Investment Securities - Schedule Of FHN's Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	$ 7,812,737
Securities available-for-sale	7,995,872	$ 4,445,403
Securities pledged as collateral	5,700,000	3,800,000
U.S. treasuries
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	300,037	100
Securities available for sale, gross unrealized gains	0	0
Securities available for sale, gross unrealized losses	(3)	0
Securities available-for-sale	300,034	100
Government agency issued MBS
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	3,731,490	2,316,381
Securities available for sale, gross unrealized gains	97,734	34,692
Securities available for sale, gross unrealized losses	(2,158)	(2,556)
Securities available-for-sale	3,827,066	2,348,517
Government agency issued CMO
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	2,675,192	1,667,773
Securities available for sale, gross unrealized gains	31,694	9,916
Securities available for sale, gross unrealized losses	(1,439)	(7,197)
Securities available-for-sale	2,705,447	1,670,492
Other U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	644,328	303,463
Securities available for sale, gross unrealized gains	13,942	3,750
Securities available for sale, gross unrealized losses	(4)	(1,121)
Securities available-for-sale	658,266	306,092
Corporates and other debt
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	39,936	40,054
Securities available for sale, gross unrealized gains	74	486
Securities available for sale, gross unrealized losses	0	0
Securities available-for-sale	40,010	40,540
States and municipalities
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	421,754	57,232
Securities available for sale, gross unrealized gains	10,342	3,324
Securities available for sale, gross unrealized losses	(6)	(30)
Securities available-for-sale	432,090	60,526
Securities available-for-sale, excluding interest only strip:
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	7,812,737	4,385,003
Securities available for sale, gross unrealized gains	153,786	52,168
Securities available for sale, gross unrealized losses	(3,610)	(10,904)
Securities available-for-sale	7,962,913	4,426,267
Interest- only strips- AFS
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale	$ 32,959	$ 19,136

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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Amortized cost of 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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Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions.

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