Revenue by Operation Segments (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Noninterest income, Customer-related Fees									$ 525	$ 508	$ 490
Noninterest Income	$ 152	$ 146	$ 132	$ 132	$ 140	$ 136	$ 138	$ 138	562	552	544
Net Interest Income	$ 559	$ 567	$ 569	$ 577	$ 576	$ 565	$ 548	$ 541	2,272	2,230	2,065
Commercial account fees									121	122	126
Card fees									92	94	96
Retail and business banking fees									78	78	78
Capital markets and foreign exchange gain (loss)									78	58	49
Wealth management and trust fees									60	55	47
Loan-related fees and income									75	74	72
Commerce Bank Of Washington [Member]
Total non-interest income from contracts with customers (ASC 606)									4	4
Other Noninterest Income, Loss (Non-ASC 606 Customer Related)									1	1
Noninterest income, Customer-related Fees									5	5	5
Noninterest Income, Other											0
Noninterest Income									5	5	5
Gains (Losses) on Sales of Other Real Estate									0	0	0
Net Interest Income									54	53	44
Revenues									59	58	49
Commercial account fees									2	2	2
Card fees									1	2	2
Retail and business banking fees									0	0	0
Capital markets and foreign exchange gain (loss)									0	0	1
Other non-interest income (non-customer related) Gain (Loss)									0	0
Wealth management and trust fees									1	0	0
Other Noninterest Income From Contracts With Customers									0	0	0
Loan-related fees and income											0
Other Segment [Member]
Total non-interest income from contracts with customers (ASC 606)									34	36
Other Noninterest Income, Loss (Non-ASC 606 Customer Related)									3	8
Noninterest income, Customer-related Fees									37	44	47
Noninterest Income, Other											50
Noninterest Income									70	85	97
Gains (Losses) on Sales of Other Real Estate									0	0	0
Net Interest Income									(21)	7	73
Revenues									49	92	170
Commercial account fees									(1)	0	0
Card fees									2	2	8
Retail and business banking fees									2	1	0
Capital markets and foreign exchange gain (loss)									4	3	8
Other non-interest income (non-customer related) Gain (Loss)									33	41
Wealth management and trust fees									18	14	14
Other Noninterest Income From Contracts With Customers									9	16	12
Loan-related fees and income											5
National Bank Of Arizona [Member]
Total non-interest income from contracts with customers (ASC 606)									29	30
Other Noninterest Income, Loss (Non-ASC 606 Customer Related)									12	9
Noninterest income, Customer-related Fees									41	39	39
Noninterest Income, Other											0
Noninterest Income									42	39	39
Gains (Losses) on Sales of Other Real Estate									0	0	0
Net Interest Income									227	218	191
Revenues									269	257	230
Commercial account fees									7	7	8
Card fees									11	10	11
Retail and business banking fees									8	9	9
Capital markets and foreign exchange gain (loss)									1	1	3
Other non-interest income (non-customer related) Gain (Loss)									1	0
Wealth management and trust fees									1	2	2
Other Noninterest Income From Contracts With Customers									1	1	2
Loan-related fees and income											4
Nevada State Bank [Member]
Total non-interest income from contracts with customers (ASC 606)									36	35
Other Noninterest Income, Loss (Non-ASC 606 Customer Related)									7	6
Noninterest income, Customer-related Fees									43	41	39
Noninterest Income, Other											0
Noninterest Income									43	41	39
Gains (Losses) on Sales of Other Real Estate									1	0	0
Net Interest Income									153	144	123
Revenues									197	185	162
Commercial account fees									8	9	9
Card fees									12	11	13
Retail and business banking fees									11	10	10
Capital markets and foreign exchange gain (loss)									1	1	1
Other non-interest income (non-customer related) Gain (Loss)									0	0
Wealth management and trust fees									3	3	3
Other Noninterest Income From Contracts With Customers									1	1	0
Loan-related fees and income											3
Vectra Bank Colorado [Member]
Total non-interest income from contracts with customers (ASC 606)									19	19
Other Noninterest Income, Loss (Non-ASC 606 Customer Related)									7	5
