Derivatives - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended		12 Months Ended
	Nov. 01, 2019	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Total trading revenues				$ 228,400	$ 132,300	$ 173,900
Term borrowings				791,368	1,170,963
Cash flow hedge length of time			5 years
Hedged amount of foreign currency denominated loans				18,400	11,000
Cash Flow Hedges | Hedged Items
Derivative Instruments, Gain (Loss) [Line Items]
Variability in cash flows related to debt instruments (primarily loans)			$ 250,000
Notional		$ 650,000
Visa Class B Shares
Derivative Instruments, Gain (Loss) [Line Items]
Derivative liabilities related to sale				22,800	31,500
Long Term Debt Hedged With Fair Value Interest Rate Derivatives Using Long Haul Method
Derivative Instruments, Gain (Loss) [Line Items]
Term borrowings				500,000
Long Term Debt Hedged With Fair Value Interest Rate Derivatives Using Long Haul Method | First Horizon Bank
Derivative Instruments, Gain (Loss) [Line Items]
Term borrowings				400,000
Additional Derivative Agreements | Derivative Instruments With Adjustable Collateral Posting Thresholds
Derivative Instruments, Gain (Loss) [Line Items]
Net fair value of derivative assets with adjustable posting thresholds				63,100	15,000
Net fair value of derivative liabilities with adjustable posting thresholds				6,400	34,900
Collateral received				148,500	80,200
Securities posted collateral				18,400	13,300
Additional Derivative Agreements | Derivative Instruments With Accelerated Termination Provisions
Derivative Instruments, Gain (Loss) [Line Items]
Net fair value of derivative assets with adjustable posting thresholds				63,100	19,000
Net fair value of derivative liabilities with adjustable posting thresholds				10,300	33,200
Collateral received				148,500	84,500
Securities posted collateral				22,700	15,200
Interest Rate Contract
Derivative Instruments, Gain (Loss) [Line Items]
Collateral cash payables				143,334	39,637
Interest Rate Contract | Cash Flow Hedges | Hedged Items
Derivative Instruments, Gain (Loss) [Line Items]
Variability in cash flows related to debt instruments (primarily loans)				900,000	900,000
Minimum | Cash Flow Hedges | Hedged Items
Derivative Instruments, Gain (Loss) [Line Items]
Cash flow hedge length of time		3 years
Maximum | Cash Flow Hedges | Hedged Items
Derivative Instruments, Gain (Loss) [Line Items]
Cash flow hedge length of time		7 years
Counterparties
Derivative Instruments, Gain (Loss) [Line Items]
Collateral cash receivables				136,600	76,000
Collateral cash payables				$ 53,000	$ 34,000
Senior Subordinated Notes | Senior Debt Maturing In December 2019
Derivative Instruments, Gain (Loss) [Line Items]
Debt redeemed	$ 400,000