Quarterly Financial Data: (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended			0 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Nov. 14, 2014	Dec. 11, 2013
Quarterly Financial Data (Unaudited)
Revenues	$ 322,909	$ 263,491	$ 254,336	$ 264,511	$ 282,137	$ 258,154	$ 245,877	$ 243,307	$ 1,105,247	$ 1,029,475	$ 797,517
Net income (loss) attributable to common stockholders	1,429,221	35,914	16,088	17,819	144,878	38,123	218,997	18,092	1,499,042	420,090	337,426
Net income (loss) attributable to common stockholders per share-basic (in dollars per share)	$ 9.52	$ 0.25	$ 0.11	$ 0.13	$ 1.03	$ 0.27	$ 1.57	$ 0.13	$ 10.46	$ 3.01	$ 2.51
Net income (loss) attributable to common stockholders per share-diluted (in dollars per share)	$ 9.51	$ 0.25	$ 0.11	$ 0.13	$ 1.03	$ 0.27	$ 1.57	$ 0.13	$ 10.45	$ 3.00	$ 2.51
Remeasurement gain on acquisition of additional interest									1,423,136	51,205	199,956
Gain (loss) from sale									0	286,414	50,811
PPRLP Queens Portfolio
Quarterly Financial Data (Unaudited)
Remeasurement gain on acquisition of additional interest												1,423,136
Chesterfield Towne Center
Quarterly Financial Data (Unaudited)
Gain (loss) from sale													$ 151,467