Consolidated Statements of Shareholders' Equity - 3 months ended Mar. 31, 2017 - USD ($) $ in Thousands	Total	Shares of Beneficial Interest at Par Value [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Parent [Member]	Noncontrolling Interest [Member]
Balance (in shares) at Dec. 31, 2016		74,606,000
Balance at Dec. 31, 2016	$ 1,052,062	$ 746	$ 1,368,636	$ (326,047)	$ 7,611	$ 1,050,946	$ 1,116
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to the controlling interests	6,634			6,634		6,634
Net loss attributable to the noncontrolling interests	(19)						(19)
Unrealized gain on interest rate hedge	735				735	735
Distributions to noncontrolling interests	(36)						(36)
Dividends	(22,607)			(22,607)		(22,607)
Equity offerings, net of issuance costs (in shares)		972,000
Equity offerings, net of issuance costs	29,959	$ 10	29,949			29,959
Shares issued under dividend reinvestment program (in shares)		34,770
Shares issued under dividend reinvestment program	1,114	$ 1,100	1,114			1,114
Share grants, net of forfeitures (in shares)		89,000
Share grants, net of share grant amortization, forfeitures and tax withholdings	395	$ 1	394			395
Balance (in shares) at Mar. 31, 2017		75,702,000
Balance at Mar. 31, 2017	$ 1,068,237	$ 757	$ 1,400,093	$ (342,020)	$ 8,346	$ 1,067,176	$ 1,061