Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Earning assets
Trading investments	$ 47,707	$ 37,465
Accrued interest receivable, difference	40,760	100,743
Tax indemnification receivable	225,073	239,578
Total earning assets, difference	1,922,273	3,409,977
Interest-bearing liabilities
Long-term borrowings, difference	263,987	(69,184)
Total interest-bearing liabilities, difference	512,514	(151,510)
Excess of net asset fair value over carrying value	2,434,787	3,258,467
Money-market and savings accounts
Interest-bearing liabilities
Deposits, difference	86,309	(24,799)
Certificates of deposit
Interest-bearing liabilities
Deposits, difference	162,218	(57,527)
Fair Value
Earning assets
Cash and cash equivalents	3,649,651	4,334,603
Trading investments	47,707	37,465
Available-for-sale investments	2,566,556	2,517,956
Accrued interest receivable	1,233,370	1,306,410
Tax indemnification receivable	8,268	8,047
Derivative instruments	1,781	1,322
Total earning assets	28,590,174	31,856,320
Interest-bearing liabilities
Long-term borrowings	4,955,323	6,000,174
Accrued interest payable	44,065	46,600
Derivative instruments	3,853	252
Total interest-bearing liabilities	24,731,979	26,956,044
Fair Value \| Money-market and savings accounts
Interest-bearing liabilities
Deposits	11,666,178	11,457,490
Fair Value \| Certificates of deposit
Interest-bearing liabilities
Deposits	8,062,560	9,451,528
Carrying Value
Earning assets
Cash and cash equivalents	3,649,651	4,334,603
Trading investments	47,707	37,465
Available-for-sale investments	2,566,556	2,517,956
Accrued interest receivable	1,192,610	1,205,667
Tax indemnification receivable	8,268	8,047
Derivative instruments	1,781	1,322
Total earning assets	26,667,901	28,446,343
Interest-bearing liabilities
Long-term borrowings	5,219,310	5,930,990
Accrued interest payable	44,065	46,600
Derivative instruments	3,853	252
Total interest-bearing liabilities	25,244,493	26,804,534
Carrying Value \| Money-market and savings accounts
Interest-bearing liabilities
Deposits	11,752,487	11,432,691
Carrying Value \| Certificates of deposit
Interest-bearing liabilities
Deposits	8,224,778	9,394,001
Private Education Loans
Earning assets
Loans held for investment, net, difference	1,867,009	3,294,462
Private Education Loans \| Fair Value
Earning assets
Loans held for investment, net	20,378,259	22,919,836
Private Education Loans \| Carrying Value
Earning assets
Loans held for investment, net	18,511,250	19,625,374
FFELP Loans
Earning assets
Loans held for investment, net, difference	12,199	12,690
FFELP Loans \| Fair Value
Earning assets
Loans held for investment, net	675,651	705,644
FFELP Loans \| Carrying Value
Earning assets
Loans held for investment, net	663,452	692,954
Credit Cards
Earning assets
Loans held for investment, net, difference	2,305	2,082
Credit Cards \| Fair Value
Earning assets
Loans held for investment, net	28,931	25,037
Credit Cards \| Carrying Value
Earning assets
Loans held for investment, net	$ 26,626	$ 22,955

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Assets, Fair Value Disclosure, Difference Between Carrying Value and Fair Value

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Deposits, Fair Value Disclosure, Difference Between Carrying Value and Fair Value

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Excess of net asset fair value over carrying value.

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Long-term Debt, Fair Value, Difference Between Carrying Value and Fair Value

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Fair value portion of accrued expenses.

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Fair value of financial obligations, including, but not limited to, debt instruments, derivative liabilities, federal funds purchased and sold under agreements to repurchase, securities loaned or sold under agreements to repurchase, financial instruments sold not yet purchased, guarantees, line of credit, loans and notes payable, servicing liability, and trading liabilities.

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Carrying amount as of the balance sheet date of income taxes previously overpaid to tax authorities (such as U.S. Federal, state and local tax authorities) representing refunds of overpayments or recoveries based on agreed-upon resolutions of disputes. Also called income tax refund receivable.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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