Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Cash flows from operating activities:
Net loss	$ (4,680)	$ 251	$ (11,335)	$ (2,395)
Adjustment to reconcile net loss to net cash provided by operating activities:
Depreciation and amortization			82,042	62,606
Non-cash portion of interest expense			907	1,009
Intangible amortization in rental income, net			6,331	4,600
Straight-line rent adjustments, net			(2,318)	(2,314)
Dividends on forfeited equity compensation			15	128
Loss on impairments			8,378	0
Gain on sales of rental property			(1,713)	(2,153)
Non-cash compensation expense			5,667	5,337
Change in assets and liabilities:
Tenant accounts receivable, net			(1,477)	934
Restricted cash			(582)	(596)
Prepaid expenses and other assets			(4,407)	(3,218)
Accounts payable, accrued expenses and other liabilities			6,821	1,426
Tenant prepaid rent and security deposits			1,728	1,033
Total adjustments			101,392	68,792
Net cash provided by operating activities			90,057	66,397
Cash flows from investing activities:
Acquisitions of land and buildings and improvements			(188,263)	(228,063)
Additions to building and other capital improvements			(10,053)	(5,038)
Proceeds from sales of rental property, net			9,186	7,492
Restricted cash			(1,052)	734
Acquisition deposits, net			1,425	(1,920)
Acquisitions of deferred leasing intangibles			(56,102)	(61,413)
Net cash used in investing activities			(244,859)	(288,208)
Cash flows from financing activities:
Redemption of common units for cash			(64)	(342)
Proceeds from unsecured credit facility			190,750	187,500
Repayment of unsecured credit facility			(144,000)	(162,000)
Proceeds from unsecured term loans			0	50,000
Proceeds from unsecured notes			120,000	50,000
Repayment of mortgage notes payable			(15,370)	(3,321)
Payment of loan fees and costs			(3,019)	(1,718)
Dividends and distributions			(78,541)	(60,663)
Proceeds from sales of common stock			74,892	164,077
Offering costs			(1,228)	(2,647)
Withholding taxes for settlement of outperformance program			0	(475)
Net cash provided by financing activities			143,420	220,411
Decrease in cash and cash equivalents			(11,382)	(1,400)
Cash and cash equivalents-beginning of period			23,878	6,690	$ 6,690
Cash and cash equivalents-end of period	12,496	5,290	12,496	5,290	23,878
Supplemental disclosure:
Cash paid for interest			22,445	16,286
Supplemental schedule of non-cash investing and financing activities
Issuance of units for acquisitions of land and buildings and improvements			16,873	0
Issuance of units for acquisitions of deferred leasing intangibles			5,029	0
Additions to building and other capital improvements			(565)	0
Transfer of other assets to building and other capital improvements			565	0
Acquisitions of land and buildings and improvements			(34,460)	0
Acquisitions of deferred leasing intangibles			(10,203)	0
Change in additions of land and building and improvements included in accounts payable, accrued expenses, and other liabilities			(1,687)	(3,913)
Assumption of mortgage notes payable			22,343	0
Fair market value adjustment to mortgage notes payable acquired			418	0
Change in loan fees and costs and offering costs included in accounts payable, accrued expenses, and other liabilities			(64)	(159)
Dividends and distributions declared but not paid	$ 8,228	$ 6,565	$ 8,228	$ 6,565	$ 7,355