Significant Accounting Policies Organization and Principles of Consolidation (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended			12 Months Ended
	Dec. 31, 2011	Feb. 24, 2012	Dec. 31, 2010	Dec. 31, 2011 Parent Company [Member]	Dec. 31, 2010 Parent Company [Member]	Dec. 31, 2011 Wholly Owned Properties [Member]	Dec. 31, 2011 Unconsolidated Properties [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Operations Commenced Date	1993
Parent Company, Ownership Percentage of Outstanding Common Partnership Units of Operating Partnership				99.80%	99.80%
Tax Basis of Investments, Unrealized Appreciation (Depreciation), Net	$ 95.1		$ 71.5
Number of Real Estate Properties						217	147
General Partners' Capital Account, Units Outstanding		89,921,858	81,886,872
Partners' Capital Account, Units	90,099,022		82,064,036