CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
REVENUES (Note 2):
Rental income	$ 892,306	$ 826,472	$ 656,631
Tenant reimbursements	0	0	80,982
Other property income	6,091	10,982	9,685
Revenues	898,397	837,454	747,298
EXPENSES:
Property expenses	155,118	160,037	133,787
Real estate taxes (Note 2)	92,218	78,097	70,820
Provision for bad debts (Note 2)	0	0	5,685
Ground leases (Notes 2, 5 and 18)	8,891	8,113
Ground leases (Notes 2, 5 and 18)			6,176
General and administrative expenses (Notes 15 and 19)	99,264	88,139	90,471
Leasing costs (Notes 2 and 5)	4,493	7,615
Depreciation and amortization (Notes 2 and 5)	299,308	273,130	254,281
Total expenses	659,292	615,131	561,220
OTHER (EXPENSES) INCOME:
Interest income and other net investment gain (loss) (Note 19)	3,424	4,641	(559)
Interest expense (Note 9)	(70,772)	(48,537)	(49,721)
Gains on sales of depreciable operating properties (Note 4)	35,536	36,802	142,926
Loss on early extinguishment of debt (Note 9)	0	0	(12,623)
Net gain on sales of land (Note 4)	0	0	11,825
Total other (expenses) income	(31,812)	(7,094)	91,848
NET INCOME	207,293	215,229	277,926
Net income attributable to noncontrolling common units of the Operating Partnership (Notes 2 and 11)	(2,869)	(3,766)	(5,193)
Net income attributable to noncontrolling interests in consolidated property partnerships (Notes 2 and 11)	(17,319)	(16,020)	(14,318)
Total income attributable to noncontrolling interests	(20,188)	(19,786)	(19,511)
NET INCOME AVAILABLE TO COMMON STOCKHOLDERS (UNITHOLDERS)	$ 187,105	$ 195,443	$ 258,415
Net income available to common stockholders per share - basic (in dollars per share)	$ 1.63	$ 1.87	$ 2.56
Net income available to common stockholders per share - diluted (in dollars per share)	$ 1.63	$ 1.86	$ 2.55
Weighted average shares of common stock outstanding – basic (in shares)	113,241,341	103,200,568	99,972,359
Weighted average shares of common stock outstanding – diluted (in shares)	113,719,622	103,849,168	100,482,365
Revenue, Product and Service [Extensible List]	us-gaap:ServiceMember