Guarantor and Non-Guarantor Financial Information - Condensed Consolidated Statements of Cash Flows (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by Operating Activities	$ 94,833	$ 108,895
Cash Flows from Investing Activities:
Investment in real estate assets and real estate related intangibles, net of accruals	(107,772)	(98,116)
Redemption of noncontrolling interest in unconsolidated variable interest entity	(4,000)	0
Intercompany note receivable	0	0
Net sales proceeds from wholly-owned properties	87,925	46,240
Net sales proceeds received from unconsolidated joint ventures	0	6,017
Investments in unconsolidated joint ventures	0	(42)
Deferred lease costs paid	(10,678)	(11,370)
Net cash used in investing activities	(34,525)	(57,271)
Cash Flows from Financing Activities:
Deferred financing costs paid	(830)	(1,016)
Proceeds from debt	1,054,857	846,564
Repayments of debt	(1,012,576)	(779,070)
Intercompany note payable		0
Net costs of issuance of common stock	(326)	0
Repurchases of common stock as part of announced plan	(39,914)	(54,685)
(Distributions to)/repayments from affiliates	0	0
Dividends paid and discount on dividend reinvestments	(64,828)	(61,827)
Net cash used in financing activities	(63,617)	(50,034)
Net (decrease)/increase in cash and cash equivalents	(3,309)	1,590
Cash and cash equivalents, end of period	8,997	8,563
Cash and cash equivalents, beginning of period	12,306	6,973
Issuer [Member]
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by Operating Activities	(15,322)	9,461
Cash Flows from Investing Activities:
Investment in real estate assets and real estate related intangibles, net of accruals	(10,269)	(13,072)
Redemption of noncontrolling interest in unconsolidated variable interest entity	0
Intercompany note receivable	72,000	(100)
Net sales proceeds from wholly-owned properties	87,925	29,519
Net sales proceeds received from unconsolidated joint ventures		6,017
Investments in unconsolidated joint ventures		(42)
Deferred lease costs paid	(2,027)	(2,403)
Net cash used in investing activities	147,629	19,919
Cash Flows from Financing Activities:
Deferred financing costs paid	(423)	(1,016)
Proceeds from debt	895,578	846,564
Repayments of debt	(907,000)	(204,000)
Intercompany note payable		0
Net costs of issuance of common stock	0
Repurchases of common stock as part of announced plan	0	0
(Distributions to)/repayments from affiliates	(122,441)	(668,859)
Dividends paid and discount on dividend reinvestments	0	0
Net cash used in financing activities	(134,286)	(27,311)
Net (decrease)/increase in cash and cash equivalents	(1,979)	2,069
Cash and cash equivalents, end of period	6,164	5,330
Cash and cash equivalents, beginning of period	8,143	3,261
Guarantor [Member]
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by Operating Activities	1,215	1,259
Cash Flows from Investing Activities:
Investment in real estate assets and real estate related intangibles, net of accruals	0	0
Redemption of noncontrolling interest in unconsolidated variable interest entity	0
Intercompany note receivable	0	0
Net sales proceeds from wholly-owned properties	0	0
Net sales proceeds received from unconsolidated joint ventures		0
Investments in unconsolidated joint ventures		0
Deferred lease costs paid	0	0
Net cash used in investing activities	0	0
Cash Flows from Financing Activities:
Deferred financing costs paid	0	0
Proceeds from debt	0	0
Repayments of debt	0	0
Intercompany note payable		0
Net costs of issuance of common stock	(326)
Repurchases of common stock as part of announced plan	(39,914)	(54,685)
(Distributions to)/repayments from affiliates	102,205	115,245
Dividends paid and discount on dividend reinvestments	(64,820)	(61,819)
Net cash used in financing activities	(2,855)	(1,259)
Net (decrease)/increase in cash and cash equivalents	(1,640)	0
Cash and cash equivalents, end of period	150	150
Cash and cash equivalents, beginning of period	1,790	150
Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by Operating Activities	91,753	84,096
Cash Flows from Investing Activities:
Investment in real estate assets and real estate related intangibles, net of accruals	(97,503)	(85,044)
Redemption of noncontrolling interest in unconsolidated variable interest entity	(4,000)
Intercompany note receivable	0	0
Net sales proceeds from wholly-owned properties	0	16,721
Net sales proceeds received from unconsolidated joint ventures		0
Investments in unconsolidated joint ventures		0
Deferred lease costs paid	(8,651)	(8,967)
Net cash used in investing activities	(110,154)	(77,290)
Cash Flows from Financing Activities:
Deferred financing costs paid	(407)	0
Proceeds from debt	159,279	0
Repayments of debt	(177,576)	(575,070)
Intercompany note payable		100
Net costs of issuance of common stock	0
Repurchases of common stock as part of announced plan	0	0
(Distributions to)/repayments from affiliates	37,423	567,693
Dividends paid and discount on dividend reinvestments	(8)	(8)
Net cash used in financing activities	18,711	(7,285)
Net (decrease)/increase in cash and cash equivalents	310	(479)
Cash and cash equivalents, end of period	2,683	3,083
Cash and cash equivalents, beginning of period	2,373	3,562
Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by Operating Activities	17,187	14,079
Cash Flows from Investing Activities:
Investment in real estate assets and real estate related intangibles, net of accruals	0	0
Redemption of noncontrolling interest in unconsolidated variable interest entity	0
Intercompany note receivable	(72,000)	100
Net sales proceeds from wholly-owned properties	0	0
Net sales proceeds received from unconsolidated joint ventures		0
Investments in unconsolidated joint ventures		0
Deferred lease costs paid	0	0
Net cash used in investing activities	(72,000)	100
Cash Flows from Financing Activities:
Deferred financing costs paid	0	0
Proceeds from debt	0	0
Repayments of debt	72,000	0
Intercompany note payable		(100)
Net costs of issuance of common stock	0
Repurchases of common stock as part of announced plan	0	0
(Distributions to)/repayments from affiliates	(17,187)	(14,079)
Dividends paid and discount on dividend reinvestments	0	0
Net cash used in financing activities	54,813	(14,179)
Net (decrease)/increase in cash and cash equivalents	0	0
Cash and cash equivalents, end of period	0	0
Cash and cash equivalents, beginning of period	$ 0	$ 0