Quarterly report pursuant to Section 13 or 15(d)

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

v3.8.0.1
Short-term and Long-term Investments, Restricted Deposits (Narrative) (Details)
9 Months Ended
Sep. 30, 2017
position
Schedule Of Investments And Restricted Deposits By Type [Line Items]  
Positions from which gross unrealized losses were generated 1,201
Total unrealized investment positions 3,192
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) 4 years 1 month 6 days
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 95.00%