Derivatives (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Mar. 31, 2017	Mar. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Derivative liabilities not subject to master netting agreements	$ 24,200	$ 15,200
Derivative assets not subject to master netting agreements	19,700	10,200
Collateral cash receivables	26,700	27,800
Collateral cash payables	34,500	28,300
Total trading revenues	38,000			$ 42,700
Term borrowings	1,214,967	1,218,097
Hedged amount of foreign currency denominated loans	5,900	1,500
Cash flow hedge length of time					5 years
Cash Flow Hedge | Hedged Items
Derivative Instruments, Gain (Loss) [Line Items]
Variability in cash flows related to debt instruments (primarily loans)				$ 650,000	$ 250,000
Visa Class B Shares
Derivative Instruments, Gain (Loss) [Line Items]
Derivative liabilities related to sale	5,600	5,600
Long Term Debt Hedged With Fair Value Interest Rate Derivatives Using Long Haul Method
Derivative Instruments, Gain (Loss) [Line Items]
Net fair value of interest rate derivatives hedging		200
Term borrowings	500,000
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]
Net fair value of interest rate derivatives hedging		100
Term borrowings	400,000
Additional Derivative Agreements | Derivative Instruments With Accelerated Termination Provisions
Derivative Instruments, Gain (Loss) [Line Items]
Collateral received	109,300	118,600
Securities posted collateral	13,800	6,700
Net fair value of derivative assets with adjustable posting thresholds	18,800	22,800
Net fair value of derivative liabilities with adjustable posting thresholds	40,400	19,400
Additional Derivative Agreements | Derivative Instruments With Adjustable Collateral Posting Thresholds
Derivative Instruments, Gain (Loss) [Line Items]
Collateral received	112,000	119,300
Securities posted collateral	18,400	18,900
Net fair value of derivative assets with adjustable posting thresholds	21,300	23,300
Net fair value of derivative liabilities with adjustable posting thresholds	49,000	34,500
Interest Rate Contract | Cash Flow Hedge | Hedged Items
Derivative Instruments, Gain (Loss) [Line Items]
Variability in cash flows related to debt instruments (primarily loans)	$ 900,000	900,000
Minimum
Derivative Instruments, Gain (Loss) [Line Items]
Cash flow hedge length of time				3 years
Maximum
Derivative Instruments, Gain (Loss) [Line Items]
Cash flow hedge length of time				7 years
Corporate Segment
Derivative Instruments, Gain (Loss) [Line Items]
Long-lived asset impairment		3,000	$ 2,000
Regional Banking
Derivative Instruments, Gain (Loss) [Line Items]
Long-lived asset impairment		$ 800