Recently Issued Accounting Literature (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change				$ 14,519
Increase to retained earnings	$ (4,198,088)			$ (4,183,253)
Decrease in fair value of marketable securities	(32,986)	$ 0
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase to retained earnings			$ 111,225
Decrease in fair value of marketable securities	(32,986)
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Capitalized leasing costs	$ 1,348	$ 974
Retained Earnings | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of accounting change			$ 14,519