Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended										12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Hurricane related expenses per diluted share							$ 0.05
Hurricane related expenses							$ 4,987
Hurricane Related Expenses per basic share							$ 0.05
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property											$ 0		$ 43,231		$ 295,397
Property revenues	$ 244,919	$ 241,770	$ 237,133	$ 230,683	$ 229,827		$ 228,178		$ 223,370	$ 219,521	954,505		900,896		876,447
Net income attributable to common shareholders	$ 39,196	$ 38,866	$ 38,671	$ 39,395	$ 87,989		$ 34,384		$ 39,188	$ 34,861	$ 156,128		$ 196,422		$ 819,823
Total earnings per common share - basic	$ 0.41	$ 0.41	$ 0.40	$ 0.41	$ 0.92	[1]	$ 0.38	[2]	$ 0.43	$ 0.39	$ 1.63	[3]	$ 2.14	[3]	$ 9.08
Total earnings per common share - diluted	$ 0.41	$ 0.40	$ 0.40	$ 0.41	0.91	[1]	$ 0.38	[2]	$ 0.43	$ 0.39	$ 1.63	[3]	$ 2.13	[3]	$ 9.05
Income loss due to disposition of operating properties per basic share					0.46
Income loss due to disposition of operating properties per diluted share					$ 0.45

[1] Includes a $43,231, or $0.46 basic and $0.45 diluted per share, impact related to a gain on sale of one operating property.

[2] Includes a $4,987, or $0.05 basic and diluted per share, impact related to expenses due to Hurricanes Harvey and Irma.

[3] 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.