Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Earning assets
Accrued interest receivable	$ 128,363
Accrued interest receivable, difference		$ 93,861
Tax indemnification receivable	108,743	41,570
Total earning assets	2,751,177
Total earning assets, difference		2,152,077
Interest-bearing liabilities
Long-term borrowings, difference	(123,363)	5,373
Total interest-bearing liabilities, difference	(248,555)	63,304
Excess of net asset fair value over carrying value	2,502,622	2,215,381
Money-market and savings accounts
Interest-bearing liabilities
Deposits, difference	(27,806)	19,151
Certificates of deposit
Interest-bearing liabilities
Deposits, difference	(97,386)	38,780
Fair Value
Earning assets
Cash and cash equivalents	3,851,608	2,559,106
Available-for-sale investments	455,968	176,245
Accrued interest receivable	1,638,821	1,285,842
Tax indemnification receivable	38,226	39,207
Derivative instruments	275	2,090
Total earning assets	33,324,376	28,391,625
Interest-bearing liabilities
Short-term borrowings	297,800	0
Long-term borrowings	4,725,251	4,278,931
Accrued interest payable	83,172	61,341
Derivative instruments	1,238	2,032
Total interest-bearing liabilities	27,860,792	23,226,455
Fair Value \| Money-market and savings accounts
Interest-bearing liabilities
Deposits	10,356,228	9,370,957
Fair Value \| Certificates of deposit
Interest-bearing liabilities
Deposits	12,397,103	9,513,194
Carrying Value
Earning assets
Cash and cash equivalents	3,851,608	2,559,106
Available-for-sale investments	455,968	176,245
Accrued interest receivable	1,510,458	1,191,981
Tax indemnification receivable	38,226	39,207
Derivative instruments	275	2,090
Total earning assets	30,573,199	26,239,548
Interest-bearing liabilities
Short-term borrowings	297,800	0
Long-term borrowings	4,601,888	4,284,304
Accrued interest payable	83,172	61,341
Derivative instruments	1,238	2,032
Total interest-bearing liabilities	27,612,237	23,289,759
Carrying Value \| Money-market and savings accounts
Interest-bearing liabilities
Deposits	10,328,422	9,390,108
Carrying Value \| Certificates of deposit
Interest-bearing liabilities
Deposits	12,299,717	9,551,974
Private Education Loans
Earning assets
Loans held for investment, net	2,547,095
Loans held for investment, net, difference		2,018,576
Private Education Loans \| Fair Value
Earning assets
Loans held for investment, net	25,402,823	22,313,419
Private Education Loans \| Carrying Value
Earning assets
Loans held for investment, net	22,855,728	20,294,843
FFELP Loans
Earning assets
Loans held for investment, net	11,459
Loans held for investment, net, difference		11,296
FFELP Loans \| Fair Value
Earning assets
Loans held for investment, net	810,141	859,185
FFELP Loans \| Carrying Value
Earning assets
Loans held for investment, net	798,682	847,889
Personal Loans
Earning assets
Loans held for investment, net	64,260
Loans held for investment, net, difference		28,344
Personal Loans \| Fair Value
Earning assets
Loans held for investment, net	1,126,514	1,156,531
Personal Loans \| Carrying Value
Earning assets
Loans held for investment, net	$ 1,062,254	$ 1,128,187

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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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