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Investments - Held to Maturity Investments (Details)
$ in Millions
Sep. 30, 2017
USD ($)
Amortized Cost  
Due in one year or less $ 100
Due after one year through five years 17
Held-to-maturity Securities 117
Estimated Fair Value  
Due in one year or less 100
Due after one year through five years 17
Estimated Fair Value, Total $ 117