Accounting for the Impairment or Disposal of Long-Lived Assets (Discontinued Operations) (Details) - USD ($) $ 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
Discontinued Operation
Operating expense reimbursement									$ 151,276	$ 136,184	$ 167,315
Total revenue	$ 180,281	$ 170,389	$ 174,091	$ 180,128	$ 179,825	$ 170,783	$ 160,747	$ 154,029	704,889	665,384	675,323
Real estate taxes									91,663	83,152	92,920
Other Cost and Expense, Operating									11,994	6,167	5,017
Impairment - real estate assets									32,431	3,946	3,879
Total expense									646,404	539,862	589,424
Interest and other income									12,048	7,734	13,846
Gain on property dispositions									303,200	14,600	0
Income taxes									(7,364)	(1,992)	(1,969)
Income from discontinued operations	$ 115,832	$ 104,332	$ 7,948	$ 98,426	$ 22,579	$ 3,389	$ 8,358	$ 6,913	326,538	41,239	27,129
Income available to common shareholders									318,930	40,274	26,497
Discontinued operations
Discontinued Operation
Revenues									43,393	61,405	48,343
Operating expense reimbursement									12,663	27,024	23,042
Total revenue									56,056	88,429	71,385
Rental property									6,959	17,826	13,783
Real estate taxes									7,354	11,582	6,873
Other Cost and Expense, Operating									(9)	(272)	110
Interest expense									5,812	6,858	9,950
Depreciation and amortization									5,356	19,190	13,642
Impairment - real estate assets									7,257	6,686	0
Total expense									32,729	61,870	44,358
Interest and other income									52	102	104
Gain on property dispositions									303,159	14,578	0
Income taxes									0	0	(2)
Income from discontinued operations									326,538	41,239	27,129
Noncontrolling interest - operating partnership									(7,608)	(965)	(632)
Income available to common shareholders									318,930	40,274	26,497
Capital expenditures on cash basis									$ 6,100	$ 62,400	$ 18,900