Dispositions (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($) property	Dec. 31, 2014 USD ($) property	Dec. 31, 2013 USD ($) property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Investments in real estate partnerships (note 4)		$ 306,206	$ 333,167
Proceeds from sale of real estate investments		108,822	118,787	$ 212,632
Gain (Loss) on Sale of Properties, Net of Applicable Income Taxes		35,606	55,077	59,656
Revenues included in Discontinued Operations	$ 140,399
Disposal Group, Including Discontinued Operation, Revenue				14,924
Disposal Group, Including Discontinued Operation, Operating Expense				7,592
Parent Company [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on sale of operating properties, net of tax		0	0	57,953
Investments in real estate partnerships (note 4)		306,206	333,167
Proceeds from sale of real estate investments		108,822	118,787	212,632
Gain (Loss) on Sale of Properties, Net of Applicable Income Taxes		35,606	55,077	1,703
Provision for impairment		0	1,257	6,000
Revenues included in Discontinued Operations		569,763	537,898	489,007
Operating Expenses		365,098	353,348	324,687
Operating income		$ 0	$ 0	$ 7,332
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		5	11	12
Land [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Real Estate Properties Sold | property		2	6	10