Business Segment Information - Amounts of Consolidated Revenue, Expense, Tax and Assets by Segment (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2023	Mar. 31, 2023	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2023	Jun. 30, 2022
Disaggregation of Revenue [Line Items]
Net interest income (expense)	$ 630		$ 542		$ 1,318	$ 1,021
Provision for credit losses	50		30		100	(10)
Noninterest income	400		201		571	430
Noninterest expense	555		488		1,033	982
Income (loss) before income taxes	425		225		756	479
Income tax expense (benefit)	96		48		171	105
Net income (loss)	329	$ 255	177	$ 198	585	374
Average assets	82,304		86,326		80,582	87,450
Gain on merger termination	225		0		225	0
TD Transaction
Disaggregation of Revenue [Line Items]
Merger and integration expense	30		25		51	34
Regional Banking
Disaggregation of Revenue [Line Items]
Net interest income (expense)	612		465		1,198	892
Provision for credit losses	43		52		85	22
Noninterest income	108		114		215	228
Noninterest expense	321		299		641	604
Income (loss) before income taxes	356		228		687	494
Income tax expense (benefit)	84		53		161	116
Net income (loss)	272		175		526	378
Average assets	45,615		41,943		45,054	41,249
Gain on merger termination	0				0
Specialty Banking
Disaggregation of Revenue [Line Items]
Net interest income (expense)	129		141		255	285
Provision for credit losses	10		(18)		20	(21)
Noninterest income	48		96		101	201
Noninterest expense	88		114		181	247
Income (loss) before income taxes	79		141		155	260
Income tax expense (benefit)	19		35		38	63
Net income (loss)	60		106		117	197
Average assets	20,024		20,237		19,732	20,240
Gain on merger termination	0				0
Corporate
Disaggregation of Revenue [Line Items]
Net interest income (expense)	(111)		(64)		(135)	(156)
Provision for credit losses	(3)		(4)		(5)	(11)
Noninterest income	244		(9)		255	1
Noninterest expense	146		75		211	131
Income (loss) before income taxes	(10)		(144)		(86)	(275)
Income tax expense (benefit)	(7)		(40)		(28)	(74)
Net income (loss)	(3)		(104)		(58)	(201)
Average assets	16,665		24,146		15,796	25,961
Gain on merger termination	225				225
Corporate | First Horizon Foundation
Disaggregation of Revenue [Line Items]
Payment for cash contribution	50				50
Corporate | TD Transaction
Disaggregation of Revenue [Line Items]
Merger and integration expense	30				51
Gain on merger termination	$ 225				$ 225
Corporate | BKC Merger and TD Merger Agreement
Disaggregation of Revenue [Line Items]
Merger and integration expense			$ 38			$ 75