Discontinued Operations and Dispositions of Real Estate - 2015 Dispositions (Details) $ in Thousands	1 Months Ended		12 Months Ended
	Mar. 31, 2017 USD ($) property	Jan. 31, 2017 USD ($) facility	Dec. 31, 2017 USD ($) parcel facility	Dec. 31, 2016 USD ($) facility	Dec. 31, 2015 USD ($) parcel facility
Senior housing triple-net | Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties disposed	64		4	7	9
Total consideration for disposition of real estate	$ 1,125,000			$ 88,000	$ 60,000
Life science
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties disposed | facility		4
Total consideration for disposition of real estate		$ 76,000
Life science | Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties disposed | facility				5
Total consideration for disposition of real estate				$ 386,000	$ 51,000
Life science | Disposed of by Sale | Land
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties disposed | parcel			1,000		2
Total consideration for disposition of real estate			$ 27,000
Medical office | Disposed of by Sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties disposed | facility			4	3	1
Total consideration for disposition of real estate				$ 20,000	$ 400