Intangible Assets on Real Estate Acquisitions (Details) (USD $)	9 Months Ended
Sep. 30, 2011 sqfeet Year property segment	Sep. 30, 2010	Dec. 31, 2010
Intangible assets on real estate acquisitions
Gross Carrying Amount	$ 229,690,000		$ 237,578,000
Accumulated Amortization	131,736,000		123,843,000
Net Carrying Amount	97,954,000		113,735,000
Amortization of the intangible assets	20,700,000	18,800,000
Weighted average amortization periods of intangible assets (in years)	10
Estimated future amortization expense associated with the intangible asset categories
Remainder of 2011	5,700,000
2012	18,500,000
2013	14,300,000
2014	12,000,000
2015	10,000,000
2016	8,600,000
In-place lease value
Intangible assets on real estate acquisitions
Gross Carrying Amount	156,477,000		162,708,000
Accumulated Amortization	97,818,000		92,380,000
Net Carrying Amount	58,659,000		70,328,000
Weighted average amortization periods of intangible assets (in years)	7
Tenant relationship value
Intangible assets on real estate acquisitions
Gross Carrying Amount	48,556,000		50,320,000
Accumulated Amortization	22,569,000		21,603,000
Net Carrying Amount	25,987,000		28,717,000
Weighted average amortization periods of intangible assets (in years)	8
Above-market cost arrangements
Intangible assets on real estate acquisitions
Gross Carrying Amount	12,415,000		12,415,000
Accumulated Amortization	2,489,000		1,387,000
Net Carrying Amount	9,926,000		11,028,000
Weighted average amortization periods of intangible assets (in years)	26
Above-market leases
Intangible assets on real estate acquisitions
Gross Carrying Amount	10,909,000		10,802,000
Accumulated Amortization	8,555,000		8,193,000
Net Carrying Amount	2,354,000		2,609,000
Weighted average amortization periods of intangible assets (in years)	5
Market concentration premium
Intangible assets on real estate acquisitions
Gross Carrying Amount	1,333,000		1,333,000
Accumulated Amortization	305,000		280,000
Net Carrying Amount	$ 1,028,000		$ 1,053,000
Weighted average amortization periods of intangible assets (in years)	31

The amount of amortization expense expected to be recognized during the fifth full fiscal year following the date of the most recent balance sheet.

Name:	ofc_FutureAmortizationExpenseFifthFullFiscalYear
Namespace Prefix:	ofc
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of amortization expense expected to be recognized during the first full fiscal year following the date of the most recent balance sheet.

+ Details

Name:	ofc_FutureAmortizationExpenseFirstFullFiscalYear
Namespace Prefix:	ofc
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of amortization expense expected to be recognized during the fourth full fiscal year following the date of the most recent balance sheet.

+ Details

Name:	ofc_FutureAmortizationExpenseFourthFullFiscalYear
Namespace Prefix:	ofc
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of amortization expense expected to be recognized during the second full fiscal year following the date of the most recent balance sheet.

+ Details

Name:	ofc_FutureAmortizationExpenseSecondFullFiscalYear
Namespace Prefix:	ofc
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of amortization expense expected to be recognized during the third full fiscal year following the date of the most recent balance sheet.

+ Details

Name:	ofc_FutureAmortizationExpenseThirdFullFiscalYear
Namespace Prefix:	ofc
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The accumulated amount of amortization of a major finite-lived intangible asset class. A major class is composed of intangible assets that can be grouped together because they are similar, either by their nature or by their use in the operations of a company.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 142

-Paragraph 45

-Subparagraph a(1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=6905858&loc=d3e16323-109275

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Aggregate amount of intangible asset amortization recognized as expense during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 142

-Paragraph 45

-Subparagraph a(2)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=6905858&loc=d3e16323-109275

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsAmortizationExpense
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsFutureAmortizationExpenseAbstract
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Sum of the gross carrying amounts before accumulated amortization as of the balance sheet date of all intangible assets having statutory or estimated useful lives. The aggregate gross carrying amount (including any previously recognized impairment charges) of a major finite-lived intangible asset class. A major class is composed of intangible assets that can be grouped together because they are similar, either by their nature or by their use in the operations of a company.

+ References

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsGross
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsLineItems
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The calculated weighted-average useful life of all finite-lived intangible assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (a)(3)

-URI http://asc.fasb.org/extlink&oid=6905858&loc=d3e16265-109275

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsWeightedAverageUsefulLife
Namespace Prefix:	us-gaap
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of amortization expense expected to be recognized during the remainder of the current fiscal year.

+ Details

Name:	us-gaap_FutureAmortizationExpenseRemainderOfFiscalYear
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Sum of the carrying amounts of all intangible assets, excluding goodwill, as of the balance sheet date, net of accumulated amortization and impairment charges.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 142

-Paragraph 42, 45

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 50

-Paragraph 2

-Subparagraph ((a)(1),(b))

-URI http://asc.fasb.org/extlink&oid=6905858&loc=d3e16323-109275

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 30

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6388964&loc=d3e16212-109274

+ Details

Name:	us-gaap_IntangibleAssetsNetExcludingGoodwill
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant