Equity and Noncontrolling Interests - Changes in AOCI (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	$ 1,797	$ 2,084	$ 1,535	$ 2,189
Aggregate adjustment for the period, net of tax	(46)	(203)	54	(129)
Stockholders' equity, ending balance	1,842	1,883	1,842	1,883
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	(582)	(504)	(665)	(569)
Stockholders' equity, ending balance	(619)	(690)	(619)	(690)
Accumulated Foreign Currency Adjustment Attributable to Parent
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	(579)	(508)	(634)	(558)
Aggregate adjustment for the period, net of tax	(26)	(124)	29	(74)
Stockholders' equity, ending balance	(605)	(632)	(605)	(632)
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	0	7	(28)	(8)
Current period changes in fair value, net of tax	(10)	(59)	15	(42)
Reclassification to income, net of tax	(1)	(3)	2	(5)
Stockholders' equity, ending balance	(11)	(55)	(11)	(55)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	(3)	(3)	(3)	(3)
Stockholders' equity, ending balance	$ (3)	$ (3)	$ (3)	$ (3)