Components of Other Comprehensive Income (Loss) - Schedule of Changes in AOCI (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		$ 5,076	$ 4,786	$ 4,581
Net unrealized gains (losses)		92	126	(64)
Amounts reclassified from AOCI		7	11	11
Other comprehensive income (loss)		99	137	(53)
Ending balance		8,307	5,076	4,786
Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(96)	(1)
Ending balance			(96)	(1)
Adjusted Balance
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		4,980	4,785	4,581
Ending balance			4,980	4,785
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	[1]	(239)	(376)	(265)
Other comprehensive income (loss)	[1]	99	137	(53)
Ending balance	[1]	(140)	(239)	(376)
Balance, as adjusted for ASU					$ (323)
Accumulated Other Comprehensive Income (Loss) | Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	[1]			(58)
Accumulated Other Comprehensive Income (Loss) | Adjusted Balance
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	[1]	(239)	(376)	(323)
Ending balance	[1]		(239)	(376)
Securities AFS
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		31	(76)	(22)
Net unrealized gains (losses)		74	107	(49)
Amounts reclassified from AOCI		3	0	0
Other comprehensive income (loss)		77	107	(49)
Ending balance		108	31	(76)
Balance, as adjusted for ASU					(27)
Securities AFS | Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance				(5)
Cash Flow Hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		3	(12)	(6)
Net unrealized gains (losses)		15	11	(6)
Amounts reclassified from AOCI		(6)	4	2
Other comprehensive income (loss)		9	15	(4)
Ending balance		12	3	(12)
Balance, as adjusted for ASU					(8)
Cash Flow Hedges | Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance				(2)
Pension and Post-retirement Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance		(273)	(288)	(237)
Net unrealized gains (losses)		3	8	(9)
Amounts reclassified from AOCI		10	7	9
Other comprehensive income (loss)		13	15	0
Ending balance		$ (260)	$ (273)	(288)
Balance, as adjusted for ASU					$ (288)
Pension and Post-retirement Plans | Adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance				$ (51)

[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.