Quarterly Financial Data (unaudited) (Details) $ / shares in Units, $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2019 USD ($) $ / shares		Sep. 30, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($) $ / shares	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2019 USD ($) $ / shares		Dec. 31, 2018 USD ($) $ / shares		Dec. 31, 2017 USD ($) $ / shares	Dec. 12, 2019
Current Fiscal Year End Date										--12-31
Gain (Loss) on Extinguishment of Debt | $	$ (11,995)									$ (11,995)		$ 0		$ (323)
Gain (loss) on extinguishment of debt per basic share	$ 0.12
Gain (loss) on extinguishment of debt per diluted share	$ 0.12
Gain on sale of operating properties | $	$ 49,901									49,901		0		43,231
Property revenues | $	263,461		$ 260,672	$ 255,761	$ 248,567	$ 244,919	$ 241,770	$ 237,133	$ 230,683	1,028,461		954,505		900,896
Net income attributable to common shareholders | $	$ 95,014		$ 43,597	$ 42,399	$ 38,613	$ 39,196	$ 38,866	$ 38,671	$ 39,395	$ 219,623		$ 156,128		$ 196,422
Total earnings per common share - basic	$ 0.96	[1]	$ 0.44	$ 0.43	$ 0.40	$ 0.41	$ 0.41	$ 0.40	$ 0.41	$ 2.23	[2]	$ 1.63	[2]	$ 2.14
Total earnings per common share - diluted	0.95	[1]	$ 0.44	$ 0.43	$ 0.40	$ 0.41	$ 0.40	$ 0.40	$ 0.41	$ 2.22	[2]	$ 1.63	[2]	$ 2.13
Income loss due to disposition of operating property per basic share	0.50
Income loss due to disposition of operating property per diluted share	$ 0.49
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Number of operating properties sold	2									2				1	3

[1] Includes a $11,995 or $0.12 basic and diluted per share, impact related to the loss on early retirement of debt in October 2019. Also includes a $49,901 or $0.50 basic and $0.49 diluted per share, impact related to a gain on sale of two consolidated operating properties.

[2] Net income per share is computed independently for each of the quarters presented. Therefore, the sum of quarterly net income per share amounts may not equal the total computed for the year.