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Fair Value Measurements - Summary of Investment Portfolio (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2018
Fair Value Disclosures [Abstract] |  |  |  |  |  |
Original cost |  | $ 335 |  |  | $ 335 |
Accumulated adjustments, net |  | (47) |  |  | (38) |
Adjusted cost | [1] | 288 |  |  | 297 |
Investment income |  | 3 | [2] | $ 2 | 8 | [2]
Net unrealized gain |  | 24 |  |  | 54 |
Realized gains |  | 0 |  |  | 6 |
Settlement funds |  | 2 |  |  | 10 |
Disbursements |  | (8) |  | $ (5) | (33) |
Fair value |  | $ 309 |  |  | $ 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.

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- Definition Amortized cost of 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://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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