Dispositions (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) property	Dec. 31, 2015 USD ($) property	Dec. 31, 2014 USD ($) property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Document Fiscal Year Focus		2016
Investments in real estate partnerships (note 4)		$ 296,699	$ 306,206
Proceeds from sale of real estate investments		137,479	108,822	$ 118,787
Gain (Loss) on Sale of Properties, Net of Applicable Income Taxes		47,321	35,606	55,077
Provision for impairment		1,700
Revenues included in Discontinued Operations	$ 149,628
Parent Company [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Investments in real estate partnerships (note 4)		296,699	306,206
Proceeds from sale of real estate investments		135,269	108,822	118,787
Gain (Loss) on Sale of Properties, Net of Applicable Income Taxes		47,321	35,606	55,077
Provision for impairment		4,200	0	1,257
Revenues included in Discontinued Operations		614,371	569,763	537,898
Operating Expenses		$ 403,152	$ 365,098	$ 353,348
Operating Segments [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties Sold | property		11	5	11
Land [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties Sold | property		16	2	6