CONSOLIDATED CONDENSED STATEMENTS OF EQUITY (Parenthetical) - USD ($) $ in Millions	3 Months Ended						12 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Preferred stock - cash dividends declared per share (in dollars per share)		$ 1,550	$ 1,550	$ 1,550	$ 1,550	$ 1,550
Common stock - cash dividends declared per share (in dollars per share)	$ 0.15	$ 0.15	$ 0.15	$ 0.14	$ 0.14	$ 0.14
Accounting Standards Update [Extensible List]							us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member
First Horizon Share Repurchase Program
Common stock repurchased under share repurchase program				$ 28.2	$ 50.2	$ 51.5
Preferred Stock
Number of shares issued in transaction (in shares)		1,500
Preferred stock issuance, net of offering cost (in dollars per share)		$ 100,000