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Fair Value Measurements (Narrative) (Details) - USD ($)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Marketable securities, gain (loss) $ 29,000,000 $ 14,000,000 $ 28,000,000
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High $ 1.00    
Corporate and Asset-Backed | Standard & Poor's, A Rating [Member] | Fitch, A Rating [Member] | Moody's, A2 Rating [Member]      
Percent of investments with credit quality ratings less than single A and A2 1.00%    
Available-for-sale Securities [Member]      
Interest Receivable $ 1,000,000 $ 2,000,000