Dispositions (Real Estate Assets Held for Sale) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($) property parcel	Dec. 31, 2015 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties held for sale | property	8	7
Assets held for sale	$ 100,324	$ 54,961
Liabilities held for sale	61,202	1,462
Impairment of real estate	$ 37,500	$ 35,200	$ 42,200
Triple-Net Leased Properties
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties held for sale | property	0	0
Assets held for sale	$ 0	$ 0
Liabilities held for sale	$ 0	$ 0
Office Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties held for sale | property	8	7
Assets held for sale	$ 100,324	$ 53,151
Liabilities held for sale	$ 61,202	$ 1,462
Senior Living Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties held for sale | property	0	0
Assets held for sale	$ 0	$ 1,810
Liabilities held for sale	$ 0	$ 0
Seniors Housing Communities And Hospitals
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of land parcel classified as held for sale | parcel		1