Fair Value Measurements (Details 2) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012	Dec. 31, 2011
Liabilities:
Interest rate derivatives	$ 1,855		$ 3,309
Redeemable noncontrolling interest	18,417		17,758		10,298	8,908
Corporate Office Properties, L.P.
Liabilities:
Interest rate derivatives	1,855		3,309
Fair value measurement on a recurring basis
Assets:
Common stocks			298	[1]
Interest rate derivatives	274	[2]	6,594	[2]
Warrants to purchase common stock in KEYW	164	[2]	301	[2]
Assets	6,320		14,660
Liabilities:
Deferred compensation plan liability	5,882	[3]	7,467	[3]
Interest rate derivatives	1,855		3,309
Liabilities	7,737		10,776
Redeemable noncontrolling interest	18,417		17,758
Fair value measurement on a recurring basis | Mutual funds
Assets:
Marketable securities in deferred compensation plan	5,756	[1]	7,090	[1]
Fair value measurement on a recurring basis | Common Shares
Assets:
Marketable securities in deferred compensation plan			176	[1]
Fair value measurement on a recurring basis | Other
Assets:
Marketable securities in deferred compensation plan	126	[1]	201	[1]
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Common stocks			298	[1]
Assets	5,882		7,765
Liabilities:
Liabilities	0		0
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mutual funds
Assets:
Marketable securities in deferred compensation plan	5,756	[1]	7,090	[1]
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Common Shares
Assets:
Marketable securities in deferred compensation plan			176	[1]
Fair value measurement on a recurring basis | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Assets:
Marketable securities in deferred compensation plan	126	[1]	201	[1]
Fair value measurement on a recurring basis | Significant Other Observable Inputs (Level 2)
Assets:
Interest rate derivatives	274	[2]	6,594	[2]
Warrants to purchase common stock in KEYW	164	[2]	301	[2]
Assets	438		6,895
Liabilities:
Deferred compensation plan liability	5,882	[3]	7,467	[3]
Interest rate derivatives	1,855		3,309
Liabilities	7,737		10,776
Fair value measurement on a recurring basis | Significant Unobservable Inputs (Level 3)
Liabilities:
Redeemable noncontrolling interest	18,417		17,758
Fair value measurement on a recurring basis | Corporate Office Properties, L.P.
Assets:
Common stocks			298	[4]
Interest rate derivatives	274	[5]	6,594	[5]
Warrants to purchase common stock in KEYW	164	[5]	301	[5]
Assets	438		7,193
Liabilities:
Interest rate derivatives	1,855		3,309
Redeemable noncontrolling interest	18,417		17,758
Fair value measurement on a recurring basis | Corporate Office Properties, L.P. | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Common stocks			298	[4]
Assets	0		298
Fair value measurement on a recurring basis | Corporate Office Properties, L.P. | Significant Other Observable Inputs (Level 2)
Assets:
Interest rate derivatives	274	[5]	6,594	[5]
Warrants to purchase common stock in KEYW	164	[5]	301	[5]
Assets	438		6,895
Liabilities:
Interest rate derivatives	1,855		3,309
Fair value measurement on a recurring basis | Corporate Office Properties, L.P. | Significant Unobservable Inputs (Level 3)
Assets:
Assets	0		0
Liabilities:
Redeemable noncontrolling interest	$ 18,417		$ 17,758

[1] Included in the line entitled “restricted cash and marketable securities” on COPT’s consolidated balance sheet.

[2] Included in the line entitled “prepaid expenses and other assets” on COPT’s consolidated balance sheet.

[3] Included in the line entitled “other liabilities” on COPT’s consolidated balance sheet.

[4] Included in the line entitled “restricted cash and marketable securities” on COPLP’s consolidated balance sheet.

[5] Included in the line entitled “prepaid expenses and other assets” on COPLP’s consolidated balance sheet.