Schedule III - Summary of Real Estate and Accumulated Depreciation (Details) (USD $) In Thousands, unless otherwise specified			12 Months Ended							12 Months Ended																																		12 Months Ended																																		12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Post Street 150 [Member] year	Dec. 31, 2011 Andorra [Member] year	Dec. 31, 2011 Assembly Square Market Place Assembly Row [Member] year	Dec. 31, 2011 THE AVENUE at White Marsh [Member] year	Dec. 31, 2011 Bala Cynwyd [Member] year	Dec. 31, 2011 Barracks Road [Member] year	Dec. 31, 2011 Bethesda Row [Member]	Dec. 31, 2011 Bethesda Row [Member] Minimum [Member] year	Dec. 31, 2011 Bethesda Row [Member] Maximum [Member] year	Dec. 31, 2011 Brick Plaza [Member] year	Dec. 31, 2011 Bristol [Member] year	Dec. 31, 2011 Chelsea [Member] year	Dec. 31, 2011 Colorado Blvd [Member] year	Dec. 31, 2011 Congressional Plaza [Member] year	Dec. 31, 2011 Courthouse Center [Member] year	Dec. 31, 2011 Courtyard Shops [Member] year	Dec. 31, 2011 Crossroads [Member] year	Dec. 31, 2011 Crow Canyon [Member] year	Dec. 31, 2011 Dedham Plaza [Member] year	Dec. 31, 2011 Del Mar Village [Member] year	Dec. 31, 2011 Eastgate [Member] year	Dec. 31, 2011 Elllisburg Circle [Member] year	Dec. 31, 2011 Former Mervyn’s Parcel (Escondido Promenade) [Member] year	Dec. 31, 2011 Falls Plaza [Member] year	Dec. 31, 2011 Federal Plaza [Member] year	Dec. 31, 2011 Fifth Avenue [Member] year	Dec. 31, 2011 Finley Square [Member] year	Dec. 31, 2011 Flourtown [Member] year	Dec. 31, 2011 Forest Hills [Member] year	Dec. 31, 2011 Fresh Meadows [Member] year	Dec. 31, 2011 Friendship Ctr [Member] year	Dec. 31, 2011 Gaithersburg Square [Member] year	Dec. 31, 2011 Garden Market [Member] year	Dec. 31, 2011 Governor Plaza [Member] year	Dec. 31, 2011 Gratiot Plaza [Member] year	Dec. 31, 2011 Greenwich Avenue [Member] year	Dec. 31, 2011 Hauppauge [Member] year	Dec. 31, 2011 Linden Square [Member] year	Dec. 31, 2011 Loehmann’s Plaza [Member] year	Dec. 31, 2011 Melville Mall [Member] year	Dec. 31, 2011 Mercer Mall [Member]	Dec. 31, 2011 Mercer Mall [Member] Minimum [Member] year	Dec. 31, 2011 Mercer Mall [Member] Maximum [Member] year	Dec. 31, 2011 Mid Pike Plaza [Member] year	Dec. 31, 2011 Montrose Crossing [Member] year	Dec. 31, 2011 Mount Vernon [Member] year	Dec. 31, 2011 Town Center of New Britain [Member] year	Dec. 31, 2011 North Dartmouth [Member] year	Dec. 31, 2011 Northeast [Member] year	Dec. 31, 2011 North Lake Commons [Member] year	Dec. 31, 2011 Old Keene Mill [Member] year	Dec. 31, 2011 Old Town Center [Member] year	Dec. 31, 2011 Pan Am Shopping Center [Member] year	Dec. 31, 2011 Leesburg Plaza [Member] year	Dec. 31, 2011 Lawrence Park [Member] year	Dec. 31, 2011 Lancaster [Member] year	Dec. 31, 2011 Langhorne Square [Member] year	Dec. 31, 2011 Laurel [Member] year	Dec. 31, 2011 Hermosa Ave [Member] year	Dec. 31, 2011 Hollywood Blvd [Member] year	Dec. 31, 2011 Houston Street [Member] year	Dec. 31, 2011 Huntington [Member] year	Dec. 31, 2011 HuntingtonSquare [Member] year	Dec. 31, 2011 Idylwood Plaza [Member] year	Dec. 31, 2011 Kings Court [Member] year	Dec. 31, 2011 Pentagon Row [Member] year	Dec. 31, 2011 Perring