Summary of Significant Accounting Policies (Narrative) (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Period in which short-term investment become cash equivalents			90 days
Interest capitalized	$ 9,700,000	$ 5,300,000	$ 25,300,000	$ 13,700,000
Compensation costs, leasing and construction activities	19,500,000	20,800,000	55,100,000	58,400,000
Cash flows from capitalized leasing costs			6,400,000	11,100,000
Impact of Change in Foreign Exchange Rates			(16,253,000)
Unrecognized tax benefits	0		0		$ 0
Income tax penalties and interest expense	$ 0	0	$ 0	0
Revenue recognized from applying Topic 840	97.00%
Revenue recognized from applying Topic 606			3.00%
Gross proceeds	$ 291,700,000		$ 291,700,000
Gain on sale of property			$ 80,400,000
Number of reportable segments | segment			1
Total operating revenues	768,924,000	$ 609,925,000	$ 2,268,211,000	$ 1,726,483,000
Net investments in real estate	$ 14,225,697,000		$ 14,225,697,000		$ 13,841,186,000
Sales | Geographic Concentration Risk
Summary Of Significant Accounting Policies [Line Items]
Concentration risk	9.40%	11.50%	9.80%	11.70%
Total long-lived assets
Summary Of Significant Accounting Policies [Line Items]
Concentration risk					12.10%
Total long-lived assets | Geographic Concentration Risk
Summary Of Significant Accounting Policies [Line Items]
Concentration risk			11.60%
Outside the United States
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	$ 139,700,000	$ 131,700,000	$ 421,300,000	$ 380,000,000
Net investments in real estate	3,300,000,000		3,300,000,000		$ 3,100,000,000
United States
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	629,300,000	478,200,000	1,800,000,000	1,300,000,000
Net investments in real estate	10,800,000,000		10,800,000,000		10,500,000,000
United Kingdom
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	72,600,000	$ 70,100,000	221,900,000	$ 202,800,000
Net investments in real estate	$ 1,600,000,000		$ 1,600,000,000		$ 1,700,000,000