Components of Other Comprehensive Income/(loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020		Mar. 31, 2019
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	$ 5,076,008		$ 4,785,380
Net unrealized gains/(losses)	101,433		52,551
Amounts reclassified from AOCI	2,011		2,914
Other comprehensive income/(loss)	103,444		55,465
Balance, end of period	5,055,580		4,846,521
Securities AFS
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	31,079		(75,736)
Net unrealized gains/(losses)	88,278		48,615
Amounts reclassified from AOCI	0		0
Other comprehensive income/(loss)	88,278		48,615
Balance, end of period	119,357		(27,121)
Cash Flow Hedges
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	3,227		(12,112)
Net unrealized gains/(losses)	13,155		3,936
Amounts reclassified from AOCI	(94)		1,451
Other comprehensive income/(loss)	13,061		5,387
Balance, end of period	16,288		(6,725)
Pension and Post-retirement Plans
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(273,914)		(288,768)
Net unrealized gains/(losses)	0		0
Amounts reclassified from AOCI	2,105		1,463
Other comprehensive income/(loss)	2,105		1,463
Balance, end of period	(271,809)		(287,305)
Total
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance, beginning of period	(239,608)	[1]	(376,616)	[2]
Other comprehensive income/(loss)	103,444	[1]	55,465	[2]
Balance, end of period	$ (136,164)	[1]	$ (321,151)	[2]

[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.