Regulatory Capital and Restrictions - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jan. 01, 2021 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Retained earnings	$ 2,261	$ 1,798
Dividend paid to parent company	$ 180	$ 345
First Horizon Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Capital, Actual Ratio	0.1093	0.1018
Retained earnings	$ 1,800
Percent of capital stock and surplus threshold for credit extension to parent and certain financial subsidiaries	10.00%
Maximum amount of credit bank may extend to parent and certain financial institutions	$ 804
Covered transactions	$ 1
Percent of capital stock and surplus threshold for credit extension to affiliates	20.00%
Maximum amount of credit bank may extend to all affiliates	$ 1,600
FHN Financial Securities Corp.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Covered transactions	394
First Horizon Advisors, Inc.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Covered transactions	42
All Affiliates Member
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Covered transactions	436
Minimum | First Horizon Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Cash reserve required		$ 396
Vault cash included in cash reserves	$ 397
Subsequent Event | First Horizon Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Positive (negative) amount available for dividend payments			$ 897