Plaza [Member] year	Dec. 31, 2011 Pike 7 [Member] year	Dec. 31, 2011 Plaza El Segundo [Member] year	Dec. 31, 2011 Queen Anne Plaza [Member] year	Dec. 31, 2011 Quince Orchard Plaza [Member] year	Dec. 31, 2011 Rockville Town Square [Member] year	Dec. 31, 2011 Rollingwood Apartments [Member] year	Dec. 31, 2011 Sams Park and Shop [Member] year	Dec. 31, 2011 Santana Row [Member]	Dec. 31, 2011 Santana Row [Member] Minimum [Member] year	Dec. 31, 2011 Santana Row [Member] Maximum [Member] year	Dec. 31, 2011 Saugus [Member] year	Dec. 31, 2011 Shirlington [Member] year	Dec. 31, 2011 Shoppers' World [Member] year	Dec. 31, 2011 Shoppes At Nottingham Square [Member] year	Dec. 31, 2011 Third Street Promenade [Member] year	Dec. 31, 2011 Tower [Member] year	Dec. 31, 2011 Tower Shops January 19 2011 [Member] year	Dec. 31, 2011 Troy [Member] year	Dec. 31, 2011 Tyson Station [Member] year	Dec. 31, 2011 Westgate Mall [Member] year	Dec. 31, 2011 White Marsh Plaza [Member] year	Dec. 31, 2011 White Marsh Other [Member] year	Dec. 31, 2011 Wildwood [Member] year	Dec. 31, 2011 Willow Grove [Member] year	Dec. 31, 2011 Willow Lawn [Member] year	Dec. 31, 2011 Wynnewood [Member] year
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$ 810,616	$ 589,441				$ 56,629		$ 38,995	$ 23,995			$ 28,757		$ 7,292				$ 7,078		$ 19,951							$ 0												$ 14,700		$ 36,621	$ 22,115	$ 47,270				$ 80,000	$ 10,554								$ 28,320	$ 27,640	$ 4,907								$ 16,276		$ 52,572			$ 185,568			$ 4,552	$ 23,236						$ 6,364	$ 5,407								$ 9,354		$ 24,295			$ 28,168
Initial cost to company, Land	919,112		11,685	2,432	75,406	20,682	3,565	4,363	36,971			0	3,856	9,417	5,262	2,793	1,750	16,862	4,635	8,638	12,287	14,218	1,608	4,028	19,117	1,798	10,216	2,149	9,252	1,345	2,885	24,625	12,696	7,701	2,677	2,068	525	7,484	8,791	79,382	1,237	35,622	4,488			0	35,746	10,068	1,282	27,214	1,152	2,782	638	3,420	8,694	8,184	5,723	0	720	7,458	1,116	8,300	14,680	0	0	4,308	0	0	2,800	9,709	53,723	3,319	3,197	0	552	4,840	41,969			4,383	9,761	10,211	4,441	22,645	7,170	28,823	3,126	388	6,319	3,478	60,400	9,111	1,499	3,192	8,055
Initial cost to company, Buildings and Improvements	1,517,305		9,181	12,346	34,196	72,432	14,466	16,459	35,406			24,715	15,959	19,466	4,071	7,424	1,869	21,851	11,611	54,575	12,918	39,559	5,775	11,309	15,829	1,270	17,895	584	9,544	3,943	2,885	25,255	20,803	5,271	4,829	4,905	1,601	5,445	15,262	19,247	15,096	32,882	70,076			10,335	105,010	33,501	12,285	0	10,596	8,604	998	2,765	12,929	10,722	7,160	2,103	2,974	22,525	280	16,920	1,976	16,008	10,075	10,026	10,714	2,955	6,461	22,799	164,427	8,457	7,949	8,092	2,246	6,319	1,161			8,291	14,808	18,863	12,849	12,709	10,518	36,313	5,193	453	107,284	21,413	1,843	1,061	6,643	7,723	13,759
Cost Capitalized Subsequent to Acquisition	1,998,127		16,992	9,916	121,227	2,954	20,231	33,223	141,873			35,959	8,274	1,418	7,664	63,505	920	1,394	14,633	6,581	8,095	1,443	19,134	14,691	9,525	9,370	35,067	3,323	13,680	10,641	2,441	25,655	1,811	12,174	4,786	19,065	16,876	1,040	3,942	47,895	16,655	467	34,007			43,036	0	35,033	890	(17,846)	11,764	2,770	4,660	28,263	6,941	16,086	17,836	10,725	16,572	19,055	4,082	14,042	48,751	22,814	363	2,049	888	85,934	18,392	3,093	252	3,932	11,976	36,061	5,952	1,642	537,483			1,962	32,113	1,884	32	41,625	3,382	6,144	20,001	3,321	4,859	134	(23,108)	7,962	20,588	70,385	14,805
