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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.