Components of Other Comprehensive Income/(Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		$ 4,785,380	$ 4,580,488	$ 2,705,084
Net unrealized gains/(losses)		125,056	(64,577)	(20,000)
Amounts reclassified from AOCI		11,952	10,997	2,375
Other comprehensive income/(loss)		137,008	(53,580)	(17,625)
Ending balance		5,076,008	4,785,380	4,580,488
Balance, as adjusted for ASU 2018-02				4,580,488
Balance as adjusted			4,784,369	4,580,555	$ 2,705,084
Accumulated Other Comprehensive Income/(Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	[1]	(376,616)	(265,279)	(247,654)
Other comprehensive income/(loss)	[1]	137,008	(53,580)	(17,625)
Ending balance	[1]	(239,608)	(376,616)	(265,279)
Balance, as adjusted for ASU 2018-02	[1]			(322,825)
Balance as adjusted	[1]		(376,616)	(323,036)	$ (247,654)
Securities AFS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(75,736)	(21,997)	(17,232)
Net unrealized gains/(losses)		106,614	(48,858)	(4,467)
Amounts reclassified from AOCI		201	(39)	(298)
Other comprehensive income/(loss)		106,815	(48,897)	(4,765)
Ending balance		31,079	(75,736)	(21,997)
Balance, as adjusted for ASU 2018-02				(26,834)
Balance as adjusted				(26,839)
Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(12,112)	(6,366)	(1,265)
Net unrealized gains/(losses)		11,234	(6,284)	(2,156)
Amounts reclassified from AOCI		4,105	2,142	(2,945)
Other comprehensive income/(loss)		15,339	(4,142)	(5,101)
Ending balance		3,227	(12,112)	(6,366)
Balance, as adjusted for ASU 2018-02				(7,764)
Balance as adjusted				(7,970)
Pension and Post-retirement Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(288,768)	(236,916)	(229,157)
Net unrealized gains/(losses)		7,208	(9,435)	(13,377)
Amounts reclassified from AOCI		7,646	8,894	5,618
Other comprehensive income/(loss)		14,854	(541)	(7,759)
Ending balance		$ (273,914)	$ (288,768)	(236,916)
Balance, as adjusted for ASU 2018-02				(288,227)
Balance as adjusted				(288,227)
Accounting Standards Update 2018-02 | Accumulated Other Comprehensive Income/(Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adjustment to reflect adoption	[1]			(57,546)
Accounting Standards Update 2018-02 | Securities AFS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adjustment to reflect adoption				(4,837)
Accounting Standards Update 2018-02 | Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adjustment to reflect adoption				(1,398)
Accounting Standards Update 2018-02 | Pension and Post-retirement Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adjustment to reflect adoption				(51,311)
Accounting Standards Update 2016-01 and 2017-12
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adjustment to reflect adoption				67
Accounting Standards Update 2016-01 and 2017-12 | Accumulated Other Comprehensive Income/(Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adjustment to reflect adoption	[1]			(211)
Accounting Standards Update 2016-01 and 2017-12 | Securities AFS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adjustment to reflect adoption				(5)
Accounting Standards Update 2016-01 and 2017-12 | Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adjustment to reflect adoption				(206)
Accounting Standards Update 2016-01 and 2017-12 | Pension and Post-retirement Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adjustment to reflect adoption				$ 0

[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.