Recently Issued Accounting Literature (Narrative) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change						$ 14,519,000
Earnings less than distributions	$ (4,206,381,000)		$ (4,206,381,000)			$ (4,183,253,000)
Increase (decrease) in fair value of marketable securities		$ 0	(17,102,000)	$ 0
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Earnings less than distributions					$ (111,225,000)
Increase (decrease) in fair value of marketable securities	15,884,000		(17,102,000)
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Capitalized leasing costs	$ 1,358,000	$ 1,241,000	$ 2,706,000	$ 2,214,000
Retained Earnings | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change					$ 14,519,000