Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Maximum exposure to losses	$ 22,600
Useful life of property	40 years
Depreciation expense from operation	$ 161,200	$ 130,800	$ 113,500
Real estate cost capitalized excluding interest cost	18,400	17,800	17,400
Real estate interest cost capitalized incurred	30,200	33,200	26,800
(Gain) loss on sale of real estate, net	0	14,769	0
Gain (loss) on disposition of property	$ 0	14,769	(6,994)
Term Loan Maturing 2025
Summary Of Significant Accounting Policies [Line Items]
Interest rate	4.75%
Senior Notes 4.75 Due 2025
Summary Of Significant Accounting Policies [Line Items]
Interest rate	3.875%
Real Estate Assets
Summary Of Significant Accounting Policies [Line Items]
Depreciation expense from operation	$ 147,800	118,900	101,200
Non-Real Estate Assets
Summary Of Significant Accounting Policies [Line Items]
Depreciation expense from operation	$ 13,400	11,900	12,300
Minimum | Real Property
Summary Of Significant Accounting Policies [Line Items]
Useful life of property	20 years
Maximum | Real Property
Summary Of Significant Accounting Policies [Line Items]
Useful life of property	40 years
Maximum | Leasehold Improvements
Summary Of Significant Accounting Policies [Line Items]
Useful life of property	20 years
Restructuring Charges
Summary Of Significant Accounting Policies [Line Items]
Gain (loss) on disposition of property			$ 7,000
Atlanta Metro facility
Summary Of Significant Accounting Policies [Line Items]
(Gain) loss on sale of real estate, net		1,400
Owned Properties | Manassas, Virginia
Summary Of Significant Accounting Policies [Line Items]
(Gain) loss on sale of real estate, net		13,400
Transaction cost		$ 5,800