Segment Information (Summary of net income and EBITDA reconciliation by segment) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting [Abstract]
Net income	$ 363,849	$ 219,162	$ 3,173,586	$ 324,782
Depreciation and amortization expense	96,437	113,169	326,181	333,701
General and administrative expense	33,237	31,977	130,129	108,937
Transaction related costs, impairment losses and other	1,576	2,510	103,315	16,683
Income from partially owned entities	(25,946)	(7,206)	(56,139)	(6,059)
(Income) loss from real estate fund investments	(2,190)	190	13,780	37,973
Interest and other investment income, net	(3,045)	(2,893)	(15,930)	(9,401)
Interest and debt expense	61,448	88,951	226,940	264,774
Net gain on transfer to Fifth Avenue and Times Square JV	0	0	(2,571,099)	0
Net gains on disposition of wholly owned and partially owned assets	(309,657)	(141,269)	(641,664)	(164,828)
Income tax expense	23,885	1,943	80,542	4,964
Loss (income) from discontinued operations	8	(61)	85	(381)
NOI from partially owned entities	86,024	60,094	236,400	193,359
NOI attributable to noncontrolling interests in consolidated subsidiaries	(18,096)	(16,943)	(51,915)	(51,415)
NOI at share	307,530	349,624	954,211	1,053,089
Non-cash adjustments for straight-line rents, amortization of acquired below-market leases, net, and other	(4,037)	(8,743)	530	(39,172)
NOI at share - cash basis	$ 303,493	$ 340,881	$ 954,741	$ 1,013,917