Real Estate Fund Investments - Schedule of Amounts Recognized in Income Statement (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Real Estate Properties [Line Items]
Rental income	$ 146,470		$ 435,630
Tenant reimbursement income	14,405		39,956
Fee and other income	6,851		16,294
Total revenues	167,726		491,880
Operating	63,354		183,019
Depreciation and amortization	70,654		223,658
General and administrative	6,666		28,412
Total expenses	141,159		444,921
Operating income (loss)	26,567		46,959
Unrealized gains on interest rate swaps	15,772		49,497
Interest and debt expense	(42,821)		(126,945)
Net income before income taxes	10,146		3,784
Income tax benefit (expense)	(789)		(2,706)
Net income	$ 9,357		$ 1,078
1633 Broadway [Member]
Real Estate Properties [Line Items]
Rental income		$ 39,105		$ 112,075
Tenant reimbursement income		4,667		10,580
Fee and other income		1,214		2,438
Total revenues		44,986		125,093
Building operating		15,441		42,320
Related party management fees		768		2,253
Operating		16,209		44,573
Depreciation and amortization		3,234		8,801
General and administrative		23		33
Total expenses		19,466		53,407
Operating income (loss)		25,520		71,686
Unrealized gains on interest rate swaps		10,946		23,891
Interest and debt expense		(13,005)		(38,630)
Net income before income taxes		23,461		56,947
Net income		23,461		56,947
900 Third Avenue [Member]
Real Estate Properties [Line Items]
Rental income		8,708		26,472
Tenant reimbursement income		800		2,314
Fee and other income		198		810
Total revenues		9,706		29,596
Building operating		4,249		12,332
Related party management fees		262		787
Operating		4,511		13,119
Depreciation and amortization		1,318		5,049
General and administrative		6		37
Total expenses		5,835		18,205
Operating income (loss)		3,871		11,391
Unrealized gains on interest rate swaps		2,833		5,768
Interest and debt expense		(3,735)		(11,088)
Net income before income taxes		2,969		6,071
Net income		2,969		6,071
31 West 52nd Street [Member]
Real Estate Properties [Line Items]
Rental income		19,885		56,106
Tenant reimbursement income		1,601		4,090
Fee and other income		489		3,104
Total revenues		21,975		63,300
Building operating		6,424		17,941
Related party management fees		353		1,008
Operating		6,777		18,949
Depreciation and amortization		8,886		19,960
General and administrative		25		59
Total expenses		15,688		38,968
Operating income (loss)		6,287		24,332
Unrealized gains on interest rate swaps		4,261		8,549
Interest and debt expense		(5,622)		(16,691)
Net income before income taxes		4,926		16,190
Net income		4,926		16,190
1301 Avenue of the Americas [Member]
Real Estate Properties [Line Items]
Rental income		30,117		83,866
Tenant reimbursement income		2,488		6,922
Fee and other income		870		2,527
Total revenues		33,475		93,315
Building operating		13,113		38,801
Related party management fees		427		1,264
Operating		13,540		40,065
Depreciation and amortization		12,342		30,897
General and administrative		45		96
Total expenses		25,927		71,058
Operating income (loss)		7,548		22,257
Unrealized gains on interest rate swaps		4,441		10,566
Interest and debt expense		(15,699)		(46,248)
Net income before income taxes		(3,710)		(13,425)
Net income		(3,710)		(13,425)
One Market Plaza [Member]
Real Estate Properties [Line Items]
Rental income		17,555		53,678
Tenant reimbursement income		346		1,007
Fee and other income		504		2,548
Total revenues		18,405		57,233
Building operating		7,264		21,265
Related party management fees		191		585
Operating		7,455		21,850
Depreciation and amortization		8,641		26,327
General and administrative		170		464
Total expenses		16,266		48,641
Operating income (loss)		2,139		8,592
Unrealized gains on interest rate swaps		10,765		21,209
Interest and debt expense		(13,491)		(40,563)
Net income before income taxes		(587)		(10,762)
Net income		(587)		(10,762)
Liberty Place [Member]
Real Estate Properties [Line Items]
Rental income		1,469		5,198
Tenant reimbursement income		475		1,659
Fee and other income		16		39
Total revenues		1,960		6,896
Building operating		1,161		3,329
Related party management fees		54		186
Operating		1,215		3,515
General and administrative		(4)		12
Total expenses		1,211		3,527
Operating income (loss)		749		3,369
Interest and debt expense		(945)		(2,835)
Unrealized depreciation on investment in real estate		(17)		(506)
Net income before income taxes		(213)		28
Income tax benefit (expense)		22		(4)
Net income		(191)		24
1899 Pennsylvania Avenue [Member]
Real Estate Properties [Line Items]
Rental income		2,027		6,071
Tenant reimbursement income		962		3,069
Fee and other income		45		101
Total revenues		3,034		9,241
Building operating		1,303		3,908
Related party management fees		68		209
Operating		1,371		4,117
Depreciation and amortization		998		2,921
General and administrative		(3)		13
Total expenses		2,366		7,051
Operating income (loss)		668		2,190
Interest and debt expense		(1,129)		(3,386)
Net income before income taxes		(461)		(1,196)
Income tax benefit (expense)		97		279
Net income		(364)		(917)
2099 Pennsylvania Avenue [Member]
Real Estate Properties [Line Items]
Rental income		51		127
Tenant reimbursement income		2		5
Fee and other income		9		19
Total revenues		62		151
Building operating		1,231		3,454
Related party management fees		19		21
Operating		1,250		3,475
General and administrative		340		739
Total expenses		1,590		4,214
Operating income (loss)		(1,528)		(4,063)
Interest and debt expense		(1,489)		(3,697)
Unrealized depreciation on investment in real estate		(5)		(535)
Net income before income taxes		(3,022)		(8,295)
Net income		(3,022)		(8,295)
425 Eye Street [Member]
Real Estate Properties [Line Items]
Rental income		2,639		7,939
Tenant reimbursement income		214		1,041
Total revenues		2,853		8,980
Building operating		1,415		4,319
Related party management fees		91		275
Operating		1,506		4,594
Depreciation and amortization		1,400		4,193
General and administrative		(30)		84
Total expenses		2,876		8,871
Operating income (loss)		(23)		109
Interest and debt expense		(1,277)		(3,797)
Net income before income taxes		(1,300)		(3,688)
Income tax benefit (expense)		21		(2,294)
Net income		$ (1,279)		$ (5,982)