Investments Available-for-Sale - Schedule of Contractual Maturities of Available-for-Sale Securities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019
Fair Value
Fair Value	$ 1,636	$ 1,668
US Treasury Securities [Member]
Amortized Cost
Due within one to five years	582
Due within five to ten years	618
Due after ten years	391
Amortized Cost	1,591	1,591
Fair Value
Due within one to five years	612
Due within five to ten years	626
Due after ten years	398
Fair Value	$ 1,636	$ 1,668

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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