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Restricted Investments - Maturities of Held-to-Maturity (Details)
$ in Millions
Dec. 31, 2016
USD ($)
Contractual maturities of our held-to-maturity restricted investments  
Amortized Cost, Due in one year or less $ 100
Amortized Cost, Due one year through five years 10
Amortized Cost, Total 110
Estimated Fair Value, Due in one year or less 100
Estimated Fair Value, Due one year through five years 10
Estimated Fair Value, Total $ 110