Summary of Significant Accounting Policies (Narrative) (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) segment	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Period in which short-term investment become cash equivalents	90 days
Interest capitalized	$ 7,400,000	$ 4,600,000
Compensation costs, leasing and construction activities	18,300,000	18,700,000
Cash flows from capitalized leasing costs	2,300,000	3,200,000
Impact of Change in Foreign Exchange Rates	15,515,000
Unrecognized tax benefits	0		$ 0
Income tax penalties and interest expense	$ 0	0
Revenue recognized from applying Topic 840	97.00%
Revenue recognized from applying Topic 606	3.00%
Gross proceeds	$ 139,300,000
Gain on sale of property	$ 39,400,000
Number of reportable segments | segment	1
Total operating revenues	$ 744,368,000	$ 550,569,000
Net investments in real estate	$ 14,089,926,000		$ 13,841,186,000
Sales | Geographic Concentration Risk
Summary Of Significant Accounting Policies [Line Items]
Concentration risk	10.10%	11.80%
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	12.80%
Outside the United States
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	$ 140,900,000	$ 121,400,000
Net investments in real estate	3,300,000,000		$ 3,100,000,000
United States
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	603,500,000	429,100,000
Net investments in real estate	10,600,000,000		10,500,000,000
United Kingdom
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	75,200,000	$ 65,200,000
Net investments in real estate	$ 1,800,000,000		$ 1,700,000,000