Investment Securities - Schedule Of FHN's Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	$ 5,293,746
Securities available-for-sale	5,476,156	$ 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	(119)	0
Securities held to maturity, fair value	9,881	10,001
Securities pledged as collateral	4,700,000	3,800,000
U.S. treasuries
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	999,974	100
Securities available for sale, gross unrealized gains	5	0
Securities available for sale, gross unrealized losses	(4)	0
Securities available-for-sale	999,975	100
Government agency issued MBS
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	2,383,956	2,316,381
Securities available for sale, gross unrealized gains	103,506	34,692
Securities available for sale, gross unrealized losses	(80)	(2,556)
Securities available-for-sale	2,487,382	2,348,517
Government agency issued CMO
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	1,383,703	1,667,773
Securities available for sale, gross unrealized gains	31,570	9,916
Securities available for sale, gross unrealized losses	(8)	(7,197)
Securities available-for-sale	1,415,265	1,670,492
Other U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	403,008	303,463
Securities available for sale, gross unrealized gains	14,284	3,750
Securities available for sale, gross unrealized losses	0	(1,121)
Securities available-for-sale	417,292	306,092
Corporates and other debt
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	39,950	40,054
Securities available for sale, gross unrealized gains	463	486
Securities available for sale, gross unrealized losses	0	0
Securities available-for-sale	40,413	40,540
States and municipalities
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	83,155	57,232
Securities available for sale, gross unrealized gains	7,228	3,324
Securities available for sale, gross unrealized losses	0	(30)
Securities available-for-sale	90,383	60,526
Securities available-for-sale, excluding interest only strip:
Debt Securities, Available-for-sale [Line Items]
Securities available for sale, amortized cost	5,293,746	4,385,003
Securities available for sale, gross unrealized gains	157,056	52,168
Securities available for sale, gross unrealized losses	(92)	(10,904)
Securities available-for-sale	5,450,710	4,426,267
Interest- only strips- AFS
Debt Securities, Available-for-sale [Line Items]
Securities available-for-sale	25,446	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	(119)	0
Securities held to maturity, fair value	$ 9,881	$ 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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