Noninterest income, Customer-related Fees									26	24	24
Noninterest Income, Other											1
Noninterest Income									26	24	25
Gains (Losses) on Sales of Other Real Estate									0	0	0
Net Interest Income									137	131	121
Revenues									163	155	146
Commercial account fees									6	6	6
Card fees									6	6	6
Retail and business banking fees									4	4	4
Capital markets and foreign exchange gain (loss)									1	1	2
Other non-interest income (non-customer related) Gain (Loss)									0	0
Wealth management and trust fees									2	2	1
Other Noninterest Income From Contracts With Customers									0	0	1
Loan-related fees and income											4
Zions Bank [Member]
Total non-interest income from contracts with customers (ASC 606)									142	141
Other Noninterest Income, Loss (Non-ASC 606 Customer Related)									7	8
Noninterest income, Customer-related Fees									149	149	146
Noninterest Income, Other											2
Noninterest Income									150	150	148
Gains (Losses) on Sales of Other Real Estate									3	1	0
Net Interest Income									703	673	605
Revenues									856	824	753
Commercial account fees									42	42	46
Card fees									51	50	27
Retail and business banking fees									23	24	25
Capital markets and foreign exchange gain (loss)									6	6	8
Other non-interest income (non-customer related) Gain (Loss)									1	1
Wealth management and trust fees									17	16	15
Other Noninterest Income From Contracts With Customers									3	3	4
Loan-related fees and income											21
Amegy Corporation [Member]
Total non-interest income from contracts with customers (ASC 606)									108	104
Other Noninterest Income, Loss (Non-ASC 606 Customer Related)									30	22
Noninterest income, Customer-related Fees									138	126	116
Noninterest Income, Other											0
Noninterest Income									138	126	116
Gains (Losses) on Sales of Other Real Estate									0	0	0
Net Interest Income									497	491	453
Revenues									635	617	569
Commercial account fees									34	34	33
Card fees									28	29	19
Retail and business banking fees									17	16	16
Capital markets and foreign exchange gain (loss)									16	14	16
Other non-interest income (non-customer related) Gain (Loss)									0	0
Wealth management and trust fees									11	10	9
Other Noninterest Income From Contracts With Customers									2	1	0
Loan-related fees and income											23
California Bank Trust [Member]
Total non-interest income from contracts with customers (ASC 606)									63	61
Other Noninterest Income, Loss (Non-ASC 606 Customer Related)									23	19
Noninterest income, Customer-related Fees									86	80	74
Noninterest Income, Other											1
Noninterest Income									88	82	75
Gains (Losses) on Sales of Other Real Estate									0	0	(1)
Net Interest Income									522	513	455
Revenues									610	595	529
Commercial account fees									23	22	22
Card fees									16	15	10
Retail and business banking fees									13	14	14
Capital markets and foreign exchange gain (loss)									5	4	10
Other non-interest income (non-customer related) Gain (Loss)									2	2
Wealth management and trust fees									4	4	3
Other Noninterest Income From Contracts With Customers									2	2	3
Loan-related fees and income											12
Consolidated Entities [Member]
Total non-interest income from contracts with customers (ASC 606)									435	430
Other Noninterest Income, Loss (Non-ASC 606 Customer Related)									90	78
Noninterest income, Customer-related Fees									525	508	490
Noninterest Income, Other											54
Noninterest Income									562	552	544
Gains (Losses) on Sales of Other Real Estate									4	1	(1)
Net Interest Income									2,272	2,230	2,065
Revenues									2,838	2,783	2,608
Commercial account fees									121	122	126
Card fees									127	125	96
Retail and business banking fees									78	78	78
Capital markets and foreign exchange gain (loss)									34	30	49
Other non-interest income (non-customer related) Gain (Loss)									37	44
Wealth management and trust fees									57	51	47
Other Noninterest Income From Contracts With Customers									$ 18	$ 24	22
Loan-related fees and income											$ 72