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Fair Value Measurements - Summary of Investment Income, Realized Gains and Disbursements Related to Investments (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Sep. 30, 2017
Fair Value Disclosures [Abstract]          
Investment income $ 2 $ 2 $ 6 [1] $ 7 $ 9 [1]
Realized gains 0 0 4 2 2
Disbursements $ (7) $ (7) $ (25) $ (19) $ (27)
[1] Investment income for the demand deposit includes interest income as well as dividend income transferred from the equity and corporate bond mutual funds.