Property Dispositions and Assets Held for Sale (Assets Held-for-Sale) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total other assets held for sale, net	$ 20,791	$ 84,293
Disposal Group, Held-for-sale, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total real estate assets held for sale, net	110,552	561,449
Straight-line rent receivables	10,756	44,666
Prepaid expenses and other assets	430	2,067
Deferred lease costs, less accumulated amortization of $2,446 and $20,169 as of December 31, 2018 and 2017, respectively	9,605	37,560
Accumulated amortization on deferred lease costs	2,446	20,169
Total other assets held for sale, net	20,791	84,293
Intangible lease liabilities, less accumulated amortization of $0 and $935 as of December 31, 2018 and 2017, respectively	0	380
Accumulated amortization on Intangible lease liabilities	0	935
Disposal Group, Held-for-sale, Not Discontinued Operations | Land
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Real estate assets held for sale, net:	30,562	128,668
Disposal Group, Held-for-sale, Not Discontinued Operations | Building and building improvements
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Real estate assets held for sale, net:	77,936	430,136
Accumulated depreciation (building improvements)	48,453	270,552
Disposal Group, Held-for-sale, Not Discontinued Operations | Construction in progress
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Real estate assets held for sale, net:	$ 2,054	$ 2,645