Real Estate Investments - Discontinued Operations and Assets Held for Sale - Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Balance Sheet data (as of the period ended):
Land	$ 186,231	$ 183,060
Buildings, improvements and lease intangibles	3,033,213	3,048,251
Personal property	9,970	9,914
Real estate properties, Total	3,254,650	3,258,279
Accumulated depreciation	(730,125)	(700,671)
Total real estate properties, net	2,524,525	2,557,608
Other assets, net (including receivables)	191,524	185,337
Assets held for sale and discontinued operations, net	14,192	9,146
Liabilities of discontinued operations	168	372
Discontinued Operations [Member]
Balance Sheet data (as of the period ended):
Land	2,524	422
Buildings, improvements and lease intangibles	21,513	12,822
Personal property	21	13
Real estate properties, Total	24,058	13,257
Accumulated depreciation	(10,322)	(4,464)
Total real estate properties, net	13,736	8,793
Other assets, net (including receivables)	456	353
Assets of discontinued operations, net	456	353
Assets held for sale and discontinued operations, net	14,192	9,146
Accounts payable and accrued liabilities	149	86
Other liabilities	19	286
Liabilities of discontinued operations	$ 168	$ 372