Components of Other Comprehensive Income/(loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Thousands	3 Months Ended								6 Months Ended
	Jun. 30, 2020		Mar. 31, 2020		Jun. 30, 2019		Mar. 31, 2019		Jun. 30, 2020		Jun. 30, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	$ 5,055,580		$ 5,076,008		$ 4,846,521		$ 4,785,380		$ 5,076,008		$ 4,785,380
Net unrealized gains/(losses)	610				55,566				102,043		108,117
Amounts reclassified from AOCI	756				3,096				2,767		6,010
Other comprehensive income/(loss)	1,366		103,444		58,662		55,465		104,810		114,127
Balance, end of period	5,208,385		5,055,580		4,926,081		4,846,521		5,208,385		4,926,081
Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	119,357		31,079		(27,121)		(75,736)		31,079		(75,736)
Net unrealized gains/(losses)	(1,030)				47,991				87,248		96,606
Amounts reclassified from AOCI	0				201				0		201
Other comprehensive income/(loss)	(1,030)				48,192				87,248		96,807
Balance, end of period	118,327		119,357		21,071		(27,121)		118,327		21,071
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	16,288		3,227		(6,725)		(12,112)		3,227		(12,112)
Net unrealized gains/(losses)	1,640				7,575				14,795		11,511
Amounts reclassified from AOCI	(1,623)				1,334				(1,717)		2,785
Other comprehensive income/(loss)	17				8,909				13,078		14,296
Balance, end of period	16,305		16,288		2,184		(6,725)		16,305		2,184
Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(271,809)		(273,914)		(287,305)		(288,768)		(273,914)		(288,768)
Net unrealized gains/(losses)	0				0				0		0
Amounts reclassified from AOCI	2,379				1,561				4,484		3,024
Other comprehensive income/(loss)	2,379				1,561				4,484		3,024
Balance, end of period	(269,430)		(271,809)		(285,744)		(287,305)		(269,430)		(285,744)
Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(136,164)	[1]	(239,608)	[1]	(321,151)	[2]	(376,616)	[2]	(239,608)	[1]	(376,616)	[2]
Other comprehensive income/(loss)	1,366	[1]	103,444	[1]	58,662	[1]	55,465	[2]
Balance, end of period	$ (134,798)	[1]	$ (136,164)	[1]	$ (262,489)	[2]	$ (321,151)	[2]	$ (134,798)	[1]	$ (262,489)	[2]

[1] Due to the nature of the preferred stock issued by FHN and its subsidiaries, all components of Other comprehensive income/(loss) have been attributed solely to FHN as the controlling interest holder.

[2] Due to the nature of the preferred stock issued by FHN and its subsidiaries, all components of Other comprehensive income/(loss) have been attributed solely to FHN as the controlling interest holder.