Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Cash Flows [Abstract]
Net income	$ 23,092	$ 85,571	$ 69,436
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	142,472	127,690	116,614
Other amortization	3,879	3,351	3,749
Stock-based compensation	10,028	7,598	6,029
Amortization of straight-line rent receivable	(6,072)	(7,201)	(9,600)
Amortization of straight-line rent liability	1,497	67	771
Gain on sales of real estate assets	(39,524)	(41,044)	(67,229)
Loss on extinguishment of debt	44,985		27,998
Impairment of real estate assets	5,385	121	4,325
Pension termination			5,260
Impairment of internally-developed software			654
Equity income from unconsolidated joint ventures	(7)
Provision for bad debts, net	159	(21)	(194)
Changes in operating assets and liabilities:
Other assets	(2,156)	(1,332)	(2,932)
Accounts payable and accrued liabilities	(7,307)	449	(2,202)
Other liabilities	3,335	(23,977)	1,304
Net cash provided by operating activities	179,766	151,272	153,983
INVESTING ACTIVITIES
Acquisitions of real estate	(274,668)	(224,944)	(154,858)
Development of real estate	(14,911)	(34,719)	(17,354)
Additional long-lived assets	(80,613)	(71,433)	(48,769)
Investment in unconsolidated joint ventures	(8,701)
Proceeds from sales of real estate	119,426	93,253	153,281
Proceeds from mortgages and notes receivable repayments	19	19	1,918
Net cash used in investing activities	(259,448)	(237,824)	(65,782)
FINANCING ACTIVITIES
Net borrowings (repayments) on unsecured credit facility	82,000	(99,000)	121,000
Repayment on term loan		(50,000)
Borrowings of notes and bonds payable	297,459	11,500	249,793
Repayments on notes and bonds payable	(5,829)	(37,910)	(72,724)
Redemption of notes and bonds payable	(442,774)		(326,830)
Dividends paid	(142,327)	(131,759)	(120,266)
Net proceeds from issuance of common stock	248,554	450,503	66,942
Common stock redemptions	(1,686)	(1,756)	(1,367)
Settlement of swaps			(1,684)
Debt issuance and assumption costs	(4,007)	(4,621)	(2,482)
Net cash provided by (used in) financing activities	31,390	136,957	(87,618)
Increase (decrease) in cash, cash equivalents and restricted cash	(48,292)	50,405	583
Cash, cash equivalents and restricted cash at beginning of period	54,507	4,102	3,519
Cash, cash equivalents and restricted cash at end of period	6,215	54,507	4,102
Supplemental Cash Flow Information:
Interest paid	64,395	55,878	69,773
Mortgage notes payable assumed upon acquisition (adjusted to fair value)	46,374	13,951	28,783
Invoices accrued for construction, tenant improvements and other capitalized costs	8,303	11,734	10,431
Capitalized interest	$ 871	$ 1,258	$ 239