Revenue Recognition and Deferred Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting, Revenue Reconciling Item [Line Items]
Service Charges and Fees on Deposit Accounts	$ 41	$ 42	$ 81	$ 84
Other service charges, commissions, and fees	58	55	113	110
Wealth Management and Trust Income	13	14	26	25
Capital Market and Foreign Exchange Gain (Loss)	9	7	16	15
Noninterest income, Customer-related Fees	130	125	250	247
Noninterest Income	132	138	264	276
Net interest income	569	548	1,145	1,090
Gains (losses) from fair value changes	(9)	(1)	(9)	(6)
Consolidated Entities [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Service Charges and Fees on Deposit Accounts	41	42	81	84
Other service charges, commissions, and fees	46	46	88	89
Wealth Management and Trust Income	13	13	26	25
Capital Market and Foreign Exchange Gain (Loss)	2	3	5	6
Other Noninterest Income From Contracts With Customers			1	1
Total non-interest income from contracts with customers (ASC 606)	102	104	201	205
Other non-interest income (Non-ASC 606 customer related)	28	21	49	42
Noninterest income, Customer-related Fees	130	125	250	247
Other non-interest income (non-customer related)	2	13	14	29
Noninterest Income	132	138	264	276
Gains (Losses) on Sales of Other Real Estate	0	0	2	1
Net interest income	569	548	1,145	1,090
Revenues	701	686	1,411	1,367
Zions Bank [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Service Charges and Fees on Deposit Accounts	14	15	27	29
Other service charges, commissions, and fees	18	18	36	35
Wealth Management and Trust Income	4	4	8	7
Capital Market and Foreign Exchange Gain (Loss)	2	1	3	3
Other Noninterest Income From Contracts With Customers			0	0
Total non-interest income from contracts with customers (ASC 606)	38	38	74	74
Other non-interest income (Non-ASC 606 customer related)	1	0	(2)	(1)
Noninterest income, Customer-related Fees	39	38	72	73
Other non-interest income (non-customer related)	0	0	0	0
Noninterest Income	39	38	72	73
Gains (Losses) on Sales of Other Real Estate	0	0	1	0
Net interest income	179	176	358	342
Revenues	218	214	431	415
Amegy Corporation [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Service Charges and Fees on Deposit Accounts	11	11	22	22
Other service charges, commissions, and fees	9	9	18	19
Wealth Management and Trust Income	3	3	5	5
Capital Market and Foreign Exchange Gain (Loss)	(2)	(1)	(3)	(3)
Other Noninterest Income From Contracts With Customers			0	0
Total non-interest income from contracts with customers (ASC 606)	21	22	42	43
Other non-interest income (Non-ASC 606 customer related)	13	9	26	21
Noninterest income, Customer-related Fees	34	31	68	64
Other non-interest income (non-customer related)	0	0	0	0
Noninterest Income	34	31	68	64
Gains (Losses) on Sales of Other Real Estate	0	0	0	0
Net interest income	132	127	263	254
Revenues	166	158	331	318
California Bank Trust [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Service Charges and Fees on Deposit Accounts	7	7	14	14
Other service charges, commissions, and fees	7	6	13	12
Wealth Management and Trust Income	1	1	2	2
Capital Market and Foreign Exchange Gain (Loss)	1	1	2	2
Other Noninterest Income From Contracts With Customers			0	0
Total non-interest income from contracts with customers (ASC 606)	16	15	31	30
Other non-interest income (Non-ASC 606 customer related)	6	4	10	8
Noninterest income, Customer-related Fees	22	19	41	38
Other non-interest income (non-customer related)	0	0	0	2
Noninterest Income	22	19	41	40
Gains (Losses) on Sales of Other Real Estate	0	0	0	0
Net interest income	140	131	278	262
Revenues	162	150	319	302
