Components of Other Comprehensive Income/(Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019		Mar. 31, 2018		Jan. 01, 2019		Jan. 01, 2018		Dec. 31, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period		$ 4,785,380		$ 4,580,488
Balance, adjusted										$ 4,580,488
Beginning balance, as adjusted						$ 4,784,369		$ 4,580,555
Net unrealized gains/(losses)		52,551		(68,142)
Amounts reclassified from AOCI		2,914		1,093
Other comprehensive income/(loss)		55,465		(67,049)
Balance, end of period		4,846,521		4,572,528
Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period		(75,736)
Balance, adjusted										(26,834)
Beginning balance, as adjusted										(26,839)
Net unrealized gains/(losses)		48,615		(59,504)
Amounts reclassified from AOCI		0		(39)
Other comprehensive income/(loss)		48,615		(59,543)
Balance, end of period		(27,121)		(86,382)
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period		(12,112)
Balance, adjusted										(7,764)
Beginning balance, as adjusted										(7,970)
Net unrealized gains/(losses)		3,936		(8,638)
Amounts reclassified from AOCI		1,451		(155)
Other comprehensive income/(loss)		5,387		(8,793)
Balance, end of period		(6,725)		(16,763)
Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period		(288,768)
Balance, adjusted										(288,227)
Beginning balance, as adjusted										(288,227)
Net unrealized gains/(losses)		0		0
Amounts reclassified from AOCI		1,463		1,287
Other comprehensive income/(loss)		1,463		1,287
Balance, end of period		(287,305)		(286,940)
Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period		(376,616)	[1]	(265,279)	[2]
Balance, adjusted	[2]									(322,825)
Beginning balance, as adjusted						$ (376,616)	[1]	(323,036)	[2]	(323,036)
Other comprehensive income/(loss)		55,465	[1]	(67,049)	[2]
Balance, end of period		$ (321,151)	[1]	$ (390,085)	[2]
Accounting Standards Update 2016-01 and 2017-12
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12								67
Accounting Standards Update 2016-01 and 2017-12 | Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12										(5)
Accounting Standards Update 2016-01 and 2017-12 | Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12										(206)
Accounting Standards Update 2016-01 and 2017-12 | Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12										0
Accounting Standards Update 2016-01 and 2017-12 | Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12								$ (211)	[2]	$ (211)

[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.