Investment in Unconsolidated Joint Ventures (Summary of Financial Information for Joint Ventures) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Net Investment in Properties	$ 741,228	$ 758,582	$ 779,752
Total Assets	922,694	935,990	942,107
Mortgage Loans	457,141	460,023	461,548
Total Liabilities	549,997	558,310	569,895
Equity / (Deficit)	372,697	377,679	372,212
Our investment in and share of equity in earnings of unconsolidated joint ventures	106,402	106,107	94,729
Revenues	137,518	129,649	102,303
Property Operating Expense	(44,126)	(36,146)	(26,873)
Net Operating Income	93,392	93,503	75,430
Net Income (Loss)	38,214	37,350	30,577
Investment in and share of net income (loss)	$ 17,104	$ 15,491	$ 13,289
2001 Sixth Avenue
Schedule of Equity Method Investments [Line Items]
% Ownership	50.00%	50.00%	50.00%
Net Investment in Properties	$ 27,342	$ 33,757	$ 37,620
Total Assets	43,258	44,732	42,537
Mortgage Loans	101,394	102,998	104,523
Total Liabilities	106,241	107,807	110,749
Equity / (Deficit)	(62,983)	(63,075)	(68,212)
Revenues	45,518	43,734	39,807
Property Operating Expense	(15,574)	(15,205)	(14,707)
Net Operating Income	29,944	28,529	25,100
Net Income (Loss)	$ 16,374	$ 14,171	$ 11,982
2020 Fifth Avenue
Schedule of Equity Method Investments [Line Items]
% Ownership	50.00%	50.00%	50.00%
Net Investment in Properties	$ 45,973	$ 46,633	$ 47,239
Total Assets	55,005	55,257	55,123
Mortgage Loans	47,000	47,000	47,000
Total Liabilities	47,612	47,857	47,795
Equity / (Deficit)	7,393	7,400	7,328
Revenues	8,788	8,474	8,308
Property Operating Expense	(1,500)	(1,177)	(1,086)
Net Operating Income	7,288	7,297	7,222
Net Income (Loss)	$ 4,821	$ 4,840	$ 4,844
33 Chun Choi Street (Hong Kong)
Schedule of Equity Method Investments [Line Items]
% Ownership	50.00%	50.00%	50.00%
Net Investment in Properties	$ 134,249	$ 138,742	$ 143,014
Total Assets	184,855	179,525	165,912
Mortgage Loans	0	0	0
Total Liabilities	3,291	4,173	10,210
Equity / (Deficit)	181,564	175,352	155,702
Revenues	18,856	17,700	8,671
Property Operating Expense	(6,636)	(5,358)	(2,625)
Net Operating Income	12,220	12,342	6,046
Net Income (Loss)	$ 6,315	$ 4,480	$ 2,976
PREI
Schedule of Equity Method Investments [Line Items]
% Ownership	20.00%	20.00%	20.00%
Net Investment in Properties	$ 409,876	$ 419,498	$ 429,358
Total Assets	468,298	481,175	492,494
Mortgage Loans	207,270	208,000	208,000
Total Liabilities	288,325	293,276	296,480
Equity / (Deficit)	179,973	187,898	196,014
Revenues	41,075	40,011	39,467
Property Operating Expense	(8,503)	(6,157)	(6,144)
Net Operating Income	32,572	33,854	33,323
Net Income (Loss)	$ 13,615	$ 15,121	$ 12,378
GCEAR
Schedule of Equity Method Investments [Line Items]
% Ownership	20.00%	20.00%	20.00%
Net Investment in Properties	$ 116,949	$ 119,952	$ 122,521
Total Assets	162,863	175,301	186,041
Mortgage Loans	101,477	102,025	102,025
Total Liabilities	104,393	105,197	104,661
Equity / (Deficit)	58,470	70,104	81,380
Revenues	19,742	19,730	6,050
Property Operating Expense	(7,808)	(8,249)	(2,311)
Net Operating Income	11,934	11,481	3,739
Net Income (Loss)	$ (1,396)	$ (1,262)	$ (1,603)
1101 Space Park Drive
Schedule of Equity Method Investments [Line Items]
% Ownership	17.00%
Net Investment in Properties	$ 6,839
Total Assets	8,415
Mortgage Loans	0
Total Liabilities	135
Equity / (Deficit)	8,280
Revenues	3,539
Property Operating Expense	(4,105)
Net Operating Income	(566)
Net Income (Loss)	$ (1,515)