CONSOLIDATED STATEMENT OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities:
Net income	$ 1,052	$ 614	$ 552
Adjustments to reconcile net income to net cash provided by operating activities:
Provision (benefit) for loan losses	(86)	(168)	(149)
Depreciation and amortization (including amortization and accretion on investment securities)	244	262	239
Gains on securities and other, net	(53)	(28)	(42)
Losses on early extinguishment of debt	4	58	0
Share-based compensation	46	41	30
Deferred tax expense	339	450	275
Other	18	(7)	(5)
Net effect of changes in assets and liabilities:
Decrease (increase) in receivables from brokers, dealers and clearing organizations	418	(134)	(528)
(Increase) decrease in margin receivables	(489)	(2,340)	667
Decrease (increase) in other assets	159	(49)	(3)
(Decrease) increase in payables to brokers, dealers and clearing organizations	(594)	559	(593)
Increase in customer payables	668	1,290	1,615
(Decrease) increase in other liabilities	(40)	34	(14)
Net cash provided by operating activities	1,686	582	2,044
Cash flows from investing activities:
Purchases of available-for-sale securities	(8,386)	(9,819)	(6,705)
Proceeds from sales of available-for-sale securities	7,423	1,645	3,194
Proceeds from maturities of and principal payments on available-for-sale securities	1,944	1,588	1,540
Purchases of held-to-maturity securities	(4,163)	(10,519)	(4,389)
Proceeds from maturities of and principal payments on held-to-maturity securities	2,395	2,556	2,068
Proceeds from sales of loans	30	40	0
Decrease in loans receivable	609	983	1,176
Capital expenditures for property and equipment	(112)	(102)	(75)
Proceeds from sale of real estate owned and repossessed assets	24	29	20
Acquisitions, net of cash acquired	(150)	0	(723)
Net cash flow from derivative contracts	221	66	(109)
Other	(25)	(43)	4
Net cash used in investing activities	(190)	(13,576)	(3,999)
Cash flows from financing activities:
Increase in deposits	1,781	11,060	2,237
Common stock dividends	(36)	0	0
Preferred stock dividends	(36)	(25)	0
Net decrease in securities sold under agreements to repurchase	0	0	(82)
Net (decrease) increase in advances from FHLB	(500)	500	0
Proceeds from issuance of senior notes	420	999	0
Payments on senior notes	0	(1,049)	0
Payments on trust preferred securities	(413)	0	0
Proceeds from issuance of preferred stock	0	300	400
Repurchases of common stock	(1,139)	(362)	(452)
Other	(32)	(36)	(28)
Net cash provided by financing activities	45	11,387	2,075
Increase (decrease) in cash, cash equivalents and segregated cash	1,541	(1,607)	120
Cash, cash equivalents and segregated cash, beginning of period	1,803	3,410	3,290
Cash, cash equivalents and segregated cash, end of period	3,344	1,803	3,410
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and equivalents, end of period	2,333	931	1,950
Segregated cash, end of period	1,011	872	1,460
Cash, cash equivalents and segregated cash, end of period	3,344	1,803	3,410
Supplemental disclosures:
Cash paid for interest	157	126	77
Cash paid for income taxes, net of refunds	11	8	6
Non-cash investing and financing activities:
Transfers of loans held-for-investment to loans held-for-sale	0	57	0
Transfers from loans to other real estate owned and repossessed assets	15	27	34
Conversion of convertible debentures to common stock	0	3	5
Transfer of available-for-sale securities to held-to-maturity securities	1,161	0	492
Transfer of held-to-maturity securities to available-for-sale securities	$ 4,672	$ 0	$ 0