Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Earning assets
Trading investments	$ 11,360	$ 0
Accrued interest receivable, difference	10,226	98,746
Tax indemnification receivable	286,006	88,844
Total earning assets	1,754,833
Total earning assets, difference		2,265,887
Interest-bearing liabilities
Long-term borrowings, difference	135,411	(80,286)
Total interest-bearing liabilities, difference	15,803	(226,078)
Excess of net asset fair value over carrying value	1,770,636	2,039,809
Money-market and savings accounts
Interest-bearing liabilities
Deposits, difference	(46,898)	(28,528)
Certificates of deposit
Interest-bearing liabilities
Deposits, difference	(72,710)	(117,264)
Fair Value
Earning assets
Cash and cash equivalents	7,292,929	5,563,877
Trading investments	11,360	0
Available-for-sale investments	614,582	487,669
Accrued interest receivable	1,291,972	1,491,471
Tax indemnification receivable	27,727	27,558
Derivative instruments	3,653	715
Total earning assets	32,682,681	34,406,223
Interest-bearing liabilities
Short-term borrowings	0	289,230
Long-term borrowings	4,572,625	4,434,323
Accrued interest payable	93,768	75,158
Derivative instruments	275	1,164
Total interest-bearing liabilities	29,230,239	29,228,573
Fair Value \| Money-market and savings accounts
Interest-bearing liabilities
Deposits	10,360,970	10,363,691
Fair Value \| Certificates of deposit
Interest-bearing liabilities
Deposits	14,202,601	14,065,007
Carrying Value
Earning assets
Cash and cash equivalents	7,292,929	5,563,877
Trading investments	11,360	0
Available-for-sale investments	614,582	487,669
Accrued interest receivable	1,281,746	1,392,725
Tax indemnification receivable	27,727	27,558
Derivative instruments	3,653	715
Total earning assets	30,927,848	32,140,336
Interest-bearing liabilities
Short-term borrowings	0	289,230
Long-term borrowings	4,708,036	4,354,037
Accrued interest payable	93,768	75,158
Derivative instruments	275	1,164
Total interest-bearing liabilities	29,246,042	29,002,495
Carrying Value \| Money-market and savings accounts
Interest-bearing liabilities
Deposits	10,314,072	10,335,163
Carrying Value \| Certificates of deposit
Interest-bearing liabilities
Deposits	14,129,891	13,947,743
Private Education Loans
Earning assets
Loans held for investment, net, difference	1,650,067	2,092,426
Private Education Loans \| Fair Value
Earning assets
Loans held for investment, net	21,826,300	24,988,941
Private Education Loans \| Carrying Value
Earning assets
Loans held for investment, net	20,176,233	22,896,515
FFELP Loans
Earning assets
Loans held for investment, net, difference	(9,322)	11,239
FFELP Loans \| Fair Value
Earning assets
Loans held for investment, net	755,449	795,055
FFELP Loans \| Carrying Value
Earning assets
Loans held for investment, net	764,771	783,816
Personal Loans
Earning assets
Loans held for investment, net, difference	104,306	63,476
Personal Loans \| Fair Value
Earning assets
Loans held for investment, net	851,750	1,047,119
Personal Loans \| Carrying Value
Earning assets
Loans held for investment, net	747,444	983,643
Credit Cards
Earning assets
Loans held for investment, net, difference	(444)	0
Credit Cards \| Fair Value
Earning assets
Loans held for investment, net	6,959	3,818
Credit Cards \| Carrying Value
Earning assets
Loans held for investment, net	$ 7,403	$ 3,818

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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 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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This element represents the portion of the balance sheet assertion valued at fair value by the entity 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. This item represents the amount of short-term debt existing as of the balance sheet date.

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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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