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Operating Segments, Geographic Information and Significant Customers (Schedule of Revenue by Major Customers) (Details)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Humana      
Revenue, Major Customer [Line Items]      
Major customer revenue, percentage 22.00% 20.00% 22.00%
UnitedHealthcare      
Revenue, Major Customer [Line Items]      
Major customer revenue, percentage [1] 19.00% 23.00% 19.00%
Aetna      
Revenue, Major Customer [Line Items]      
Major customer revenue, percentage [2] 14.00% 10.00% 11.00%
[1] UnitedHealthcare also includes other carriers owned by UnitedHealthcare.
[2] Aetna includes other carriers owned by Aetna.