Derivatives - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2017	Mar. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Total trading revenues	$ 34,300	$ 45,000			$ 102,300	$ 133,300
Term borrowings	1,200,134				1,200,134		$ 1,218,097
Cash flow hedge length of time				5 years
Hedged amount of foreign currency denominated loans	10,600				10,600		1,500
Cash Flow Hedges | Hedging Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Variability in cash flows related to debt instruments (primarily loans)				$ 250,000
Notional amount			$ 650,000
Cash Flow Hedges | Hedging Instruments | Minimum
Derivative Instruments, Gain (Loss) [Line Items]
Initial duration			3 years
Cash Flow Hedges | Hedging Instruments | Maximum
Derivative Instruments, Gain (Loss) [Line Items]
Initial duration			7 years
Long Term Debt Hedged With Fair Value Interest Rate Derivatives Using Long Haul Method
Derivative Instruments, Gain (Loss) [Line Items]
Term borrowings	500,000				500,000
Net fair value of interest rate derivatives hedging	0				0		200
Derivative Instruments With Adjustable Collateral Posting Thresholds | Additional Derivative Agreements
Derivative Instruments, Gain (Loss) [Line Items]
Net fair value of derivative assets with adjustable posting thresholds	22,000				22,000		23,300
Net fair value of derivative liabilities with adjustable posting thresholds	57,700				57,700		34,500
Collateral received	92,300				92,300		119,300
Securities posted collateral	20,400				20,400		18,900
Derivative Instruments With Accelerated Termination Provisions | Additional Derivative Agreements
Derivative Instruments, Gain (Loss) [Line Items]
Net fair value of derivative assets with adjustable posting thresholds	18,100				18,100		22,800
Net fair value of derivative liabilities with adjustable posting thresholds	54,500				54,500		19,400
Collateral received	88,200				88,200		118,600
Securities posted collateral	20,900				20,900		6,700
FTBNA | Long Term Debt Hedged With Fair Value Interest Rate Derivatives Using Long Haul Method
Derivative Instruments, Gain (Loss) [Line Items]
Term borrowings	400,000				400,000
Net fair value of interest rate derivatives hedging	0				0		100
Certain Counterparties
Derivative Instruments, Gain (Loss) [Line Items]
Collateral cash receivables	81,700				81,700		60,300
Collateral cash payables	50,300				50,300		49,700
Visa Class B Shares
Derivative Instruments, Gain (Loss) [Line Items]
Derivative liabilities related to sale	34,200				34,200		$ 5,600
Derivative liabilities related to sale, transaction executed	$ 28,100				$ 28,100