Quarterly report pursuant to Section 13 or 15(d)

Short-term and Long-term Investments, Restricted Deposits - Narrative (Details)

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Short-term and Long-term Investments, Restricted Deposits - Narrative (Details)
3 Months Ended
Mar. 31, 2019
position
Schedule Of Investments And Restricted Deposits By Type [Line Items]  
Positions from which gross unrealized losses were generated 1,966
Total unrealized investment positions 4,339
Commercial mortgage-backed securities  
Schedule Of Investments And Restricted Deposits By Type [Line Items]  
Investments recorded at fair value that carry rating of AA Plus, weighted average (in years) 3 years 9 months
Rated Securities | External Credit Rating, Investment Grade  
Schedule Of Investments And Restricted Deposits By Type [Line Items]  
Percentage of investments in rated securities carry an investment grade rating by nationally recognized statistical rating organizations 97.00%