Summary of Significant Accounting Policies (Narrative) (Details)	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Mar. 31, 2018	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) segment	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Period in which short-term investment become cash equivalents				90 days
Interest capitalized	$ 8,200,000		$ 3,800,000	$ 15,500,000	$ 8,400,000
Compensation costs, leasing and construction activities	17,400,000		18,900,000	35,700,000	37,600,000
Cash flows from capitalized leasing costs				5,500,000	6,100,000
Impact of Change in Foreign Exchange Rates				(11,270,000)
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	199,400,000			199,400,000
Gain on sale of property				$ 53,700,000
Number of reportable segments | segment				1
Total operating revenues	754,919,000		$ 565,989,000	$ 1,499,287,000	$ 1,116,558,000
Net investments in real estate	$ 14,134,486,000			$ 14,134,486,000		$ 13,841,186,000
Sales | Geographic Concentration Risk
Summary Of Significant Accounting Policies [Line Items]
Concentration risk	9.80%		11.90%	10.00%	11.90%
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				11.90%
Outside the United States
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	$ 140,800,000		$ 126,900,000	$ 281,700,000	$ 248,300,000
Net investments in real estate	3,200,000,000			3,200,000,000		$ 3,100,000,000
United States
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	614,100,000		439,100,000	1,200,000,000	900,000,000
Net investments in real estate	10,700,000,000			10,700,000,000		10,500,000,000
United Kingdom
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	74,000,000		$ 67,500,000	149,200,000	$ 132,700,000
Net investments in real estate	$ 1,700,000,000			$ 1,700,000,000		$ 1,700,000,000