Summary of Significant Accounting Policies - Organization and Principles of Consolidation (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Operations Commenced Date	Dec. 31, 1993
General Partners' Capital Account, Units Outstanding	90,394,486	89,921,858
Partners' Capital Account, Units	90,571,650	90,099,022
Number Of Criteria Used To Determine Estimated Fair Value Of Identifiable Intangible Assets	3
Number Of Joint Ventures That Grant Each Partner A Right To Dissolve The Real Estate Partnership	6
Maximum Period Of TIme In Which Company Capitalizes Interest Costs	12 months
Restricted Cash and Cash Equivalents	$ 6,500,000	$ 6,000,000
Tax Basis of Investments, Unrealized Appreciation (Depreciation), Net	247,600,000	95,100,000
Parent Company [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Parent Company, Ownership Percentage of Outstanding Common Partnership Units of Operating Partnership	99.80%	99.80%
Restricted Cash and Cash Equivalents	$ 6,472,000	$ 6,050,000
Wholly Owned Properties [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of Real Estate Properties	204
Unconsolidated Properties [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of Real Estate Properties	144
Minimum [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Equity Method Investment, Accretion Period	10 years
Maximum [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Equity Method Investment, Accretion Period	40 years
Building and Improvements [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Property, Plant and Equipment, Useful Life	40 years
Tenant Improvements [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Property, Plant and Equipment, Useful Life	10 years
Furniture and Fixtures [Member] | Minimum [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Furniture and Fixtures [Member] | Maximum [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Property, Plant and Equipment, Useful Life	7 years