Investments in Unconsolidated Joint Ventures (Summary of Financial Information for Joint Ventures) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Equity / (Deficit)	$ 1,287,109
Investments in unconsolidated joint ventures	1,287,109	$ 175,108	$ 163,477
Equity in earnings of unconsolidated joint ventures	8,067	32,979	25,516
Corporate Joint Venture
Schedule of Equity Method Investments [Line Items]
Net Investment in Properties	2,627,689	1,070,661	1,061,950
Total Assets	4,877,750	1,371,431	1,375,006
Mortgage Loans	1,360,384	728,338	712,690
Total Liabilities	1,680,946	876,157	869,879
Equity / (Deficit)	3,196,804	495,274	505,127
Revenues	379,600	214,815	151,965
Property Operating Expense	(143,143)	(75,826)	(49,230)
Net Operating Income	236,457	138,989	102,735
Net Income (Loss)	$ 8,357	$ 47,222	$ 43,944
2001 Sixth Avenue
Schedule of Equity Method Investments [Line Items]
Ownership percentage in joint ventures	50.00%	50.00%	50.00%
Net Investment in Properties	$ 30,748	$ 32,786	$ 26,933
Total Assets	47,485	49,278	50,481
Mortgage Loans	134,583	134,527	134,472
Total Liabilities	140,354	139,569	138,564
Equity / (Deficit)	(92,869)	(90,291)	(88,083)
Revenues	56,266	52,806	49,369
Property Operating Expense	(19,254)	(17,264)	(16,719)
Net Operating Income	37,012	35,542	32,650
Net Income (Loss)	$ 27,422	$ 25,612	$ 20,833
2020 Fifth Avenue
Schedule of Equity Method Investments [Line Items]
Ownership percentage in joint ventures	50.00%	50.00%	50.00%
Net Investment in Properties	$ 43,918	$ 44,644	$ 45,309
Total Assets	54,325	54,855	54,594
Mortgage Loans	48,000	48,000	47,000
Total Liabilities	48,703	48,333	47,249
Equity / (Deficit)	5,622	6,522	7,345
Revenues	9,868	9,417	9,088
Property Operating Expense	(2,544)	(2,156)	(1,820)
Net Operating Income	7,324	7,261	7,268
Net Income (Loss)	$ 4,649	$ 4,689	$ 4,881
CenturyLink
Schedule of Equity Method Investments [Line Items]
Ownership percentage in joint ventures	50.00%	50.00%	50.00%
Net Investment in Properties	$ 148,941	$ 151,256	$ 133,435
Total Assets	187,241	201,527	192,071
Total Liabilities	9,947	9,337	5,598
Equity / (Deficit)	177,294	192,190	186,473
Revenues	24,680	21,394	19,235
Property Operating Expense	(9,251)	(7,164)	(6,504)
Net Operating Income	15,429	14,230	12,731
Net Income (Loss)	$ 6,712	$ 6,958	$ 5,467
Mitsubishi
Schedule of Equity Method Investments [Line Items]
Ownership percentage in joint ventures	50.00%	50.00%	50.00%
Net Investment in Properties	$ 554,828	$ 332,373	$ 325,977
Total Assets	753,743	469,159	452,063
Mortgage Loans	231,046	228,075	221,851
Total Liabilities	303,130	285,424	288,962
Equity / (Deficit)	450,613	183,735	163,101
Revenues	84,344	59,300	7,927
Property Operating Expense	(39,300)	(26,360)	(4,218)
Net Operating Income	45,044	32,940	3,709
Net Income (Loss)	$ 18,751	$ 15,884	$ 1,108
Ascenty
Schedule of Equity Method Investments [Line Items]
Ownership percentage in joint ventures	51.00%
Net Investment in Properties	$ 548,114
Total Assets	2,178,663
Mortgage Loans	629,500
Total Liabilities	764,603
Equity / (Deficit)	1,414,060
Revenues	112,052
Property Operating Expense	(40,250)
Net Operating Income	71,802
Net Income (Loss)	$ (54,606)
Mapletree JV
Schedule of Equity Method Investments [Line Items]
Ownership percentage in joint ventures	20.00%
Net Investment in Properties	$ 765,443
Total Assets	1,042,661
Total Liabilities	23,796
Equity / (Deficit)	1,018,865
Revenues	17,852
Property Operating Expense	(6,774)
Net Operating Income	11,078
Net Income (Loss)	$ (1,872)
PREI
Schedule of Equity Method Investments [Line Items]
Ownership percentage in joint ventures	20.00%	20.00%	20.00%
Net Investment in Properties	$ 365,993	$ 375,016	$ 399,967
Total Assets	421,635	433,024	456,912
Mortgage Loans	210,915	210,626	207,687
Total Liabilities	281,344	283,899	285,050
Equity / (Deficit)	140,291	149,125	171,862
Revenues	42,157	42,058	41,464
Property Operating Expense	(9,918)	(8,457)	(7,978)
Net Operating Income	32,239	33,601	33,486
Net Income (Loss)	$ 9,968	$ (4,159)	$ 13,889
GCEAR
Schedule of Equity Method Investments [Line Items]
Ownership percentage in joint ventures	20.00%	20.00%	20.00%
Net Investment in Properties	$ 109,803	$ 111,909	$ 114,376
Total Assets	127,444	139,268	151,191
Mortgage Loans	101,902	101,885	101,680
Total Liabilities	104,363	104,268	104,220
Equity / (Deficit)	23,081	35,000	46,971
Revenues	21,120	20,457	18,924
Property Operating Expense	(9,073)	(8,546)	(7,362)
Net Operating Income	12,047	11,911	11,562
Net Income (Loss)	(2,636)	$ (2,177)	$ (1,962)
Other
Schedule of Equity Method Investments [Line Items]
Ownership percentage in joint ventures		17.00%	17.00%
Net Investment in Properties	59,901	$ 22,677	$ 15,953
Total Assets	64,553	24,320	17,694
Mortgage Loans	4,438	5,225
Total Liabilities	4,706	5,327	236
Equity / (Deficit)	59,847	18,993	17,458
Revenues	11,261	9,383	5,958
Property Operating Expense	(6,779)	(5,879)	(4,629)
Net Operating Income	4,482	3,504	1,329
Net Income (Loss)	$ (31)	$ 415	$ (272)