Accumulated Other Comprehensive Income (Loss) - Net Change (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Investments available-for-sale:
Unrealized gain (loss), before tax	$ (32)	$ 16
Unrealized gain (loss), tax effect	8	(4)
Unrealized gain (loss), net of tax	(24)	12
Cash flow hedging instruments:
Reclassification adjustment for portion of realized loss amortized to net income, before tax	1	1
Reclassification adjustment for portion of realized loss amortized to net income, tax effect	0	0
Reclassification adjustment for portion of realized loss amortized to net income, net of tax	1	1
Total other comprehensive income (loss), before tax	(31)	17
Income tax effect	8	(4)
Total other comprehensive income (loss), net of tax	$ (23)	$ 13