Real Estate Investments - Discontinued Operations and Assets Held for Sale - Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Balance Sheet data (as of the period ended):
Land	$ 206,647	$ 198,585
Buildings, improvements and lease intangibles	3,322,293	3,135,893
Personal property	10,124	9,954
Real estate properties, Total	3,602,236	3,380,908
Accumulated depreciation	(835,276)	(761,926)
Total real estate properties, net	2,766,960	2,618,982
Assets held for sale and discontinued operations, net	14,732	724
Liabilities of discontinued operations	572	33
Discontinued Operations [Member]
Balance Sheet data (as of the period ended):
Land	2,161	422
Buildings, improvements and lease intangibles	27,198	1,350
Personal property	5	0
Real estate properties, Total	29,364	1,772
Accumulated depreciation	(15,086)	(1,070)
Total real estate properties, net	14,278	702
Other assets, net (including receivables)	454	22
Assets held for sale and discontinued operations, net	14,732	724
Notes and bonds payable	114	0
Accounts payable and accrued liabilities	298	28
Other liabilities	160	5
Liabilities of discontinued operations	$ 572	$ 33