Summary Of Significant Accounting Policies (Summary Of Cash And Cash Equivalents) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Accounting Policies [Abstract]
Cash accounts	$ 488,504	$ 427,244
Money market funds	333,975	356,924
Cash and cash equivalents	$ 822,479	$ 784,168	$ 788,064	$ 742,486
Cash accounts, % of Total	59.40%	54.50%
Money market funds, % of Total	40.60%	45.50%