Components of Other Comprehensive Income/(Loss) (Schedule of Changes in AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018		Jun. 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)	$ (24,551)	$ 12,247	(92,693)		9,649
Amounts reclassified from AOCI	2,522	249	3,615		543
Other comprehensive income/(loss)	(22,029)	12,496	(89,078)	[1]	10,192	[1]
Balance, June 30	4,549,749	2,826,888	4,549,749		2,826,888
Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, January 1	(86,382)	(18,795)	(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)	(21,094)	9,188	(80,598)		7,652
Amounts reclassified from AOCI	0	(250)	(39)		(277)
Other comprehensive income/(loss)	(21,094)	8,938	(80,637)		7,375
Balance, June 30	(107,476)	(9,857)	(107,476)		(9,857)
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, January 1	(16,763)	(3,179)	(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)	(3,457)	3,059	(12,095)		1,997
Amounts reclassified from AOCI	463	(904)	308		(1,756)
Other comprehensive income/(loss)	(2,994)	2,155	(11,787)		241
Balance, June 30	(19,757)	(1,024)	(19,757)		(1,024)
Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, January 1	(286,940)	(227,984)	(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	0	0		0
Amounts reclassified from AOCI	2,059	1,403	3,346		2,576
Other comprehensive income/(loss)	2,059	1,403	3,346		2,576
Balance, June 30	(284,881)	(226,581)	(284,881)		(226,581)
Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, January 1	(390,085)	(249,958)	(322,825)		(247,654)
Adjustment to reflect adoption of ASU 2016-01 and ASU 2017-12							(211)
Beginning balance, as adjusted							$ (323,036)
Balance, June 30	$ (412,114)	$ (237,462)	$ (412,114)		$ (237,462)

[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.