Derivatives - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended					9 Months Ended
	Nov. 01, 2019	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2017	Mar. 31, 2016	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Collateral cash payables		$ 385,723					$ 385,723		$ 145,334
Total trading revenues		98,900		$ 63,600			277,500	$ 162,700
Cash flow hedge length of time						5 years
Hedged amount of foreign currency denominated loans		9,100					9,100		18,400
Derivative assets (Note 14)		834,480					834,480		182,984
Derivative liability		89,521					89,521		44,238
Cash Flow Hedge | Hedged Items
Derivative Instruments, Gain (Loss) [Line Items]
Variability in cash flows related to debt instruments (primarily loans)						$ 250,000
Notional					$ 650,000
Derivative, expired during period			$ 200,000
Visa Class B Shares
Derivative Instruments, Gain (Loss) [Line Items]
Derivative liabilities related to sale		15,600					15,600		22,800
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		239,000					239,000		63,100
Net fair value of derivative liabilities with adjustable posting thresholds		3,600					3,600		6,400
Collateral received		349,600					349,600		148,500
Securities posted collateral		41,100					41,100		18,400
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		239,000					239,000		63,100
Net fair value of derivative liabilities with adjustable posting thresholds		24,000					24,000		10,300
Collateral received		349,600					349,600		148,500
Securities posted collateral		62,800					$ 62,800		22,700
Minimum
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, initial remaining term							11 months
Minimum | Cash Flow Hedge | Hedged Items
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, term of contract					3 years
Maximum
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, initial remaining term							34 months
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		289,500					$ 289,500		136,600
Collateral cash payables		141,900					141,900		53,000
Senior Subordinated Notes | Senior Debt Maturing In December 2019
Derivative Instruments, Gain (Loss) [Line Items]
Term borrowings		400,000					400,000
Debt redeemed	$ 400,000
Senior Subordinated Notes | Senior Debt Maturing In December 2020
Derivative Instruments, Gain (Loss) [Line Items]
Term borrowings		500,000					500,000
Derivatives, interest rate contracts
Derivative Instruments, Gain (Loss) [Line Items]
Collateral cash payables		369,510					369,510		143,334
Notional		800,000					800,000
Derivative assets (Note 14)		787,139					787,139		162,344
Derivative liability		47,047					47,047		24,431
Derivatives, interest rate contracts | Cash Flow Hedge | Hedged Items
Derivative Instruments, Gain (Loss) [Line Items]
Variability in cash flows related to debt instruments (primarily loans)		1,500,000					1,500,000		$ 900,000
Credit Risk Contract
Derivative Instruments, Gain (Loss) [Line Items]
Derivative asset, notional amount		248,500					248,500
Derivative liability, notional amount		543,000					543,000
Derivative assets (Note 14)		342					342
Derivative liability		1,500					1,500
Embedded Derivative Financial Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative assets (Note 14)		1,300					1,300
Derivative liability		$ 1,300					$ 1,300