Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Earning assets:
Loans held for sale	$ 0	$ 0
Trading investments	51,342	55,903
Accrued interest receivable, difference	62,539	35,015
Tax indemnification receivable	337,177	380,058
Derivative instruments	0	405
Total earning assets, difference	2,664,209	2,088,881
Interest-bearing liabilities:
Long-term borrowings, difference	407,677	421,881
Derivative instruments	1,815	0
Total interest-bearing liabilities, difference	900,095	838,340
Excess of net asset fair value over carrying value	3,564,304	2,927,221
Money-market and savings accounts
Interest-bearing liabilities:
Deposits, difference	124,242	104,979
Certificates of deposit
Interest-bearing liabilities:
Deposits, difference	368,176	311,480
Fair Value
Earning assets:
Loans held for sale	26,202	29,448
Cash and cash equivalents	3,716,379	4,616,117
Trading investments	51,342	55,903
Available-for-sale investments	2,311,062	2,342,089
Accrued interest receivable	1,393,556	1,237,074
Tax indemnification receivable	2,858	2,816
Derivative instruments	0	972
Total earning assets	31,190,632	29,965,153
Interest-bearing liabilities:
Long-term borrowings	5,106,299	4,813,233
Accrued interest payable	91,943	71,586
Derivative instruments	1,815	567
Total interest-bearing liabilities	26,509,305	25,915,574
Fair Value \| Money-market and savings accounts
Interest-bearing liabilities:
Deposits	11,671,878	11,854,849
Fair Value \| Certificates of deposit
Interest-bearing liabilities:
Deposits	9,637,370	9,175,339
Carrying Value
Earning assets:
Loans held for sale	26,202	29,448
Cash and cash equivalents	3,716,379	4,616,117
Trading investments	51,342	55,903
Available-for-sale investments	2,311,062	2,342,089
Accrued interest receivable	1,331,017	1,202,059
Tax indemnification receivable	2,858	2,816
Derivative instruments	0	972
Total earning assets	28,526,423	27,876,272
Interest-bearing liabilities:
Long-term borrowings	5,513,976	5,235,114
Accrued interest payable	91,943	71,586
Derivative instruments	1,815	567
Total interest-bearing liabilities	27,409,400	26,753,914
Carrying Value \| Money-market and savings accounts
Interest-bearing liabilities:
Deposits	11,796,120	11,959,828
Carrying Value \| Certificates of deposit
Interest-bearing liabilities:
Deposits	10,005,546	9,486,819
Private Education Loans
Earning assets:
Loans held for investment, net, difference	2,590,355	2,042,835
Private Education Loans \| Fair Value
Earning assets:
Loans held for investment, net	23,088,030	21,062,548
Private Education Loans \| Carrying Value
Earning assets:
Loans held for investment, net	20,497,675	19,019,713
FFELP Loans
Earning assets:
Loans held for investment, net, difference	11,315	11,031
FFELP Loans \| Fair Value
Earning assets:
Loans held for investment, net	601,203	618,186
FFELP Loans \| Carrying Value
Earning assets:
Loans held for investment, net	$ 589,888	$ 607,155

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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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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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