Summary of Significant Accounting Policies (Narrative) (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2017 USD ($) segment	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Period in which short-term investment become cash equivalents	90 days
Interest capitalized	$ 4,600,000	$ 3,800,000
Compensation costs, leasing and construction activities	18,700,000	17,700,000
Cash flows from capitalized leasing costs	12,300,000	10,400,000
Goodwill	$ 757,444,000		$ 752,970,000
Award vesting period	4 years
Unrecognized tax benefits	$ 0		$ 0
Income tax penalties and interest expense	$ 0	0
Number of reportable segments | segment	1
Total operating revenues	$ 550,569,000	$ 504,199,000
Sales
Summary Of Significant Accounting Policies [Line Items]
Concentration risk	11.80%	10.20%
Long-Lived Assets
Summary Of Significant Accounting Policies [Line Items]
Concentration risk	16.50%		16.60%
United States
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	$ 429,100,000	$ 406,300,000
Long-lived assets	6,300,000,000		$ 6,300,000,000
Outside The United States
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	121,400,000	97,900,000
Long-lived assets	2,700,000,000		2,600,000,000
United Kingdom
Summary Of Significant Accounting Policies [Line Items]
Total operating revenues	65,200,000	$ 51,600,000
Long-lived assets	$ 1,500,000,000		$ 1,500,000,000