Components of Other Comprehensive Income/(loss) (Schedule of Changes in AOCI) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 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 2017-12				$ 67	$ 0
Beginning balance, as adjusted				4,580,555	$ 2,705,084
Net unrealized gains/(losses)		(68,142)	(2,598)
Amounts reclassified from AOCI		1,093	294
Other comprehensive income/(loss)	[1]	(67,049)	(2,304)
Balance, March 31		4,572,528	2,740,460
Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, January 1		(26,834)	(17,232)
Adjustment to reflect adoption of ASU 2017-12				(5)
Beginning balance, as adjusted				(26,839)
Net unrealized gains/(losses)		(59,504)	(1,536)
Amounts reclassified from AOCI		(39)	(27)
Other comprehensive income/(loss)		(59,543)	(1,563)
Balance, March 31		(86,382)	(18,795)
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, January 1		(7,764)	(1,265)
Adjustment to reflect adoption of ASU 2017-12				(206)
Beginning balance, as adjusted				(7,970)
Net unrealized gains/(losses)		(8,638)	(1,062)
Amounts reclassified from AOCI		(155)	(852)
Other comprehensive income/(loss)		(8,793)	(1,914)
Balance, March 31		(16,763)	(3,179)
Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, January 1		(288,227)	(229,157)
Adjustment to reflect adoption of ASU 2017-12				0
Beginning balance, as adjusted				(288,227)
Net unrealized gains/(losses)		0	0
Amounts reclassified from AOCI		1,287	1,173
Other comprehensive income/(loss)		1,287	1,173
Balance, March 31		(286,940)	(227,984)
Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, January 1		(322,825)	(247,654)
Adjustment to reflect adoption of ASU 2017-12				(211)
Beginning balance, as adjusted				$ (323,036)
Balance, March 31		$ (390,085)	$ (249,958)

[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.