Fair Value Measurements - Summary of Investment Portfolio (Details) - USD ($) $ in Millions		3 Months Ended			12 Months Ended
		Dec. 31, 2019		Dec. 31, 2018	Sep. 30, 2019
Schedule of Available-for-sale Securities [Line Items]
Original cost		$ 335			$ 335
Accumulated adjustments, net	[1]	(30)			(47)
Adjusted cost		305			288
Investment income		4	[2]	$ 3	10	[2]
Net unrealized gain	[3]	38			29
Realized gains	[3]	0			32
Settlement funds		1			7
Disbursements		(10)		$ (8)	(32)
Fair value		338			334
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		300			305
Fair value		338			334
Gross Unrealized Gain		38			31
Gross Unrealized Loss		0			(2)
Corporate Bond Mutual Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		172			176
Fair value		177			183
Gross Unrealized Gain		5			7
Gross Unrealized Loss		0			0
Equity Mutual Fund [Member]
Schedule of Available-for-sale Securities [Line Items]
Adjusted cost		120			120
Fair value		153			142
Gross Unrealized Gain		33			24
Gross Unrealized Loss		0			(2)
Demand Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Demand deposit		$ 8			$ 9

[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.