Segment Reporting - Segment Result and Reconciliations to GAAP - Core Earnings Adjustments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting, Revenue Reconciling Item [Line Items]
Net interest income (loss) after provisions for loan losses	$ 322	$ 248	$ 756	$ 683
Total other income (loss)	177	196	262	637
Goodwill and acquired intangible asset impairment and amortization	5	6	16	23
Income tax expense (benefit)	52	40	71	119
Net income	207	145	227	425
Total Adjustments [Member] | Adjustments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Net interest income (loss) after provisions for loan losses	15	3	(38)	(15)
Total other income (loss)	(2)	4	(255)	(7)
Goodwill and acquired intangible asset impairment and amortization	5	6	16	23
Total Core Earnings adjustments to GAAP	8	1	(309)	(45)
Income tax expense (benefit)	(7)	(2)	(72)	(16)
Net income	15	3	(237)	(29)
Net Impact of Goodwill and Acquired Intangible Assets [Member] | Total Adjustments [Member] | Adjustments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Goodwill and acquired intangible asset impairment and amortization	5	6	16	23
Total Core Earnings adjustments to GAAP	(5)	(6)	(16)	(23)
Net Impact of Derivative Accounting [Member] | Total Adjustments [Member] | Adjustments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Net interest income (loss) after provisions for loan losses	15	3	(38)	(15)
Total other income (loss)	(2)	4	(255)	(7)
Total Core Earnings adjustments to GAAP	$ 13	$ 7	$ (293)	$ (22)