Interest and Other Investment Income, Net (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net Investment Income [Line Items]
(Decrease) increase in fair value of marketable securities:	$ (7,699)	$ 0	$ (24,801)	$ 0
Interest on cash and cash equivalents and restricted cash	4,306	1,636	12,370	4,264
Dividends on marketable securities	3,354	3,309	10,060	9,923
Interest on loans receivable	2,004	754	8,952	3,599
Other, net	928	1,632	2,820	4,781
Interest and other investment income, net	2,893	7,331	9,401	22,567
Lexington Realty Trust
Net Investment Income [Line Items]
(Decrease) increase in fair value of marketable securities:	(7,942)	0	(24,934)	0
Other
Net Investment Income [Line Items]
(Decrease) increase in fair value of marketable securities:	243	$ 0	133	$ 0
701 Seventh Avenue
Net Investment Income [Line Items]
Interest on loans receivable	$ 1,250		$ 6,707