Consolidated Statement of Equity - 3 months ended Mar. 31, 2018 - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	AOCI Including Portion Attributable to Noncontrolling Interest	Parent	Operating Partnership Noncontrolling Interest	Partially Owned Properties Noncontrolling Interest	Noncontrolling Interest	Operating Partnership	Operating Partnership Partially Owned Properties Noncontrolling Interest
Balance at Dec. 31, 2017	$ 2,547,634	$ 1,814	$ 2,772,823	$ (315,417)	$ 13,952	$ 2,473,172	$ 73,844	$ 618	$ 74,462
Stockholders' Equity Attributable to Parent [Abstract]
Net proceeds from sale of common shares	5,316	3	5,313			5,316
Restricted share award grants, net	933	2	872	59		933
Conversion of OP Units			126			126	(126)		(126)
Dividends/distributions declared	(43,126)			(41,910)		(41,910)	(1,216)		(1,216)
Preferred distributions	(487)			(487)		(487)
Distributions	(43)							(43)	(43)	$ (43)	$ (43)
Change in market value of Redeemable Noncontrolling Interest in Operating Partnership	1,470		194	1,276		1,470
Change in fair value of interest rate swap agreements	4,298			0	4,298	4,298				4,298
Net income	11,265			10,908		10,908	313	44	357	$ 11,265	$ 44
Adjustment for Noncontrolling Interests ownership in Operating Partnership			(712)			(712)	712		712
Balance at Mar. 31, 2018	$ 2,527,260	$ 1,819	$ 2,778,616	$ (345,571)	$ 18,250	$ 2,453,114	$ 73,527	$ 619	$ 74,146