Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Earning assets
Loans held for sale	$ 0	$ 340,389
Trading investments	30,594	16,923
Accrued interest receivable, difference	108,633	140,511
Tax indemnification receivable	169,655	374,706
Total earning assets, difference	3,380,177	4,182,834
Interest-bearing liabilities
Long-term borrowings, difference	(199,096)	(209,092)
Total interest-bearing liabilities, difference	(408,148)	(479,976)
Excess of net asset fair value over carrying value	2,972,029	3,702,858
Money-market and savings accounts
Interest-bearing liabilities
Deposits, difference	(54,706)	(68,927)
Certificates of deposit
Interest-bearing liabilities
Deposits, difference	(154,346)	(201,957)
Fair Value
Earning assets
Loans held for sale	0	3,226,029
Cash and cash equivalents	6,207,001	4,455,292
Trading investments	30,594	16,923
Available-for-sale investments	2,115,885	1,996,634
Accrued interest receivable	1,373,593	1,527,816
Tax indemnification receivable	15,678	18,492
Derivative instruments	1,054	729
Total earning assets	33,383,636	34,126,992
Interest-bearing liabilities
Long-term borrowings	5,117,766	5,398,309
Accrued interest payable	79,231	60,272
Derivative instruments	451	287
Total interest-bearing liabilities	28,207,823	28,394,651
Fair Value \| Money-market and savings accounts
Interest-bearing liabilities
Deposits	11,961,848	11,136,560
Fair Value \| Certificates of deposit
Interest-bearing liabilities
Deposits	11,048,527	11,799,223
Carrying Value
Earning assets
Loans held for sale	0	2,885,640
Cash and cash equivalents	6,207,001	4,455,292
Trading investments	30,594	16,923
Available-for-sale investments	2,115,885	1,996,634
Accrued interest receivable	1,264,960	1,387,305
Tax indemnification receivable	15,678	18,492
Derivative instruments	1,054	729
Total earning assets	30,003,459	29,944,158
Interest-bearing liabilities
Long-term borrowings	4,918,670	5,189,217
Accrued interest payable	79,231	60,272
Derivative instruments	451	287
Total interest-bearing liabilities	27,799,675	27,914,675
Carrying Value \| Money-market and savings accounts
Interest-bearing liabilities
Deposits	11,907,142	11,067,633
Carrying Value \| Certificates of deposit
Interest-bearing liabilities
Deposits	10,894,181	11,597,266
Private Education Loans
Earning assets
Loans held for investment, net, difference	3,256,987	3,687,203
Private Education Loans \| Fair Value
Earning assets
Loans held for investment, net	22,889,920	22,124,171
Private Education Loans \| Carrying Value
Earning assets
Loans held for investment, net	19,632,933	18,436,968
FFELP Loans
Earning assets
Loans held for investment, net, difference	13,279	13,449
FFELP Loans \| Fair Value
Earning assets
Loans held for investment, net	738,579	748,657
FFELP Loans \| Carrying Value
Earning assets
Loans held for investment, net	725,300	735,208
Credit Cards
Earning assets
Loans held for investment, net, difference	1,278	1,282
Credit Cards \| Fair Value
Earning assets
Loans held for investment, net	11,332	12,249
Credit Cards \| Carrying Value
Earning assets
Loans held for investment, net	$ 10,054	$ 10,967

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Accrued Interest Receivable, Fair Value Disclosure, Difference

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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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Financial Liabilities Fair Value Disclosure, Difference Between Carrying Value and Fair Value

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Loans Receivable, Fair Value Disclosure, Difference

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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 loans held-for-sale including, but not limited to, mortgage loans and finance receivables.

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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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