Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Cash flows from operating activities:
Net income	$ 10,882	$ 114,763	$ 52,662	$ 105,663
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization			88,938	105,799
Provision for impairment of investment properties	724	13,034	1,316	13,034
Gain on sales of investment properties			(34,519)	(157,792)
Amortization of loan fees and debt premium and discount, net			1,780	5,871
Amortization of stock-based compensation			3,729	3,549
Premium paid in connection with defeasance of mortgages payable			0	59,968
Debt prepayment fees			974	4,545
Payment of leasing fees and inducements			(3,652)	(9,757)
Changes in accounts receivable, net			(1,520)	4,094
Changes in accounts payable and accrued expenses, net			(19,554)	(15,577)
Changes in other operating assets and liabilities, net			3,287	11,383
Other, net			(4,738)	(63)
Net cash provided by operating activities			88,703	130,717
Cash flows from investing activities:
Purchase of investment properties			0	(99,434)
Capital expenditures and tenant improvements			(32,961)	(31,629)
Proceeds from sales of investment properties			187,125	404,996
Investment in developments in progress			(6,933)	(8,612)
Net cash provided by investing activities			147,231	265,321
Cash flows from financing activities:
Principal payments on mortgages payable			(12,818)	(38,571)
Proceeds from unsecured term loans			0	200,000
Repayments of unsecured term loans			100,000	0
Proceeds from unsecured revolving line of credit			196,000	474,000
Repayments of unsecured revolving line of credit			(286,000)	(378,000)
Payment of loan fees and deposits			(5,393)	(10)
Debt prepayment fees			(974)	(4,545)
Purchase of U.S. Treasury securities in connection with defeasance of mortgages payable			0	(439,403)
Distributions paid			(72,658)	(83,182)
Shares repurchased through share repurchase program			0	(75,697)
Other, net			(1,405)	(1,333)
Net cash used in financing activities			(283,248)	(346,741)
Net (decrease) increase in cash, cash equivalents and restricted cash			(47,314)	49,297
Cash, cash equivalents and restricted cash, at beginning of period			86,335	82,349	$ 82,349
Cash, cash equivalents and restricted cash, at end of period	39,021	131,646	39,021	131,646	86,335
Supplemental cash flow disclosure, including non-cash activities:
Cash paid for interest, net of interest capitalized			34,179	41,017
Distributions payable	36,363	38,318	36,363	38,318	36,311
Accrued capital expenditures and tenant improvements			7,550	6,089
Accrued leasing fees and inducements			761	840
Accrued redevelopment costs			1,074	1,019
Developments in progress placed in service			9,355	0
U.S. Treasury securities transferred in connection with defeasance of mortgages payable			0	439,403
Defeasance of mortgages payable			0	379,435
Purchase of investment properties (after credits at closing):
Net investment properties			0	(101,307)
Accounts receivable, acquired lease intangibles and other assets			0	(7,659)
Accounts payable, acquired lease intangibles and other liabilities			0	7,008
Deferred gain			0	2,524
Purchase of investment properties (after credits at closing)			0	(99,434)
Proceeds from sales of investment properties:
Net investment properties			148,448	245,853
Accounts receivable, acquired lease intangibles and other assets			10,999	8,280
Accounts payable, acquired lease intangibles and other liabilities			(6,841)	(6,253)
Deferred gain			0	(676)
Gain on sales of investment properties			34,519	157,792
Proceeds from sales of investment properties			187,125	404,996
Reconciliation of cash, cash equivalents and restricted cash reported on the Company’s condensed consolidated balance sheets to such amounts shown in the Company’s condensed consolidated statements of cash flows:
Cash and cash equivalents, at beginning of period			25,185	53,119	53,119
Restricted cash, at beginning of period (included in “Other assets, net”)			61,150	29,230	29,230
Cash, cash equivalents and restricted cash, at beginning of period			86,335	82,349	82,349
Cash and cash equivalents, at end of period	29,125	28,003	29,125	28,003	25,185
Restricted cash, at end of period (included in “Other assets, net”)	9,896	103,643	9,896	103,643	61,150
Cash, cash equivalents and restricted cash, at end of period	$ 39,021	$ 131,646	$ 39,021	$ 131,646	$ 86,335