Investment in Unconsolidated Joint Ventures (Summary of Financial Information for Joint Ventures) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Net Investment in Properties	$ 1,061,950	$ 741,228	$ 758,582
Total Assets	1,375,006	922,694	935,990
Mortgage Loans	712,690	457,141	460,023
Total Liabilities	869,879	549,997	558,310
Equity / (Deficit)	505,127	372,697	377,679
Our investment in and share of equity in earnings of unconsolidated joint ventures	163,477	106,402	106,107
Revenues	151,965	137,518	129,649
Property Operating Expense	(49,230)	(44,126)	(36,146)
Net Operating Income	102,735	93,392	93,503
Net Income (Loss)	43,944	38,214	37,350
Investment in and share of net income (loss)	$ 25,516	$ 17,104	$ 15,491
2001 Sixth Avenue
Schedule of Equity Method Investments [Line Items]
% Ownership	50.00%	50.00%	50.00%
Net Investment in Properties	$ 26,933	$ 27,342	$ 33,757
Total Assets	50,481	43,258	44,732
Mortgage Loans	134,472	101,394	102,998
Total Liabilities	138,564	106,241	107,807
Equity / (Deficit)	(88,083)	(62,983)	(63,075)
Revenues	49,369	45,518	43,734
Property Operating Expense	(16,719)	(15,574)	(15,205)
Net Operating Income	32,650	29,944	28,529
Net Income (Loss)	$ 20,833	$ 16,374	$ 14,171
2020 Fifth Avenue
Schedule of Equity Method Investments [Line Items]
% Ownership	50.00%	50.00%	50.00%
Net Investment in Properties	$ 45,309	$ 45,973	$ 46,633
Total Assets	54,594	55,005	55,257
Mortgage Loans	47,000	47,000	47,000
Total Liabilities	47,249	47,612	47,857
Equity / (Deficit)	7,345	7,393	7,400
Revenues	9,088	8,788	8,474
Property Operating Expense	(1,820)	(1,500)	(1,177)
Net Operating Income	7,268	7,288	7,297
Net Income (Loss)	$ 4,881	$ 4,821	$ 4,840
33 Chun Choi Street (Hong Kong)
Schedule of Equity Method Investments [Line Items]
% Ownership	50.00%	50.00%	50.00%
Net Investment in Properties	$ 133,435	$ 134,249	$ 138,742
Total Assets	192,071	184,855	179,525
Mortgage Loans	0	0	0
Total Liabilities	5,598	3,291	4,173
Equity / (Deficit)	186,473	181,564	175,352
Revenues	19,235	18,856	17,700
Property Operating Expense	(6,504)	(6,636)	(5,358)
Net Operating Income	12,731	12,220	12,342
Net Income (Loss)	$ 5,467	$ 6,315	$ 4,480
Mitsubishi
Schedule of Equity Method Investments [Line Items]
% Ownership	50.00%
Net Investment in Properties	$ 325,977
Total Assets	452,063
Mortgage Loans	221,851
Total Liabilities	288,962
Equity / (Deficit)	163,101
Revenues	7,927
Property Operating Expense	(4,218)
Net Operating Income	3,709
Net Income (Loss)	$ 1,108
PREI
Schedule of Equity Method Investments [Line Items]
% Ownership	20.00%	20.00%	20.00%
Net Investment in Properties	$ 399,967	$ 409,876	$ 419,498
Total Assets	456,912	468,298	481,175
Mortgage Loans	207,687	207,270	208,000
Total Liabilities	285,050	288,325	293,276
Equity / (Deficit)	171,862	179,973	187,898
Revenues	41,464	41,075	40,011
Property Operating Expense	(7,978)	(8,503)	(6,157)
Net Operating Income	33,486	32,572	33,854
Net Income (Loss)	$ 13,889	$ 13,615	$ 15,121
GCEAR
Schedule of Equity Method Investments [Line Items]
% Ownership	20.00%	20.00%	20.00%
Net Investment in Properties	$ 114,376	$ 116,949	$ 119,952
Total Assets	151,191	162,863	175,301
Mortgage Loans	101,680	101,477	102,025
Total Liabilities	104,220	104,393	105,197
Equity / (Deficit)	46,971	58,470	70,104
Revenues	18,924	19,742	19,730
Property Operating Expense	(7,362)	(7,808)	(8,249)
Net Operating Income	11,562	11,934	11,481
Net Income (Loss)	$ (1,962)	$ (1,396)	$ (1,262)
1101 Space Park Drive
Schedule of Equity Method Investments [Line Items]
% Ownership	17.00%	17.00%
Net Investment in Properties	$ 15,953	$ 6,839
Total Assets	17,694	8,415
Mortgage Loans	0	0
Total Liabilities	236	135
Equity / (Deficit)	17,458	8,280
Revenues	5,958	3,539
Property Operating Expense	(4,629)	(4,105)
Net Operating Income	1,329	(566)
Net Income (Loss)	$ (272)	$ (1,515)