Fair Value of Financial Instruments (Details Textuals) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011 Properties	Dec. 31, 2010 Properties	Dec. 31, 2009 Properties
Fair Value of Financial Instruments (Textuals) [Abstract]
Number of properties sold (in properties)	19	19	19
Trenton Properties [Member]
Fair Value of Financial Instruments (Textuals) [Abstract]
Gross deferred gain on sale of properties	12.9
Number of properties sold (in properties)	2

Amount of gain on the sale of property that does not qualify for gain recognition as of the balance sheet date.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 605

-SubTopic 40

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 5.U.Q2)

-URI http://asc.fasb.org/extlink&oid=6955272&loc=d3e221323-122791

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