Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Cash Flows [Abstract]
Net income	$ 69,436	$ 32,200	$ 6,983
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	120,363	116,049	105,318
Stock-based compensation	6,029	4,451	5,210
Straight-line rent receivable	(9,600)	(11,050)	(8,608)
Straight-line rent liability	771	721	426
Gain on sales of real estate assets	(67,229)	(9,283)	(24,718)
Gain on sale of cost method investment in real estate			(1,492)
Loss on extinguishment of debt	27,998		29,907
Impairments of real estate properties	4,325	12,029	9,889
Pension termination	5,260
Impairment of internally-developed software	654
Provision for bad debts, net	(194)	34	185
Changes in operating assets and liabilities:
Other assets	(2,932)	(16,842)	(5,660)
Accounts payable and accrued liabilities	4,190	(1,914)	740
Other liabilities	1,304	(1,025)	2,617
Net cash provided by operating activities	160,375	125,370	120,797
INVESTING ACTIVITIES
Acquisitions of real estate	(154,858)	(71,899)	(177,744)
Development of real estate	(17,354)
Additional long-lived assets	(48,769)	(70,670)	(72,784)
Funding of mortgages and notes receivable		(1,244)	(58,731)
Proceeds from acquisition of real estate upon mortgage note receivable default		204
Proceeds from sales of real estate	153,281	32,398	96,132
Proceeds from sale of cost method investment in real estate			2,717
Proceeds from mortgages and notes receivable repayments	1,918	5,623	2,464
Net cash used in investing activities	(65,782)	(105,588)	(207,946)
FINANCING ACTIVITIES
Net borrowings (repayments) on unsecured credit facility	121,000	(153,000)	128,000
Borrowings on term loan		200,000
Borrowings on notes and bonds payable	249,793		247,948
Repayments on notes and bonds payable	(72,724)	(12,357)	(19,984)
Redemption of notes and bonds payable	(333,222)		(371,839)
Dividends paid	(120,266)	(116,371)	(111,571)
Net proceeds from issuance of common stock	66,942	76,856	220,252
Common stock redemptions	(1,367)	(10,074)	(454)
Settlement of swaps	(1,684)
Capital Contributions received from noncontrolling interest			1,806
Distributions to noncontrolling interest holders		(541)	(32)
Purchase of noncontrolling interest		(8,189)
Debt issuance and assumption costs	(2,482)	(1,258)	(5,082)
Net cash provided by (used in) financing activities	(94,010)	(24,934)	89,044
Increase (decrease) in cash and cash equivalents	583	(5,152)	1,895
Cash and cash equivalents, beginning of period	3,519	8,671	6,776
Cash and cash equivalents, end of period	4,102	3,519	8,671
Supplemental Cash Flow Information:
Interest paid	69,773	68,173	71,025
Mortgage notes payable assumed upon acquisition (adjusted to fair value)	28,783	19,636	40,992
Invoices accrued for construction, tenant improvement and other capitalized costs	10,431	5,594	10,885
Capitalized interest	$ 239		183
Elimination of construction mortgage note receivable upon acquisition real estate property		81,213	97,203
Mortgage note receivable eliminated upon acquisition		39,973
Company-financed real estate property sales		$ 1,900	$ 4,241