Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Earning assets
Accrued interest receivable	$ 110,232
Accrued interest receivable, difference		$ 93,861
Income Taxes Receivable	0	41,570
Total earning assets	2,444,338
Total earning assets, difference		2,152,077
Interest-bearing liabilities
Long-term borrowings, difference	(29,422)	5,373
Total interest-bearing liabilities, difference	(57,756)	63,304
Excess of net asset fair value over carrying value	2,386,582	2,215,381
Money-market and savings accounts
Interest-bearing liabilities
Deposits, difference		19,151
Certificates of deposit
Interest-bearing liabilities
Deposits, difference	(26,878)	38,780
Fair Value
Earning assets
Cash and cash equivalents	2,156,257	2,559,106
Available-for-sale investments	207,907	176,245
Accrued interest receivable	1,409,728	1,285,842
Income Taxes Receivable	43,124	39,207
Derivative instruments	2,060	2,090
Total earning assets	29,651,568	28,391,625
Interest-bearing liabilities
Long-term borrowings	4,505,828	4,278,931
Accrued interest payable	68,746	61,341
Derivative instruments	3,981	2,032
Total interest-bearing liabilities	24,269,179	23,226,455
Fair Value \| Money-market and savings accounts
Interest-bearing liabilities
Deposits	9,690,078	9,370,957
Fair Value \| Certificates of deposit
Interest-bearing liabilities
Deposits	10,000,546	9,513,194
Carrying Value
Earning assets
Cash and cash equivalents	2,156,257	2,559,106
Available-for-sale investments	207,907	176,245
Accrued interest receivable	1,299,496	1,191,981
Income Taxes Receivable	43,124	39,207
Derivative instruments	2,060	2,090
Total earning assets	27,207,230	26,239,548
Interest-bearing liabilities
Long-term borrowings	4,476,406	4,284,304
Accrued interest payable	68,746	61,341
Derivative instruments	3,981	2,032
Total interest-bearing liabilities	24,211,423	23,289,759
Carrying Value \| Money-market and savings accounts
Interest-bearing liabilities
Deposits	9,688,622	9,390,108
Deposits, difference	(1,456)
Carrying Value \| Certificates of deposit
Interest-bearing liabilities
Deposits	9,973,668	9,551,974
Private Education Loans
Earning assets
Loans held for investment, net	2,277,481
Loans held for investment, net, difference		2,018,576
Private Education Loans \| Fair Value
Earning assets
Loans held for investment, net	23,854,015	22,313,419
Private Education Loans \| Carrying Value
Earning assets
Loans held for investment, net	21,576,534	20,294,843
Ffelp Loans
Earning assets
Loans held for investment, net	11,867
Loans held for investment, net, difference		11,296
Ffelp Loans \| Fair Value
Earning assets
Loans held for investment, net	841,070	859,185
Ffelp Loans \| Carrying Value
Earning assets
Loans held for investment, net	829,203	847,889
Personal Loans
Earning assets
Loans held for investment, net	44,758
Loans held for investment, net, difference		28,344
Personal Loans \| Fair Value
Earning assets
Loans held for investment, net	1,137,407	1,156,531
Personal Loans \| Carrying Value
Earning assets
Loans held for investment, net	$ 1,092,649	$ 1,128,187

Accrued Interest Receivable, Fair Value Disclosure

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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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