Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Cash flows from operating activities:
Net income	$ (2,288)	$ (28,153)	$ 12,722	$ 16,225
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization			125,669	153,609
Provision for impairment of investment properties	2,279	11,177	2,625	11,177	$ 12,298
Gain on sales of investment properties	0	(1,969)	0	(18,872)
Amortization of loan fees and debt premium and discount, net			3,249	1,913
Amortization of stock-based compensation			6,734	5,672
Debt prepayment fees			2,786	8,151
Payment of leasing fees and inducements			(5,787)	(6,880)
Changes in accounts receivable, net			(17,195)	3,071
Changes in right-of-use lease assets			1,391	1,438
Changes in accounts payable and accrued expenses, net			(12,392)	(4,222)
Changes in lease liabilities			(671)	(496)
Changes in other operating assets and liabilities, net			621	6,156
Other, net			(1,111)	(6,254)
Net cash provided by operating activities			118,641	170,688
Cash flows from investing activities:
Purchase of investment properties			(54,970)	(29,891)
Capital expenditures and tenant improvements			(44,955)	(59,971)
Proceeds from sales of investment properties			11,369	44,656
Investment in developments in progress			(58,939)	(17,817)
Net cash used in investing activities			(147,495)	(63,023)
Cash flows from financing activities:
Principal payments on mortgages payable			(2,160)	(109,917)
Proceeds from unsecured notes payable			493,746	100,000
Repayments of unsecured notes payable			(100,000)	0
Proceeds from unsecured term loans			0	270,000
Repayments of unsecured term loans			(250,000)	0
Proceeds from unsecured revolving line of credit			937,704	208,000
Repayments of unsecured revolving line of credit			(955,704)	(457,000)
Payment of loan fees and deposits			(5,403)	(2,519)
Debt prepayment fees			(2,786)	(8,151)
Distributions paid			(70,851)	(106,141)
Other, net			1,689	965
Net cash provided by (used in) financing activities			46,235	(104,763)
Net increase in cash, cash equivalents and restricted cash			17,381	2,902
Cash, cash equivalents and restricted cash, at beginning of period			14,447	19,601	19,601
Cash, cash equivalents and restricted cash, at end of period	31,828	22,503	31,828	22,503	14,447
Supplemental cash flow disclosure, including non-cash activities:
Cash paid for interest, net of interest capitalized			52,919	56,950
Cash paid for amounts included in the measurement of operating lease liabilities			4,406	4,523
Distributions payable	10,713	35,387	10,713	35,387	35,387
Accrued capital expenditures and tenant improvements			8,182	6,048
Accrued leasing fees and inducements			1,594	1,184
Accrued redevelopment costs			2,678	565
Amounts reclassified to developments in progress			305	34,746
Developments in progress placed in service			4,725	1,377
Change in noncontrolling interest due to termination of joint ventures			2,217	0
Lease liabilities arising from obtaining right-of-use lease assets			383	103,519
Straight-line ground rent liabilities reclassified to right-of-use lease asset			0	31,030
Straight-line office rent liability reclassified to right-of-use lease asset			0	507
Acquired ground lease intangible liability reclassified to right-of-use lease asset			0	11,898
Purchase of investment properties (after credits at closing):
Net investment properties			(58,760)	(28,486)
Right-of-use lease assets			5,999	0
Accounts receivable, acquired lease intangibles and other assets			(1,801)	(1,792)
Lease liabilities			(5,942)	0
Accounts payable, acquired lease intangibles and other liabilities			5,534	387
Purchase of investment properties (after credits at closing)			(54,970)	(29,891)
Proceeds from sales of investment properties:
Net investment properties			11,307	30,119
Right-of-use lease assets			0	8,242
Accounts receivable, acquired lease intangibles and other assets			167	1,591
Lease liabilities			0	(11,326)
Accounts payable, acquired lease intangibles and other liabilities			(105)	(2,842)
Gain on sales of investment properties	0	1,969	0	18,872
Proceeds from sales of investment properties			11,369	44,656
Reconciliation of cash, cash equivalents and restricted cash:
Cash and cash equivalents, at beginning of period			9,989	14,722	14,722
Restricted cash, at beginning of period (included within “Other assets, net”)			4,458	4,879	4,879
Cash, cash equivalents and restricted cash, at beginning of period			14,447	19,601	19,601
Cash and cash equivalents, at end of period	27,371	17,076	27,371	17,076	9,989
Restricted cash, at end of period (included within “Other assets, net”)	4,457	5,427	4,457	5,427	4,458
Cash, cash equivalents and restricted cash, at end of period	$ 31,828	$ 22,503	$ 31,828	$ 22,503	$ 14,447