Investment Securities - Schedule Of FHN's Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt securities, available-for-sale:
Securities available for sale, amortized cost	$ 4,385,003
Securities available for sale, fair value	4,445,403	$ 4,626,470
Securities pledged and sold	3,800,000	3,800,000
Securities held-to-maturity:
Amortized Cost	10,000	10,000
Securities held to maturity, gross unrealized gains	1	0
Securities held to maturity, gross unrealized losses	0	(157)
Securities held to maturity, fair value	10,001	9,843
U.S. treasuries
Debt securities, available-for-sale:
Securities available for sale, amortized cost	100	100
Securities available for sale, unrealized gains	0	0
Securities available for sale, unrealized losses	0	(2)
Securities available for sale, fair value	100	98
Government agency issued mortgage-backed securities (“MBS”)
Debt securities, available-for-sale:
Securities available for sale, amortized cost	2,316,381	2,473,687
Securities available for sale, unrealized gains	34,692	4,819
Securities available for sale, unrealized losses	(2,556)	(58,400)
Securities available for sale, fair value	2,348,517	2,420,106
Government agency issued collateralized mortgage obligations (“CMO”)
Debt securities, available-for-sale:
Securities available for sale, amortized cost	1,667,773	2,006,488
Securities available for sale, unrealized gains	9,916	888
Securities available for sale, unrealized losses	(7,197)	(48,681)
Securities available for sale, fair value	1,670,492	1,958,695
Other U.S. government agencies
Debt securities, available-for-sale:
Securities available for sale, amortized cost	303,463	149,050
Securities available for sale, unrealized gains	3,750	809
Securities available for sale, unrealized losses	(1,121)	(73)
Securities available for sale, fair value	306,092	149,786
Corporates and other debt
Debt securities, available-for-sale:
Securities available for sale, amortized cost	40,054	55,383
Securities available for sale, unrealized gains	486	388
Securities available for sale, unrealized losses	0	(461)
Securities available for sale, fair value	40,540	55,310
State and municipalities
Debt securities, available-for-sale:
Securities available for sale, amortized cost	57,232	32,473
Securities available for sale, unrealized gains	3,324	314
Securities available for sale, unrealized losses	(30)	(214)
Securities available for sale, fair value	60,526	32,573
Available-for-sale securities, excluding interest only strips
Debt securities, available-for-sale:
Securities available for sale, amortized cost	4,385,003	4,717,181
Securities available for sale, unrealized gains	52,168	7,218
Securities available for sale, unrealized losses	(10,904)	(107,831)
Securities available for sale, fair value	4,426,267	4,616,568
Interest-Only Strip (elected fair value)
Debt securities, available-for-sale:
Securities available for sale, fair value	19,136	9,902
Corporates and other debt
Securities held-to-maturity:
Amortized Cost	10,000	10,000
Securities held to maturity, gross unrealized gains	1	0
Securities held to maturity, gross unrealized losses	0	(157)
Securities held to maturity, fair value	$ 10,001	$ 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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