Investments in Unconsolidated Joint Ventures (Summary of Financial Information for Joint Ventures) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Nov. 01, 2017
Schedule of Equity Method Investments [Line Items]
Net Investment in Properties	$ 1,070,661	$ 1,061,950	$ 741,228
Total Assets	1,371,431	1,375,006	922,694
Mortgage Loans	728,338	712,690	457,141
Total Liabilities	876,157	869,879	549,997
Equity / (Deficit)	495,274	505,127	372,697
Our investment in and share of equity in earnings of unconsolidated joint ventures	175,108	163,477	106,402
Revenues	214,815	151,965	137,518
Property Operating Expense	(75,826)	(49,230)	(44,126)
Net Operating Income	138,989	102,735	93,392
Net Income (Loss)	47,222	43,944	38,214
Equity in earnings of unconsolidated joint ventures	$ 32,979	$ 25,516	$ 17,104
2001 Sixth Avenue
Schedule of Equity Method Investments [Line Items]
% Ownership	50.00%	50.00%	50.00%
Net Investment in Properties	$ 32,786	$ 26,933	$ 27,342
Total Assets	49,278	50,481	43,258
Mortgage Loans	134,527	134,472	101,394
Total Liabilities	139,569	138,564	106,241
Equity / (Deficit)	(90,291)	(88,083)	(62,983)
Revenues	52,806	49,369	45,518
Property Operating Expense	(17,264)	(16,719)	(15,574)
Net Operating Income	35,542	32,650	29,944
Net Income (Loss)	$ 25,612	$ 20,833	$ 16,374
2020 Fifth Avenue
Schedule of Equity Method Investments [Line Items]
% Ownership	50.00%	50.00%	50.00%
Net Investment in Properties	$ 44,644	$ 45,309	$ 45,973
Total Assets	54,855	54,594	55,005
Mortgage Loans	48,000	47,000	47,000
Total Liabilities	48,333	47,249	47,612
Equity / (Deficit)	6,522	7,345	7,393
Revenues	9,417	9,088	8,788
Property Operating Expense	(2,156)	(1,820)	(1,500)
Net Operating Income	7,261	7,268	7,288
Net Income (Loss)	$ 4,689	$ 4,881	$ 4,821
33 Chun Choi Street (Hong Kong)
Schedule of Equity Method Investments [Line Items]
% Ownership	50.00%	50.00%	50.00%
Net Investment in Properties	$ 151,256	$ 133,435	$ 134,249
Total Assets	201,527	192,071	184,855
Mortgage Loans	0	0	0
Total Liabilities	9,337	5,598	3,291
Equity / (Deficit)	192,190	186,473	181,564
Revenues	21,394	19,235	18,856
Property Operating Expense	(7,164)	(6,504)	(6,636)
Net Operating Income	14,230	12,731	12,220
Net Income (Loss)	$ 6,958	$ 5,467	$ 6,315
Mitsubishi
Schedule of Equity Method Investments [Line Items]
% Ownership	50.00%	50.00%		50.00%
Net Investment in Properties	$ 332,373	$ 325,977
Total Assets	469,159	452,063
Mortgage Loans	228,075	221,851
Total Liabilities	285,424	288,962
Equity / (Deficit)	183,735	163,101
Revenues	59,300	7,927
Property Operating Expense	(26,360)	(4,218)
Net Operating Income	32,940	3,709
Net Income (Loss)	$ 15,884	$ 1,108
PREI ®
Schedule of Equity Method Investments [Line Items]
% Ownership	20.00%	20.00%	20.00%
Net Investment in Properties	$ 375,016	$ 399,967	$ 409,876
Total Assets	433,024	456,912	468,298
Mortgage Loans	210,626	207,687	207,270
Total Liabilities	283,899	285,050	288,325
Equity / (Deficit)	149,125	171,862	179,973
Revenues	42,058	41,464	41,075
Property Operating Expense	(8,457)	(7,978)	(8,503)
Net Operating Income	33,601	33,486	32,572
Net Income (Loss)	$ (4,159)	$ 13,889	$ 13,615
GCEAR
Schedule of Equity Method Investments [Line Items]
% Ownership	20.00%	20.00%	20.00%
Net Investment in Properties	$ 111,909	$ 114,376	$ 116,949
Total Assets	139,268	151,191	162,863
Mortgage Loans	101,885	101,680	101,477
Total Liabilities	104,268	104,220	104,393
Equity / (Deficit)	35,000	46,971	58,470
Revenues	20,457	18,924	19,742
Property Operating Expense	(8,546)	(7,362)	(7,808)
Net Operating Income	11,911	11,562	11,934
Net Income (Loss)	$ (2,177)	$ (1,962)	$ (1,396)
1101 Space Park Drive
Schedule of Equity Method Investments [Line Items]
% Ownership	17.00%	17.00%	17.00%
Net Investment in Properties	$ 22,677	$ 15,953	$ 6,839
Total Assets	24,320	17,694	8,415
Mortgage Loans	5,225	0	0
Total Liabilities	5,327	236	135
Equity / (Deficit)	18,993	17,458	8,280
Revenues	9,383	5,958	3,539
Property Operating Expense	(5,879)	(4,629)	(4,105)
Net Operating Income	3,504	1,329	(566)
Net Income (Loss)	$ 415	$ (272)	$ (1,515)