National Bank Of Arizona [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Service Charges and Fees on Deposit Accounts	3	3	6	6
Other service charges, commissions, and fees	3	3	6	6
Wealth Management and Trust Income	0	0	1	1
Capital Market and Foreign Exchange Gain (Loss)	0	0	0	0
Other Noninterest Income From Contracts With Customers			0	0
Total non-interest income from contracts with customers (ASC 606)	6	6	13	13
Other non-interest income (Non-ASC 606 customer related)	4	3	6	4
Noninterest income, Customer-related Fees	10	9	19	17
Other non-interest income (non-customer related)	1	1	2	2
Noninterest Income	11	10	21	19
Gains (Losses) on Sales of Other Real Estate	0	0	0	0
Net interest income	62	58	122	111
Revenues	73	68	143	130
Nevada State Bank [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Service Charges and Fees on Deposit Accounts	4	4	7	8
Other service charges, commissions, and fees	3	3	6	6
Wealth Management and Trust Income	1	1	2	2
Capital Market and Foreign Exchange Gain (Loss)	0	0	1	0
Other Noninterest Income From Contracts With Customers			0	0
Total non-interest income from contracts with customers (ASC 606)	8	8	16	16
Other non-interest income (Non-ASC 606 customer related)	3	2	5	4
Noninterest income, Customer-related Fees	11	10	21	20
Other non-interest income (non-customer related)	0	0	0	0
Noninterest Income	11	10	21	20
Gains (Losses) on Sales of Other Real Estate	0	0	0	0
Net interest income	40	38	80	73
Revenues	51	48	101	93
Vectra Bank Colorado [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Service Charges and Fees on Deposit Accounts	2	2	4	4
Other service charges, commissions, and fees	2	2	3	3
Wealth Management and Trust Income	0	0	1	1
Capital Market and Foreign Exchange Gain (Loss)	0	0	0	0
Other Noninterest Income From Contracts With Customers			0	0
Total non-interest income from contracts with customers (ASC 606)	4	4	8	8
Other non-interest income (Non-ASC 606 customer related)	2	2	4	4
Noninterest income, Customer-related Fees	6	6	12	12
Other non-interest income (non-customer related)	0	0	0	0
Noninterest Income	6	6	12	12
Gains (Losses) on Sales of Other Real Estate	0	0	0	0
Net interest income	36	34	72	66
Revenues	42	40	84	78
Commerce Bank Of Washington [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Service Charges and Fees on Deposit Accounts	0	0	1	1
Other service charges, commissions, and fees	1	1	1	1
Wealth Management and Trust Income	0	0	0	0
Capital Market and Foreign Exchange Gain (Loss)	0	0	0	0
Other Noninterest Income From Contracts With Customers			0	0
Total non-interest income from contracts with customers (ASC 606)	1	1	2	2
Other non-interest income (Non-ASC 606 customer related)	0	0	0	1
Noninterest income, Customer-related Fees	1	1	2	3
Other non-interest income (non-customer related)	0	0	0	0
Noninterest Income	1	1	2	3
Gains (Losses) on Sales of Other Real Estate	0	0	0	0
Net interest income	14	12	28	24
Revenues	15	13	30	27
Other Segment [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Service Charges and Fees on Deposit Accounts	0	0	0	0
Other service charges, commissions, and fees	3	4	5	7
Wealth Management and Trust Income	4	4	7	7
Capital Market and Foreign Exchange Gain (Loss)	1	2	2	4
Other Noninterest Income From Contracts With Customers			1	1
Total non-interest income from contracts with customers (ASC 606)	8	10	15	19
Other non-interest income (Non-ASC 606 customer related)	(1)	1	0	1
Noninterest income, Customer-related Fees	7	11	15	20
Other non-interest income (non-customer related)	1	12	12	25
Noninterest Income	8	23	27	45
Gains (Losses) on Sales of Other Real Estate	0	0	1	1
Net interest income	(34)	(28)	(56)	(42)
Revenues	$ 26	$ (5)	$ (28)	$ 4