Derivatives - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Total trading revenues	$ 44,500,000	$ 38,000,000
Term borrowings	1,177,926,000				$ 1,170,963,000
Cash flow hedge length of time				5 years
Hedged amount of foreign currency denominated loans	15,200,000				11,000,000
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	29,000,000				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
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
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	27,200,000				20,700,000
Net fair value of derivative liabilities with adjustable posting thresholds	16,400,000				37,800,000
Collateral received	86,300,000				86,600,000
Securities posted collateral	18,600,000				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	27,200,000				19,000,000
Net fair value of derivative liabilities with adjustable posting thresholds	10,900,000				33,200,000
Collateral received	86,300,000				84,500,000
Securities posted collateral	13,500,000				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	79,600,000				76,000,000
Collateral cash payables	$ 33,200,000				$ 34,000,000