Acquisitions: (Details) (USD $)	12 Months Ended		1 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended		1 Months Ended						12 Months Ended		0 Months Ended	12 Months Ended	0 Months Ended	12 Months Ended	0 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Feb. 28, 2011 Desert Sky Mall sqft	Dec. 31, 2011 Desert Sky Mall	Jun. 30, 2011 Superstition Springs Land	Dec. 31, 2011 Superstition Springs Land	Jun. 03, 2011 Superstition Springs Land	Jul. 31, 2011 Fashion Outlets of Niagara	Dec. 31, 2012 Fashion Outlets of Niagara	Dec. 31, 2011 Fashion Outlets of Niagara	Jul. 22, 2011 Fashion Outlets of Niagara sqft	Dec. 31, 2011 SDG Acquisition Properties	Apr. 29, 2011 Kohl's store at Capitola Mall	Dec. 31, 2012 500 North Michigan Avenue:	Feb. 29, 2012 500 North Michigan Avenue: sqft	Oct. 03, 2012 FlatIron Crossing sqft	Dec. 31, 2012 FlatIron Crossing	Oct. 26, 2012 Arrowhead Towne Center sqft	Dec. 31, 2012 Arrowhead Towne Center	Jan. 03, 2013 Kings Plaza	Nov. 28, 2012 Kings Plaza sqft	Dec. 31, 2012 Kings Plaza
Acquisition
Additional ownership interest (as a percent)			50.00%	50.00%	50.00%		50.00%									75.00%	75.00%	33.30%	33.30%
Property square footage			890,000								530,000				327,000	1,443,000		1,196,000			1,198,000
Purchase price on acquisition			$ 27,625,000								$ 200,000,000		$ 28,500,000		$ 70,925,000	$ 310,397,000		$ 144,400,000			$ 756,000,000
Ownership interest at completion of acquisition (as a percent)			100.00%				100.00%					100.00%				100.00%		100.00%
Fair value of existing ownership interest (as a percent)																25.00%		66.70%
Purchase price funded by cash payment on acquisition			1,875,000								78,579,000					195,900,000		69,025,000			726,000,000
Issuance restricted common stock																					30,000,000
Face amount of debt																					500,000,000
Placement of mortgage note on the property																				146,000,000	354,000,000
Purchase price paid through assumption of debt by the Company	420,123,000	192,566,000	25,750,000					121,421,000								114,497,000		75,374,000
Payment of mortgage note			51,500,000													114,497,000
Increase in purchase price due to contingent consideration, high end of range											218,322,000
Fair value of contingent consideration									16,083,000	14,786,000
Allocation of the fair value
Property			46,603,000								228,720,000	371,344,000			66,033,000	443,391,000		423,349,000			714,589,000
Deferred charges			5,474,000								10,383,000	30,786,000			7,450,000	25,251,000		31,500,000			37,371,000
Restricted cash											5,367,000							4,009,000
Cash and cash equivalents			6,057,000													3,856,000
Tenant receivables			202,000									10,048,000						926,000
Other assets			4,481,000								3,090,000	32,826,000			2,143,000	2,101,000		4,234,000			29,282,000
Total assets acquired			62,817,000								247,560,000	445,004,000			75,626,000	474,599,000		464,018,000			781,242,000
Mortgage note payable			51,500,000								130,006,000	211,543,000				175,720,000		244,403,000
Accounts payable			33,000								231,000	10,416,000				366,000		815,000
Other accrued liabilities			3,017,000								38,037,000	18,578,000			4,701,000	11,071,000		10,449,000			25,242,000
Total liabilities assumed			54,550,000								168,274,000	240,537,000			4,701,000	187,157,000		255,667,000			25,242,000
Fair value of acquired net assets (at 100% ownership)			8,267,000								79,286,000	204,467,000			70,925,000	287,442,000		208,351,000			756,000,000
Business combination
Fair value of existing ownership interest (at 25% ownership)			4,164,000				12,914,000									91,542,000		139,326,000
Carrying value of investment	(974,258,000)	(1,098,560,000)	(2,296,000)													(33,382,000)		(23,597,000)
Prior gain deferral recognized																26,067,000
Gain on remeasurement	115,729,000		1,868,000	1,868,000	1,734,000	1,734,000										84,227,000	84,227,000	115,729,000	115,729,000
Incremental revenue generated from acquired property														7,570,000			11,601,000		6,826,000			7,106,000
Incremental earnings (loss) of acquired property														(502,000)			1,643,000		41,000			(1,091,000)
Pro forma information
Revenue	1,000,983,000	918,362,000
Income from continuing operations	$ 94,335,000	$ 230,668,000