Real Estate Investments - Discontinued Operations and Assets Held for Sale - Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Balance Sheet data (as of the period ended):
Land	$ 201,157	$ 198,585
Buildings, improvements and lease intangibles	3,181,748	3,135,893
Real estate properties, Total	3,441,889	3,380,908
Accumulated depreciation	(790,819)	(761,926)
Total real estate properties, net	2,651,070	2,618,982
Other assets, net (including receivables)	187,665	186,416
Assets held for sale and discontinued operations, net	706	724
Liabilities of discontinued operations	20	33
Discontinued Operations [Member]
Balance Sheet data (as of the period ended):
Land	422	422
Buildings, improvements and lease intangibles	1,350	1,350
Real estate properties, Total	1,772	1,772
Accumulated depreciation	(1,070)	(1,070)
Total real estate properties, net	702	702
Other assets, net (including receivables)	4	22
Assets of discontinued operations, net	4	22
Assets held for sale and discontinued operations, net	706	724
Accounts payable and accrued liabilities	18	28
Other liabilities	2	5
Liabilities of discontinued operations	$ 20	$ 33