Segment Information (Other Segment - EBITDA) (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Information
Corporate general and administrative expenses	$ (36,261)		$ (33,584)	$ (122,161)	$ (112,593)
Impairment loss on investment in PREIT	(4,354)		(1,599)	(7,572)	(166,701)
(Loss) income from real estate fund investments	(6,308)		1,077	(1,649)	28,750
Our share of net gain on sale of Suffolk Downs				3,797	5,074
Net gain on repayment of Suffolk Downs JV debt investments				(11,373)
Other	1,530		2,864	3,797	5,074
EBITDA	236,745		398,007	1,021,077	1,162,125
Suffolk Downs
Segment Information
Our share of net gain on sale of Suffolk Downs		$ 15,314
Net gain on repayment of Suffolk Downs JV debt investments		$ 11,373
Real estate fund investments
Segment Information
Operating results	(7,794)		807	(11,333)	13,662
(Loss) income from real estate fund investments	(6,308)		1,077	(1,649)	28,750
Operating Segments | Other
Segment Information
Corporate general and administrative expenses	(22,730)		(21,519)	(78,952)	(76,364)
Investment income and other, net	5,910		6,871	24,079	19,317
Other EBITDA, as adjusted	30,367		38,841	89,786	105,871
Impairment loss on investment in PREIT	(4,354)		(1,599)	(7,572)	(166,701)
(Loss) income from real estate fund investments	(6,308)		1,077	(1,649)	28,750
Net gain on repayment of Suffolk Downs JV debt investments				(11,373)
Total of certain items that impact EBITDA	(90,799)		82,273	77,369	78,737
EBITDA	(60,432)		121,114	167,155	184,608
Income from mark-to-market income (loss) of investments in our deferred compensation plan	1,975		204	5,233	2,625
Operating Segments | Other | JBGS
Segment Information
Transaction costs	(53,581)		(2,739)	(67,045)	(4,597)
Operating results	13,038		75,307	153,449	214,604
Operating expense	(40,543)		72,568	86,404	210,007
Operating Segments | Other | PREIT
Segment Information
Impairment loss on investment in PREIT	(44,465)		0	(44,465)	0
Operating Segments | Other | Urban Edge Properties
Segment Information
Operating expense	5,200		0	21,100	0
Other	(3,197)		8,898	(1,024)	15,768
Operating Segments | Other | Suffolk Downs
Segment Information
Our share of net gain on sale of Suffolk Downs	0		0	15,314	0
Net gain on repayment of Suffolk Downs JV debt investments	0		0	11,373	0
Operating Segments | Other | Skyline Properties
Segment Information
Skyline properties impairment loss	0		0	0	(160,700)
Operating Segments | Other | Real estate investment | The Mart and trade shows
Segment Information
Equity in net income (loss)	24,165		21,696	72,471	70,689
Operating Segments | Other | Real estate investment | 555 California Street
Segment Information
Equity in net income (loss)	11,643		11,405	35,870	35,137
Operating Segments | Other | Real estate investment | Other investments
Segment Information
Equity in net income (loss)	11,379		20,388	36,318	57,092
Investment income and other, net	5,910		6,871	24,079	19,317
Operating Segments | Other | Real estate fund investments
Segment Information
(Loss) income from real estate fund investments	$ (7,794)		$ 807	$ (11,333)	$ 13,662