Business Segment Information - Amounts of Consolidated Revenue, Expense, Tax and Assets by Segment (Details) - USD ($)	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Disaggregation of Revenue [Line Items]
Net interest income (expense)	$ 605,000,000			$ 662,000,000			$ 1,923,000,000	$ 1,683,000,000
Provision for credit losses	110,000,000			60,000,000			210,000,000	50,000,000
Noninterest income	173,000,000			213,000,000			745,000,000	642,000,000
Noninterest expense	474,000,000			469,000,000			1,508,000,000	1,450,000,000
Income (loss) before income taxes	194,000,000			346,000,000			950,000,000	825,000,000
Income tax expense (benefit)	52,000,000			78,000,000			223,000,000	183,000,000
Net income (loss)	142,000,000	$ 329,000,000	$ 255,000,000	268,000,000	$ 177,000,000	$ 198,000,000	727,000,000	642,000,000
Average assets	83,220,000,000			82,551,000,000			81,471,000,000	85,799,000,000
Gain on merger termination	0			0			225,000,000	0
Visa Class B Shares
Disaggregation of Revenue [Line Items]
Derivative valuation adjustment							15,000,000		$ 22,000,000
TD Transaction
Disaggregation of Revenue [Line Items]
Merger and integration expense	0			21,000,000			51,000,000	55,000,000
Regional Banking
Disaggregation of Revenue [Line Items]
Net interest income (expense)	583,000,000			518,000,000			1,781,000,000	1,410,000,000
Provision for credit losses	104,000,000			43,000,000			189,000,000	64,000,000
Noninterest income	109,000,000			110,000,000			325,000,000	337,000,000
Noninterest expense	318,000,000			303,000,000			960,000,000	906,000,000
Income (loss) before income taxes	270,000,000			282,000,000			957,000,000	777,000,000
Income tax expense (benefit)	63,000,000			66,000,000			224,000,000	182,000,000
Net income (loss)	207,000,000			216,000,000			733,000,000	595,000,000
Average assets	46,657,000,000			42,820,000,000			45,594,000,000	41,779,000,000
Gain on merger termination							0
Specialty Banking
Disaggregation of Revenue [Line Items]
Net interest income (expense)	135,000,000			138,000,000			390,000,000	423,000,000
Provision for credit losses	6,000,000			17,000,000			26,000,000	(4,000,000)
Noninterest income	46,000,000			64,000,000			147,000,000	265,000,000
Noninterest expense	89,000,000			105,000,000			270,000,000	352,000,000
Income (loss) before income taxes	86,000,000			80,000,000			241,000,000	340,000,000
Income tax expense (benefit)	21,000,000			19,000,000			58,000,000	83,000,000
Net income (loss)	65,000,000			61,000,000			183,000,000	257,000,000
Average assets	20,687,000,000			19,745,000,000			20,054,000,000	20,073,000,000
Gain on merger termination							0
Corporate
Disaggregation of Revenue [Line Items]
Net interest income (expense)	(113,000,000)			6,000,000			(248,000,000)	(150,000,000)
Provision for credit losses	0			0			(5,000,000)	(10,000,000)
Noninterest income	18,000,000			39,000,000			273,000,000	40,000,000
Noninterest expense	67,000,000			61,000,000			278,000,000	192,000,000
Income (loss) before income taxes	(162,000,000)			(16,000,000)			(248,000,000)	(292,000,000)
Income tax expense (benefit)	(32,000,000)			(7,000,000)			(59,000,000)	(82,000,000)
Net income (loss)	(130,000,000)			(9,000,000)			(189,000,000)	(210,000,000)
Average assets	15,876,000,000			19,986,000,000			15,823,000,000	23,947,000,000
Gain on merger termination							225,000,000
Gain on sale of mortgage servicing rights								12,000,000
Gain on disposition of business				10,000,000				10,000,000
Restructuring costs	10,000,000						10,000,000
Surrender of bank owned life insurance policies, expense	24,000,000						24,000,000
Benefit from merger-related tax items	$ 11,000,000						11,000,000
Corporate | Visa Class B Shares
Disaggregation of Revenue [Line Items]
Derivative valuation adjustment							(15,000,000)
Corporate | Insurance Business
Disaggregation of Revenue [Line Items]
Gain on disposition of business				21,000,000				21,000,000
Corporate | Fintech Investment
Disaggregation of Revenue [Line Items]
Gain on sale of investments								6,000,000
Corporate | First Horizon Foundation
Disaggregation of Revenue [Line Items]
Payment for cash contribution							50,000,000
Corporate | TD Transaction
Disaggregation of Revenue [Line Items]
Gain on merger termination							225,000,000
Merger and integration expense							$ 51,000,000
Corporate | BKC Merger and TD Merger Agreement
Disaggregation of Revenue [Line Items]
Merger and integration expense				$ 24,000,000				99,000,000
Corporate | BKC Merger and TD Merger Agreement | Visa Class B Shares
Disaggregation of Revenue [Line Items]
Derivative valuation adjustment								$ 12,000,000