CONDENSED CONSOLIDATED STATEMENT OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net income	$ 955	$ 1,052	$ 614
Adjustments to reconcile net income to net cash provided by operating activities:
Provision (benefit) for loan losses	(51)	(86)	(168)
Depreciation and amortization (including amortization and accretion on investment securities)	225	244	262
(Gains) losses on securities and other, net	23	(53)	(28)
Losses on early extinguishment of debt	0	(4)	(58)
Share-based compensation	53	46	41
Deferred tax expense	245	339	450
Other	15	18	(7)
Net effect of changes in assets and liabilities:
(Increase) decrease in receivables from brokers, dealers and clearing organizations	(635)	418	(134)
Increase in margin receivables	(115)	(489)	(2,340)
(Increase) decrease in other assets	(204)	159	(49)
(Decrease) increase in payables to brokers, dealers and clearing organizations	(55)	(594)	559
Increase in customer payables	2,732	668	1,290
(Decrease) increase in other liabilities	(341)	(40)	34
Net cash provided by operating activities	2,847	1,686	582
Cash flows from investing activities:
Purchases of available-for-sale securities	(8,382)	(8,386)	(9,819)
Proceeds from sales of available-for-sale securities	10,735	7,423	1,645
Proceeds from maturities of and principal payments on available-for-sale securities	1,588	1,944	1,588
Purchases of held-to-maturity securities	(4,688)	(4,163)	(10,519)
Proceeds from maturities of and principal payments on held-to-maturity securities	5,307	2,395	2,556
Proceeds from sales of loans	80	30	40
Decrease in loans receivable	463	609	983
Capital expenditures for property and equipment	(149)	(112)	(102)
Net increase in securities purchased under agreements to resell	(200)	0	0
Proceeds from sale of real estate owned and repossessed assets	14	24	29
Acquisitions, net of cash acquired	(29)	(150)	0
Net cash flow from derivative contracts	(277)	221	66
Other	(34)	(25)	(43)
Net cash provided by (used in) investing activities	4,428	(190)	(13,576)
Cash flows from financing activities:
(Decrease) increase in deposits	(6,707)	1,781	11,060
Common stock dividends	(135)	(36)	0
Preferred stock dividends	(40)	(36)	(25)
Net (decrease) increase in advances from FHLB	0	(500)	500
Proceeds from issuance of senior notes	0	420	999
Payments on senior notes	0	0	(1,049)
Payments on trust preferred securities	0	(413)	0
Proceeds from issuance of preferred stock	0	0	300
Repurchases of common stock	(1,085)	(1,139)	(362)
Other	(23)	(32)	(36)
Net cash (used in) provided by financing activities	(7,990)	45	11,387
Decrease in cash, cash equivalents and segregated cash	(715)	1,541	(1,607)
Cash, cash equivalents and segregated cash, beginning of period	3,344	1,803	3,410
Cash, cash equivalents and segregated cash, end of period	2,629	3,344	1,803
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and equivalents, end of period	750	2,333	931
Segregated cash, end of period	1,879	1,011	872
Cash, cash equivalents and segregated cash, end of period	2,629	3,344	1,803
Supplemental disclosures:
Cash paid for interest	255	157	126
Cash paid for income taxes, net of refunds	112	11	8
Right-of-use assets recognized upon adoption of new lease standard	193	0	0
Right-of-use assets obtained during the period	43	0	0
Non-cash investing and financing activities:
Transfers of loans held-for-investment to loans held-for-sale	78	0	57
Transfers from loans to other real estate owned and repossessed assets	17	15	27
Conversion of convertible debentures to common stock	0	0	3
Transfer of available-for-sale securities to held-to-maturity securities	744	1,161	0
Transfer of held-to-maturity securities to available-for-sale securities	$ 0	$ 4,700	$ 0