Investment Securities - Schedule Of FHN's Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	$ 4,363,474
Securities available-for-sale	4,544,907	$ 4,445,403
Schedule of Held-to-maturity Securities [Line Items]
Total	10,000	10,000
Securities held to maturity, gross unrealized gains	0	1
Securities held to maturity, gross unrealized losses	(176)	0
Securities held to maturity, fair value	9,824	10,001
Securities pledged as collateral	4,000,000	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	0
Securities available-for-sale	100	100
Government agency issued MBS
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	2,303,720	2,316,381
Securities available for sale, gross unrealized gains	98,797	34,692
Securities available for sale, gross unrealized losses	0	(2,556)
Securities available-for-sale	2,402,517	2,348,517
Government agency issued CMO
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	1,578,623	1,667,773
Securities available for sale, gross unrealized gains	48,320	9,916
Securities available for sale, gross unrealized losses	0	(7,197)
Securities available-for-sale	1,626,943	1,670,492
Other U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	366,453	303,463
Securities available for sale, gross unrealized gains	7,268	3,750
Securities available for sale, gross unrealized losses	(1,224)	(1,121)
Securities available-for-sale	372,497	306,092
Corporates and other debt
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	40,000	40,054
Securities available for sale, gross unrealized gains	621	486
Securities available for sale, gross unrealized losses	0	0
Securities available-for-sale	40,621	40,540
States and municipalities
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	74,578	57,232
Securities available for sale, gross unrealized gains	4,572	3,324
Securities available for sale, gross unrealized losses	(25)	(30)
Securities available-for-sale	79,125	60,526
Securities available-for-sale, excluding interest only strip:
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	4,363,474	4,385,003
Securities available for sale, gross unrealized gains	159,578	52,168
Securities available for sale, gross unrealized losses	(1,249)	(10,904)
Securities available-for-sale	4,521,803	4,426,267
Interest- only strips- AFS
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale	23,104	19,136
Corporate Bonds
Schedule of Held-to-maturity Securities [Line Items]
Total	10,000	10,000
Securities held to maturity, gross unrealized gains	0	1
Securities held to maturity, gross unrealized losses	(176)	0
Securities held to maturity, fair value	$ 9,824	$ 10,001

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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