Investment Securities - Schedule Of FHN's Investment Securities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt securities, available-for-sale:
Securities available for sale, amortized cost	$ 7,872
Securities available for sale, fair value	8,047	$ 4,445
Securities pledged and sold	6,400	3,800
U.S. treasuries
Debt securities, available-for-sale:
Securities available for sale, amortized cost	613	0
Securities available for sale, unrealized gains	0	0
Securities available for sale, unrealized losses	0	0
Securities available for sale, fair value	613	0
Government agency issued MBS
Debt securities, available-for-sale:
Securities available for sale, amortized cost	3,722	2,316
Securities available for sale, unrealized gains	92	35
Securities available for sale, unrealized losses	(2)	(3)
Securities available for sale, fair value	3,812	2,348
Government agency issued CMO
Debt securities, available-for-sale:
Securities available for sale, amortized cost	2,380	1,668
Securities available for sale, unrealized gains	29	10
Securities available for sale, unrealized losses	(3)	(7)
Securities available for sale, fair value	2,406	1,671
Other U.S. government agencies
Debt securities, available-for-sale:
Securities available for sale, amortized cost	672	304
Securities available for sale, unrealized gains	12	4
Securities available for sale, unrealized losses	0	(1)
Securities available for sale, fair value	684	307
Corporate and other debt
Debt securities, available-for-sale:
Securities available for sale, amortized cost	40	40
Securities available for sale, unrealized gains	1	0
Securities available for sale, unrealized losses	(1)	0
Securities available for sale, fair value	40	40
States and municipalities
Debt securities, available-for-sale:
Securities available for sale, amortized cost	445	57
Securities available for sale, unrealized gains	15	3
Securities available for sale, unrealized losses	0	0
Securities available for sale, fair value	460	60
Available-for-sale securities, excluding interest only strips
Debt securities, available-for-sale:
Securities available for sale, amortized cost	7,872	4,385
Securities available for sale, unrealized gains	149	52
Securities available for sale, unrealized losses	(6)	(11)
Securities available for sale, fair value	8,015	4,426
Interest-only strips- AFS
Debt securities, available-for-sale:
Securities available for sale, fair value	$ 32	$ 19

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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