Gross amount at which carried atclose of period, Land	922,595		11,685	2,432	75,406	20,682	3,566	4,363	44,880			3,931	3,856	9,396	5,262	1,020	1,750	16,894	4,635	12,664	12,287	14,180	1,608	4,013	19,117	1,819	10,216	2,149	9,252	1,470	3,031	24,629	12,696	5,973	2,677	2,068	525	7,484	8,791	79,269	1,248	35,622	5,032			7,517	35,746	10,204	1,262	9,366	1,153	2,628	638	3,420	8,695	8,184	5,734	75	720	7,576	1,368	8,300	14,778	11,713	0	4,308	0	0	2,800	9,653	53,705	3,319	2,928	0	572	4,840	49,725			4,383	5,798	10,225	4,441	25,125	7,280	28,823	4,028	475	6,319	3,478	37,260	9,111	1,499	7,790	8,055
Gross amount at which carried atclose of period, Buildings and Improvements	3,511,949		26,173	22,262	155,423	75,386	34,696	49,682	169,370			56,743	24,233	20,905	11,735	72,702	2,789	23,213	26,244	57,130	21,013	41,040	24,909	26,015	25,354	10,619	52,962	3,907	23,224	14,459	5,180	50,906	22,614	19,173	9,615	23,970	18,477	6,485	19,204	67,255	31,740	33,349	103,539			45,854	105,010	68,398	13,195	2	22,359	11,528	5,658	31,028	19,869	26,808	24,985	12,753	19,546	41,462	4,110	30,962	50,629	27,109	10,438	12,075	11,602	88,889	24,853	25,948	164,697	12,389	20,194	44,153	8,178	7,961	530,888			10,253	50,884	20,733	12,881	51,854	13,790	42,457	24,292	3,687	112,143	21,547	1,875	9,023	27,231	73,510	28,564
Gross amount at which carried atclose of period, Total	4,434,544		37,858	24,694	230,829	96,068	38,262	54,045	214,250			60,674	28,089	30,301	16,997	73,722	4,539	40,107	30,879	69,794	33,300	55,220	26,517	30,028	44,471	12,438	63,178	6,056	32,476	15,929	8,211	75,535	35,310	25,146	12,292	26,038	19,002	13,969	27,995	146,524	32,988	68,971	108,571			53,371	140,756	78,602	14,457	9,368	23,512	14,156	6,296	34,448	28,564	34,992	30,719	12,828	20,266	49,038	5,478	39,262	65,407	38,822	10,438	16,383	11,602	88,889	27,653	35,601	218,402	15,708	23,122	44,153	8,750	12,801	580,613			14,636	56,682	30,958	17,322	76,979	21,070	71,280	28,320	4,162	118,462	25,025	39,135	18,134	28,730	81,300	36,619
Accumulated Depreciation and Amortization	$ 1,127,588		$ 13,762	$ 14,411	$ 13,184	$ 13,311	$ 12,624	$ 31,819	$ 37,167			$ 37,698	$ 11,626	$ 2,978	$ 7,110	$ 41,991	$ 1,232	$ 2,793	$ 10,947	$ 10,326	$ 10,692	$ 4,984	$ 14,555	$ 15,921	$ 8,034	$ 6,711	$ 32,216	$ 1,958	$ 13,958	$ 7,439	$ 2,325	$ 22,950	$ 6,298	$ 13,487	$ 5,113	$ 13,546	$ 12,772	$ 3,007	$ 7,704	$ 8,188	$ 21,594	$ 4,972	$ 28,937			$ 9,918	$ 38	$ 18,116	$ 2,392	$ 1	$ 16,130	$ 5,817	$ 4,637	$ 15,821	$ 11,411	$ 8,981	$ 21,379	$ 6,360	$ 11,214	$ 29,700	$ 1,875	$ 7,087	$ 22,647	$ 8,905	$ 481	$ 6,049	$ 6,187	$ 31,473	$ 18,108	$ 11,464	$ 19	$ 7,237	$ 12,475	$ 6,146	$ 6,865	$ 3,687	$ 96,342			$ 4,262	$ 15,035	$ 3,243	$ 2,172	$ 23,698	$ 5,721	$ 1,821	$ 16,545	$ 3,116	$ 21,847	$ 3,869	$ 355	$ 7,782	$ 19,985	$ 43,740	$ 15,095
Life on which depreciation in latest income statements is computed			35	35	35	35	35	35		35	50	35	35	35	35	35	35	35	35	35	35	35	35	35	35	25	35	35	35	35	35	35	35	35	35	35	25.75	35	35	35	35	35		25	35	50	50	35	35	0	35	35	33.33	35	35	35	22	22	35	35	35	35	35	35	35	35	26	35	35	35	35	35	35	50	25	35		40	50	35	35	35	35	35	35	35	22	17	35	35	35	33.33	35	35	35