Summary of Significant Accounting Policies - Organization and Principles of Consolidation (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
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	94,108,061	92,333,000
Partners' Capital Account, Units	94,262,231	92,499,000
Number Of Joint Ventures That Grant Each Partner A Right To Dissolve The Real Estate Partnership	5
Maximum Period Of TIme In Which Company Capitalizes Interest Costs	12 months
Tax Basis of Investments, Unrealized Appreciation (Depreciation), Net	$ 129,700,000	$ 156,800,000
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	$ 8,013,000	$ 9,520,000
Wholly Owned Properties [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of Real Estate Properties	202
Unconsolidated Properties [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Number of Real Estate Properties	120
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