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Fair Value Measurements (Narrative) (Details) - USD ($)
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High $ 1.00  
Marketable Securities, Gain (Loss) $ 11,000,000 $ 5,000,000
Corporate and asset-backed debt securities | Standard & Poor's, A Rating    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Percentage Of Company's Investments With Credit Quality Rating Less Than Single A And A2 1.00%  
Corporate and asset-backed debt securities | Moody's, A Rating    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Percentage Of Company's Investments With Credit Quality Rating Less Than Single A And A2 1.00%  
Corporate and asset-backed debt securities | Fitch, A Rating    
Fair Value, Option, Qualitative Disclosures Related to Election [Line Items]    
Percentage Of Company's Investments With Credit Quality Rating Less Than Single A And A2 1.00%