Consolidated Statements of Cash Flows $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Statement of Cash Flows [Abstract]
Cash and Cash Equivalents, at Carrying Value	$ 5,409	$ 4,102	$ 3,519
Net income	85,571	69,436	32,200
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	131,041	120,363	116,049
Stock-based compensation	7,598	6,029	4,451
Amortization of straight-line rent receivable	(7,201)	(9,600)	(11,050)
Amortization of straight-line rent liability	67	771	721
Gain on sales of real estate assets	(41,044)	(67,229)	(9,283)
Loss on extinguishment of debt		27,998
Impairment of real estate assets	121	4,325	12,029
Pension termination	0	5,260
Impairment of internally-developed software	0	654	0
Provision for bad debts, net	(21)	(194)	34
Changes in operating assets and liabilities:
Other assets	(1,332)	(2,932)	(16,842)
Accounts payable and accrued liabilities	449	4,190	(1,914)
Other liabilities	(23,977)	1,304	(1,025)
Net cash provided by operating activities	151,272	160,375	125,370
INVESTING ACTIVITIES
Acquisitions of real estate	(224,944)	(154,858)	(71,899)
Development of real estate	(34,719)	(17,354)
Additional long-lived assets	(71,433)	(48,769)	(70,670)
Funding of mortgages and notes receivable			(1,244)
Proceeds from acquisition of real estate upon mortgage note receivable default			204
Proceeds from sales of real estate	93,253	153,281	32,398
Proceeds from mortgages and notes receivable repayments	19	1,918	5,623
Net cash used in investing activities	(237,824)	(65,782)	(105,588)
FINANCING ACTIVITIES
Net (repayments) borrowings on unsecured credit facility	(99,000)	121,000	(153,000)
Borrowings (repayments) on term loan	(50,000)		200,000
Borrowings of mortgage notes and bonds payable	11,500	249,793
Repayments on notes and bonds payable	(37,910)	(72,724)	(12,357)
Redemption of notes and bonds payable		(333,222)
Dividends paid	(131,759)	(120,266)	(116,371)
Net proceeds from issuance of common stock	450,503	66,942	76,856
Common stock redemptions	(1,756)	(1,367)	(10,074)
Settlement of swaps		(1,684)
Distributions to noncontrolling interest holders			(541)
Purchase of noncontrolling interest			(8,189)
Debt issuance and assumption costs	(4,621)	(2,482)	(1,258)
Net cash provided by (used in) financing activities	136,957	(94,010)	(24,934)
Increase (decrease) in cash, cash equivalents and restricted cash	50,405	583	(5,152)
Cash, cash equivalents and restricted cash at beginning of period	4,102	3,519	8,671
Cash, cash equivalents and restricted cash at end of period	54,507	4,102	3,519
Supplemental Cash Flow Information:
Interest paid	55,878	69,773	68,173
Mortgage notes payable assumed upon acquisition (adjusted to fair value)	13,951	28,783	19,636
Invoices accrued for construction, tenant improvements and other capitalized costs	11,734	10,431	5,594
Capitalized interest	$ 1,258	239
Elimination of construction mortgage note receivable upon acquisition real estate property		$ 0	81,213
Mortgage note receivable eliminated upon acquisition			39,973
Company-financed real estate property sales			$ 1,900