Regulatory Capital and Restrictions - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2020
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Undivided profits		$ 1,798,442	$ 1,542,408
Dividend paid to parent company		$ 345,000	$ 420,000
First Horizon Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Capital, Actual Ratio		10.18%	10.72%
Undivided profits		$ 1,200,000
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		$ 426,000
Covered transactions		$ 800
Percent of capital stock and surplus threshold for credit extension to affiliates		20.00%
Maximum amount of credit bank may extend to all affiliates		$ 851,900
FHN Financial Securities Corp.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Covered transactions		$ 390,400
First Horizon National Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Capital, Actual Ratio		10.15%	10.80%
First Horizon Advisors, Inc.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Covered transactions		$ 34,700
All Affiliates Member
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Covered transactions		425,900
Minimum | First Horizon Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Cash reserve required		396,100	$ 371,700
Vault cash included in cash reserves		$ 273,700	$ 273,700
Subsequent Event
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Dividend paid to parent company	$ 65,000
Subsequent Event | First Horizon Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Positive (negative) amount available for dividend payments				$ 331,200