(12,910,000)
Total	759,016,000	747,661,000
Mortgages [Member]
Debt Instrument [Line Items]
Fixed rate debt	752,840,000	735,145,000
Total principal	1,617,840,000	1,640,145,000
Unamortized premiums, net of unamortized discount	905,000	5,181,000
Deferred financing costs, net	(6,414,000)	(12,910,000)
Total	1,612,331,000	1,632,416,000
Mortgages [Member] \| 10 Bank Street [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 31,544,000	32,214,000
Stated Rate	5.72%
Effective Rate	6.22%
Mortgages [Member] \| 1542 Third Avenue [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 17,795,000	18,222,000
Stated Rate	5.90%
Effective Rate	6.62%
Mortgages [Member] \| First Stamford Place [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 235,067,000	238,765,000
Stated Rate	5.65%
Effective Rate	6.18%
Mortgages [Member] \| 1010 Third Avenue and 77 West 55th Street [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 26,502,000	27,064,000
Stated Rate	5.69%
Effective Rate	6.39%
Mortgages [Member] \| 383 Main Avenue [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 28,654,000	29,269,000
Stated Rate	5.59%
Effective Rate	6.04%
Mortgages [Member] \| 1333 Broadway [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 67,656,000	68,646,000
Stated Rate	6.32%
Effective Rate	3.75%
Mortgages [Member] \| 1400 Broadway - First Lien [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 67,714,000	68,732,000
Stated Rate	6.12%
Effective Rate	3.41%
Mortgages [Member] \| 1400 Broadway - Second Lien [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 9,389,000	9,600,000
Stated Rate	3.35%
Effective Rate	3.36%
Mortgages [Member] \| 112 West 34th Street - First Lien [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 75,261,000	76,406,000
Stated Rate	6.01%
Effective Rate	3.38%
Mortgages [Member] \| 112 West 34th Street - Second Lien [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 9,509,000	9,640,000
Stated Rate	6.56%
Effective Rate	3.68%
Mortgages [Member] \| 1350 Broadway [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 37,764,000	38,348,000
Stated Rate	5.87%
Effective Rate	3.74%
Mortgages [Member] \| Metro Center [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 95,985,000	97,950,000
Stated Rate	3.59%
Effective Rate	3.67%
Mortgages [Member] \| 10 Union Square [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 50,000,000	20,289,000
Stated Rate	3.70%
Effective Rate	3.99%
Convertible debt [Member] \| Exchangeable Senior Notes due August 15, 2019 [Member]
Debt Instrument [Line Items]
Fixed rate debt	$ 250,000,000	250,000,000
Stated Rate	2.625%				2.625%
Effective Rate	3.93%				3.80%
Senior Notes [Member] \| Series A Senior Notes due 2025 [Member]
Debt Instrument [Line Items]
Total	$ 100,000,000	100,000,000
Stated Rate	3.93%			3.93%
Effective Rate	4.03%
Senior Notes [Member] \| Series B Senior Notes due 2027 [Member]
Debt Instrument [Line Items]
Total	$ 125,000,000	125,000,000
Stated Rate	4.09%			4.09%
Effective Rate	4.19%
Senior Notes [Member] \| Series C Senior Notes due 2030 [Member]
Debt Instrument [Line Items]
Total	$ 125,000,000	125,000,000
Stated Rate	4.18%			4.18%
Effective Rate	4.28%
Revolving credit facility [Member] \| Unsecured revolving credit facility [Member]
Debt Instrument [Line Items]
Unsecured revolving credit facility	$ 0	40,000,000
Effective Rate	1.92%
Revolving credit facility [Member] \| Unsecured revolving credit facility [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.15%
Revolving credit facility [Member] \| Unsecured Term Loan Facility [Member]
Debt Instrument [Line Items]
Unsecured revolving credit facility	$ 265,000,000	$ 265,000,000	$ 265,000,000.0
Effective Rate	2.37%
Pay rate	2.1485%
Revolving credit facility [Member] \| Unsecured Term Loan Facility [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.60%