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Fair Value Measurements - Summary of Investment Portfolio (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Sep. 30, 2018
Fair Value Disclosures [Abstract]          
Original cost $ 335   $ 335   $ 335
Accumulated adjustments, net (47)   (47)   (38)
Adjusted cost [1] 288   288   297
Investment income 2 $ 2 7 [2] $ 6 8 [2]
Net unrealized gain [3]     31   54
Realized gains [3]     26   6
Settlement funds     2   10
Disbursements (5) $ (7) (25) $ (25) (33)
Fair value $ 329   $ 329   $ 342
[1] The accumulated adjustments include investment income, realized gains, disbursements and settlements recorded in previous periods.
[2] Investment income for the demand deposit includes interest income as well as dividend income transferred from the equity and corporate bond mutual funds.
[3] Presented under the original cost method.