Acquisitions, Dispositions, Impairment, and Discontinued Operations (Narrative) (Details)	1 Months Ended				3 Months Ended				12 Months Ended
	Apr. 30, 2016 USD ($)	Feb. 29, 2016 USD ($) a	Mar. 31, 2015 USD ($)	Jul. 31, 2014 USD ($) a	Sep. 30, 2016 USD ($) a	Jun. 30, 2016 USD ($)	Dec. 31, 2015 USD ($) a	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) a	Dec. 31, 2015 USD ($) a	Dec. 31, 2014 USD ($) a	Apr. 26, 2016 a
Property, Plant and Equipment [Line Items]
Number of joint ventures accounted for under equity method investments									3		2
Equity Method Investment, Ownership Percentage							31.30%			31.30%	20.00%
Area of Land | a					2.4		58.1		4.6	58.1
Proceeds from disposition included in restricted cash						$ 13,000,000
Purchase price of land acquired									$ 19,900,000	$ 59,100,000
Impairment associated with land holdings									0	0	$ 1,152,000
Gain (Loss) on Sale of Properties					$ 262,719,000	$ 32,235,000	$ 18,870,000	$ 85,192,000
Noncontrolling Interest, Increase from Business Combination									0	9,500,000	0
Gain on sale of discontinued operations, net of tax									375,237,000	0	$ 0
Corporate Joint Venture [Member]
Property, Plant and Equipment [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired			0.01%					0.01%
Number of Businesses Acquired			2
Number of Units in Real Estate Property			798					798
Noncontrolling Interest, Increase from Business Combination			$ 9,500,000
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Property, Plant and Equipment [Line Items]
Area of Land | a		6.3		2.4							26.9
Proceeds from Sale of Land Held-for-investment		$ 2,200,000		$ 800,000						1,100,000	$ 22,900,000
Gains (Losses) on Sales of Investment Real Estate									$ 400,000	$ 300,000	$ 3,600,000
Number of Real Estate Properties			2		5	1	3	2	6	3	5
Number of Units in Real Estate Property							1,376		3,184	1,376	1,847
Proceeds from Sale of Property, Plant, and Equipment									$ 523,400,000	$ 147,400,000	$ 218,300,000
Gain (Loss) on Sale of Properties									$ 294,900,000	$ 104,000,000	$ 155,700,000
Weighted average age of operating properties									24
Land										2
Dual-Phase Property Sale [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]
Property, Plant and Equipment [Line Items]
Number of Real Estate Properties					1				1
Discontinued Operations, Held-for-sale or Disposed of by Sale [Member]
Property, Plant and Equipment [Line Items]
Area of Land | a												19.6
Number of Real Estate Properties												15
Number of Units in Real Estate Property												4,918
Proceeds from Sale of Property, Plant, and Equipment	$ 630,000,000
Weighted average age of operating properties												23