Business Segment Information - Amounts of Consolidated Revenue, Expense, Tax and Assets by Segment (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Disaggregation of Revenue [Line Items]
Net interest income (expense)	$ 627			$ 605			$ 1,881	$ 1,923
Provision for credit losses	35			110			140	210
Noninterest income	200			173			581	745
Noninterest expense	511			474			1,527	1,508
Income (loss) before income taxes	281			194			795	950
Income tax expense (benefit)	58			52			171	223
Net income	223	$ 204	$ 197	142	$ 329	$ 255	624	727
Average assets	82,366			83,220			81,779	81,471
Gain on merger termination	0			0			0	225
Visa Class B Shares
Disaggregation of Revenue [Line Items]
Derivative valuation adjustment	15						15		$ 15
Regional Banking
Disaggregation of Revenue [Line Items]
Net interest income (expense)	517			558			1,572	1,705
Provision for credit losses	33			113			119	184
Noninterest income	114			106			328	316
Noninterest expense	329			310			986	937
Income (loss) before income taxes	269			241			795	900
Income tax expense (benefit)	63			56			185	210
Net income	206			185			610	690
Average assets	43,425			43,249			43,344	42,285
Gain on merger termination								0
Specialty Banking
Disaggregation of Revenue [Line Items]
Net interest income (expense)	166			160			476	467
Provision for credit losses	3			2			26	35
Noninterest income	67			49			203	156
Noninterest expense	104			96			311	292
Income (loss) before income taxes	126			111			342	296
Income tax expense (benefit)	31			27			83	72
Net income	95			84			259	224
Average assets	24,945			24,084			24,432	23,353
Gain on merger termination								0
Corporate
Disaggregation of Revenue [Line Items]
Net interest income (expense)	(56)			(113)			(167)	(249)
Provision for credit losses	(1)			(5)			(5)	(9)
Noninterest income	19			18			50	273
Noninterest expense	78			68			230	279
Income (loss) before income taxes	(114)			(158)			(342)	(246)
Income tax expense (benefit)	(36)			(31)			(97)	(59)
Net income	(78)			(127)			(245)	(187)
Average assets	13,996			15,887			14,003	15,833
Restructuring costs	2			10			10	10
Special assessment fee (credit)	(2)						10
Surrender of bank owned life insurance policies, expense				24				24
Benefit from merger-related tax items				$ 11				11
Gain on merger termination								225
Corporate | First Horizon Foundation
Disaggregation of Revenue [Line Items]
Charitable foundation contributions								50
Corporate | TD Transaction
Disaggregation of Revenue [Line Items]
Merger and integration expense								51
Gain on merger termination								225
Corporate | Visa Class B Shares
Disaggregation of Revenue [Line Items]
Derivative valuation adjustment	$ 15						$ 15	$ 15