Components of Other Comprehensive Income/(Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017		Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, January 1			$ 4,580,488		$ 2,705,084
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12							$ 67	$ 0
Beginning balance, as adjusted							4,580,555	$ 2,705,084
Net unrealized gains/(losses)	$ (28,441)	$ 4,317	(121,134)		13,966
Amounts reclassified from AOCI	2,906	761	6,521		1,304
Other comprehensive income/(loss)	(25,535)	5,078	(114,613)	[1]	15,270	[1]
Balance, September 30	4,741,931	2,883,551	4,741,931		2,883,551
Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, January 1	(107,476)	(9,857)	(26,834)		(17,232)
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12							(5)
Beginning balance, as adjusted							(26,839)
Net unrealized gains/(losses)	(25,924)	3,918	(106,522)		11,570
Amounts reclassified from AOCI	0	(1)	(39)		(278)
Other comprehensive income/(loss)	(25,924)	3,917	(106,561)		11,292
Balance, September 30	(133,400)	(5,940)	(133,400)		(5,940)
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, January 1	(19,757)	(1,024)	(7,764)		(1,265)
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12							(206)
Beginning balance, as adjusted							(7,970)
Net unrealized gains/(losses)	(2,517)	(91)	(14,612)		1,906
Amounts reclassified from AOCI	771	(643)	1,079		(2,399)
Other comprehensive income/(loss)	(1,746)	(734)	(13,533)		(493)
Balance, September 30	(21,503)	(1,758)	(21,503)		(1,758)
Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, January 1	(284,881)	(226,581)	(288,227)		(229,157)
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12							0
Beginning balance, as adjusted							(288,227)
Net unrealized gains/(losses)	0	490	0		490
Amounts reclassified from AOCI	2,135	1,405	5,481		3,981
Other comprehensive income/(loss)	2,135	1,895	5,481		4,471
Balance, September 30	(282,746)	(224,686)	(282,746)		(224,686)
Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, January 1	(412,114)	(237,462)	(322,825)		(247,654)
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12							(211)
Beginning balance, as adjusted							$ (323,036)
Balance, September 30	$ (437,649)	$ (232,384)	$ (437,649)		$ (232,384)

[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.