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Summary of Significant Accounting Policies Summary of Significant Accounting Policies (New Accounting Pronouncements And Changes In Accounting Principle) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jan. 01, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Increase to net cash provided by operating activities $ 459,898 $ 434,220 $ 407,516  
Corresponding offset to net cash used for financing activities 253,633 328,672 305,861  
Accounting Standards Update 2016-09 [Member]        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Increase to net cash provided by operating activities   8,984 16,056  
Corresponding offset to net cash used for financing activities   (8,984) (16,056)  
Increase to deferred tax assets       $ 913
Accounting Standards Update 2014-09 [Member]        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Research services by SIDCO recorded as expense 14,623 18,409 20,333  
Additional Paid-in Capital [Member]        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Net cumulative-effect adjustment (2,582) 0 0  
Additional Paid-in Capital [Member] | Accounting Standards Update 2016-09 [Member]        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Net cumulative-effect adjustment       (2,582)
Retained Earnings [Member]        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Net cumulative-effect adjustment $ 1,669 $ 0 $ 0  
Retained Earnings [Member] | Accounting Standards Update 2016-09 [Member]        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Net cumulative-effect adjustment       $ 1,669