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Income Taxes (Narrative) (Details) - USD ($)
 3 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Tax Credit Carryforward [Line Items]          
Transition tax     $ 43,379,000    
Transition tax for foreign earnings, provisional income tax expense     48,126,000    
Decrease in transition tax $ (4,747,000)        
Transition tax for accumulated foreign earnings, provisional liability, noncurrent   $ 30,263,000 44,366,000    
Change in tax rate   1,422,000 1,281,000 $ 111,000  
Transition tax for accumulated foreign earnings   (521,820,000) (1,281,000)    
Increase in income tax expense $ 141,000        
Foreign derived intangible income benefit   7,930,000 0 0  
Global intangible low-taxed income   5,955,000 0 0  
Transition tax for accumulated foreign earnings, amount subject to tax   1,084,000      
Deferred tax liabilities, withholding and dividend tax, foreign   2,225,000      
Undistributed earnings of foreign subsidiaries   930,993,000 1,266,000,000    
Penalties and interest expense   631,000 121,000 (163,000)  
Accrued penalties and interest   1,419,000 789,000    
Cash and cash equivalents   544,358,000 909,900,000 623,855,000 $ 582,532,000
Short-term investments   500,432,000 206,257,000    
Menara Networks          
Tax Credit Carryforward [Line Items]          
Operating loss carry-forwards   12,577,000 16,202,000 $ 22,242,000  
Operating loss carry-forwards, valuation allowance   0      
Her Majesty's Revenue and Customs (HMRC) | Foreign Tax Authority          
Tax Credit Carryforward [Line Items]          
Operating loss carry-forwards   2,888,000      
State          
Tax Credit Carryforward [Line Items]          
Tax credit carry-forwards   11,801,000 $ 10,294,000    
Tax credit carryforward, valuation allowance   7,439,000      
United States          
Tax Credit Carryforward [Line Items]          
Cash and cash equivalents   243,400,000      
Short-term investments   500,400,000      
Foreign locations          
Tax Credit Carryforward [Line Items]          
Cash and cash equivalents   $ 301,000,000.0