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Operating Segments, Geographic Information and Significant Customers (Schedule of Revenue by Major Customers) (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Humana      
Revenue, Major Customer [Line Items]      
Major customer revenue, percentage 20.00% 22.00% 23.00%
UnitedHealthcare      
Revenue, Major Customer [Line Items]      
Major customer revenue, percentage 23.00% 19.00% 12.00%
Aetna      
Revenue, Major Customer [Line Items]      
Major customer revenue, percentage 10.00% 11.00% 11.00%