Summary of Significant Accounting Policies (Narrative) (Details)	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($) property	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) property	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) property segment	Dec. 31, 2014 USD ($) property	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Period in which short-term investment become cash equivalents									90 days
Number of real estate properties | property	139								139
Impairment of investments in real estate									$ 0	$ 126,470,000	$ 0
Interest capitalized									12,900,000	20,400,000	26,300,000
Compensation costs, leasing and construction activities									56,800,000	50,100,000	38,400,000
Cash flows from capitalized leasing costs									49,900,000	49,000,000	57,500,000
Deferred leasing costs, net	$ 236,000,000				$ 229,300,000				236,000,000	229,300,000
Gain (loss) on cash flow hedge ineffectiveness									(1,600,000)	800,000	0
Assets or liabilities for uncertain tax positions	0				0				0	0
Interest or penalties from uncertain tax positions									0	0	0
Asset retirement obligations	1,300,000				1,700,000				$ 1,300,000	1,700,000
Number of reportable segments | segment									1
Operating revenue	500,443,000	$ 435,989,000	$ 420,295,000	$ 406,609,000	412,216,000	$ 412,186,000	$ 401,446,000	$ 390,590,000	$ 1,763,336,000	1,616,438,000	1,482,259,000
Rental property operating and maintenance									549,885,000	503,140,000	456,596,000
Other expense									60,943,000	3,070,000	827,000
Cash and cash equivalents	57,053,000				34,814,000				57,053,000	34,814,000	50,080,000	$ 52,467,000
Restricted cash	18,009,000				18,062,000				18,009,000	18,062,000
Net cash provided by (used in) financing activities									1,749,029,000	(26,974,000)	401,832,000
Reclassification
Summary Of Significant Accounting Policies [Line Items]
Rental property operating and maintenance										(400,000)	(800,000)
Other expense										400,000	800,000
Cash and cash equivalents					(6,500,000)					(6,500,000)
Restricted cash					6,500,000					6,500,000
Net cash provided by (used in) financing activities										(200,000)	(2,900,000)
United States
Summary Of Significant Accounting Policies [Line Items]
Operating revenue									1,400,000,000	1,200,000,000	1,100,000,000
Long-lived assets	6,000,000,000				5,400,000,000				6,000,000,000	5,400,000,000	5,600,000,000
Non-US
Summary Of Significant Accounting Policies [Line Items]
Operating revenue									400,000,000	400,000,000	300,000,000
Long-lived assets	2,600,000,000				2,700,000,000				2,600,000,000	2,700,000,000	2,700,000,000
United Kingdom
Summary Of Significant Accounting Policies [Line Items]
Operating revenue									200,000,000	200,000,000	200,000,000
Long-lived assets	$ 1,600,000,000				$ 1,700,000,000				$ 1,600,000,000	$ 1,700,000,000	$ 1,800,000,000
Sales | United Kingdom
Summary Of Significant Accounting Policies [Line Items]
Concentration risk									12.30%	13.30%	13.30%
Long-Lived Assets | United Kingdom
Summary Of Significant Accounting Policies [Line Items]
Concentration risk									18.80%	21.30%	21.10%
Midwest and Northeast Regions
Summary Of Significant Accounting Policies [Line Items]
Number of real estate properties | property					5					5
Impairment of investments in real estate										$ 126,470,000
Buildings and improvements | Minimum
Summary Of Significant Accounting Policies [Line Items]
Building and improvements, useful life									5 years
Buildings and improvements | Maximum
Summary Of Significant Accounting Policies [Line Items]
Building and improvements, useful life									39 years
Furniture and Fixtures | Minimum
Summary Of Significant Accounting Policies [Line Items]
Building and improvements, useful life									3 years
Furniture and Fixtures | Maximum
Summary Of Significant Accounting Policies [Line Items]
Building and improvements, useful life									5 years
Machinery and equipment | Minimum
Summary Of Significant Accounting Policies [Line Items]
Building and improvements, useful life									7 years
Machinery and equipment | Maximum
Summary Of Significant Accounting Policies [Line Items]
Building and improvements, useful life									15 years