Components of Other Comprehensive Income/(loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Thousands		3 Months Ended												9 Months Ended
		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Sep. 30, 2019		Sep. 30, 2018		Jan. 01, 2019		Jan. 01, 2018		Dec. 31, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, adjusted																						$ 4,580,488
Balance, beginning of period		$ 4,926,081		$ 4,846,521		$ 4,785,380		$ 4,549,749		$ 4,572,528		$ 4,580,488		$ 4,785,380		$ 4,580,488
Beginning balance, as adjusted																		$ 4,784,369		$ 4,580,555
Net unrealized gains/(losses)		18,744						(28,441)						126,862		(121,134)
Amounts reclassified from AOCI		3,091						2,906						9,100		6,521
Other comprehensive income/(loss)		21,835		58,662		55,465		(25,535)		(22,029)		(67,049)		135,962		(114,613)
Balance, end of period		4,996,043		4,926,081		4,846,521		4,741,931		4,549,749		4,572,528		4,996,043		4,741,931
Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, adjusted	[1]																					(26,834)
Balance, beginning of period		21,071				(75,736)		(107,476)						(75,736)
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12																						(5)
Beginning balance, as adjusted																						(26,839)
Net unrealized gains/(losses)		16,889						(25,924)						113,495		(106,522)
Amounts reclassified from AOCI		0						0						201		(39)
Other comprehensive income/(loss)		16,889						(25,924)						113,696		(106,561)
Balance, end of period		37,960		21,071				(133,400)		(107,476)				37,960		(133,400)
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, adjusted	[1]																					(7,764)
Balance, beginning of period		2,184				(12,112)		(19,757)						(12,112)
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12																						(206)
Beginning balance, as adjusted																						(7,970)
Net unrealized gains/(losses)		1,855						(2,517)						13,367		(14,612)
Amounts reclassified from AOCI		989						771						3,773		1,079
Other comprehensive income/(loss)		2,844						(1,746)						17,140		(13,533)
Balance, end of period		5,028		2,184				(21,503)		(19,757)				5,028		(21,503)
Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, adjusted	[1]																					(288,227)
Balance, beginning of period		(285,744)				(288,768)		(284,881)						(288,768)
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12																						0
Beginning balance, as adjusted																						(288,227)
Net unrealized gains/(losses)		0						0						0		0
Amounts reclassified from AOCI		2,102						2,135						5,126		5,481
Other comprehensive income/(loss)		2,102						2,135						5,126		5,481
Balance, end of period		(283,642)		(285,744)				(282,746)		(284,881)				(283,642)		(282,746)
Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, adjusted	[1]																					(322,825)
Balance, beginning of period		(262,489)	[2]	(321,151)	[2]	(376,616)	[2]	(412,114)	[1]	(390,085)	[1]	(265,279)	[1]	(376,616)	[2]	(265,279)	[1]
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12																						(211)
Beginning balance, as adjusted																		$ (376,616)	[2]	$ (323,036)	[1]	$ (323,036)
Other comprehensive income/(loss)		21,835	[2]	58,662	[2]	55,465	[2]	(25,535)	[1]	(22,029)	[1]	(67,049)	[1]
Balance, end of period		$ (240,654)	[2]	$ (262,489)	[2]	$ (321,151)	[2]	$ (437,649)	[1]	$ (412,114)	[1]	$ (390,085)	[1]	$ (240,654)	[2]	$ (437,649)	[1]

[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.