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Fair Value Measurements - Summary of Restricted Investment Securities Portfolio (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Sep. 30, 2018
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |  |
Adjusted cost | [1] | $ 288 | $ 297
Total, fair value |  | 309 | 342
Corporate bond mutual fund [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Adjusted cost |  | 120 | 120
Unrealized gain |  | 0 | 0
Unrealized loss |  | (6) | (5)
Total, fair value |  | 114 | 115
Equity mutual fund [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Adjusted cost |  | 148 | 148
Unrealized gain |  | 30 | 59
Unrealized loss |  | 0 | 0
Total, fair value |  | 178 | 207
Demand Deposits [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Demand deposit |  | 17 | 20
Available-for-sale Securities [Member] |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |
Adjusted cost |  | 285 | 288
Unrealized gain |  | 30 | 59
Unrealized loss |  | (6) | (5)
Total, fair value |  | $ 309 | $ 342
[1] | The accumulated adjustments include investment income, realized gains, disbursements and settlements recorded in previous periods.

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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