Investment Securities - Schedule Of FHN's Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	$ 4,350,191
Securities available-for-sale	4,415,845	$ 4,626,470
Schedule of Held-to-maturity Securities [Line Items]
Total	10,000	10,000
Securities held to maturity, gross unrealized gains	0	0
Securities held to maturity, gross unrealized losses	(13)	(157)
Securities held to maturity, fair value	9,987	9,843
Securities pledged as collateral	3,700,000	3,800,000
U.S. treasuries
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	100	100
Securities available for sale, gross unrealized gains	0	0
Securities available for sale, gross unrealized losses	0	(2)
Securities available-for-sale	100	98
Government agency issued MBS
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	2,249,973	2,473,687
Securities available for sale, gross unrealized gains	37,204	4,819
Securities available for sale, gross unrealized losses	(2,745)	(58,400)
Securities available-for-sale	2,284,432	2,420,106
Government agency issued CMO
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	1,775,941	2,006,488
Securities available for sale, gross unrealized gains	13,978	888
Securities available for sale, gross unrealized losses	(5,699)	(48,681)
Securities available-for-sale	1,784,220	1,958,695
Other U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	237,414	149,050
Securities available for sale, gross unrealized gains	3,919	809
Securities available for sale, gross unrealized losses	0	(73)
Securities available-for-sale	241,333	149,786
Corporates and other debt
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	40,105	55,383
Securities available for sale, gross unrealized gains	425	388
Securities available for sale, gross unrealized losses	(90)	(461)
Securities available-for-sale	40,440	55,310
States and municipalities
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	46,658	32,473
Securities available for sale, gross unrealized gains	3,435	314
Securities available for sale, gross unrealized losses	(22)	(214)
Securities available-for-sale	50,071	32,573
Securities available-for-sale, excluding interest only strip:
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	4,350,191	4,717,181
Securities available for sale, gross unrealized gains	58,961	7,218
Securities available for sale, gross unrealized losses	(8,556)	(107,831)
Securities available-for-sale	4,400,596	4,616,568
Interest- only strips- AFS
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale	15,249	9,902
Corporate Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	10,000	10,000
Securities held to maturity, gross unrealized gains	0	0
Securities held to maturity, gross unrealized losses	(13)	(157)
Securities held to maturity, fair value	$ 9,987	$ 9,843

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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