Derivatives - Narrative (Details) - USD ($)	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2017	Mar. 31, 2016	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Total trading revenues	$ 54,500,000	$ 29,900,000			$ 99,000,000.0	$ 68,000,000.0
Term borrowings	1,186,646,000				1,186,646,000		$ 1,170,963,000
Cash flow hedge length of time				5 years
Hedged amount of foreign currency denominated loans	16,000,000.0				16,000,000.0		11,000,000.0
Cash Flow Hedge | Hedged Items
Derivative Instruments, Gain (Loss) [Line Items]
Variability in cash flows related to debt instruments (primarily loans)				$ 250,000,000
Notional			$ 650,000,000
Visa Class B Shares
Derivative Instruments, Gain (Loss) [Line Items]
Derivative liabilities related to sale	26,500,000				26,500,000		31,500,000
Long Term Debt Hedged With Fair Value Interest Rate Derivatives Using Long Haul Method
Derivative Instruments, Gain (Loss) [Line Items]
Term borrowings	500,000,000.0				500,000,000.0
Long Term Debt Hedged With Fair Value Interest Rate Derivatives Using Long Haul Method | First Tennessee Bank National Association
Derivative Instruments, Gain (Loss) [Line Items]
Term borrowings	400,000,000.0				400,000,000.0
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	62,200,000				62,200,000		20,700,000
Net fair value of derivative liabilities with adjustable posting thresholds	15,800,000				15,800,000		37,800,000
Collateral received	116,500,000				116,500,000		86,600,000
Securities posted collateral	30,600,000				30,600,000		16,200,000
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	62,200,000				62,200,000		19,000,000.0
Net fair value of derivative liabilities with adjustable posting thresholds	6,700,000				6,700,000		33,200,000
Collateral received	116,500,000				116,500,000		84,500,000
Securities posted collateral	21,500,000				21,500,000		15,200,000
Minimum | Cash Flow Hedge | Hedged Items
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, term of contract			3 years
Maximum | Cash Flow Hedge | Hedged Items
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, term of contract			7 years
Counterparties
Derivative Instruments, Gain (Loss) [Line Items]
Collateral cash receivables	96,200,000				96,200,000		76,000,000.0
Collateral cash payables	$ 44,100,000				$ 44,100,000		$ 34,000,000.0