Summary Of Significant Accounting Policies (Summary Of Cash And Cash Equivalents) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Cash accounts	$ 568,587	$ 488,504
Money market funds	312,914	333,975
Cash and cash equivalents	$ 881,501	$ 822,479	$ 784,168	$ 788,064
Cash accounts, % of Total	64.50%	59.40%
Money market funds, % of Total	35.50%	40.60%