Fair value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Earning assets:
Accrued Interest Receivable, Fair Value Disclosure, Different Between Carrying Value And Fair Value	$ 93,861
Income taxes receivable, net	41,570	$ 0
Derivative instruments	701	0
Total earning assets, difference	2,152,077	2,105,495
Interest-bearing liabilities:
Long-term borrowings, difference	5,373	(24,601)
Total interest-bearing liabilities, difference	63,304	(34,688)
Excess of net asset fair value over carrying value	2,215,381	2,070,807
Certificates of Deposit
Interest-bearing liabilities:
Certificates of deposits, difference	38,780	(10,087)
Fair Value
Earning assets:
Cash and cash equivalents	2,559,106	1,534,339
Available-for-sale investments	176,245	244,088
Accrued interest receivable	1,285,842	967,482
Income taxes receivable, net	39,207	168,011
Derivative instruments	2,090	812
Total	28,391,625	23,587,868
Interest-bearing liabilities:
Short-term borrowings	0	0
Long-term borrowings	4,278,931	3,299,871
Accrued interest payable	61,341	35,363
Derivative instruments	2,032	2,584
Total interest-bearing liabilities	23,226,455	18,852,235
Fair Value \| Money-market and savings accounts
Interest-bearing liabilities:
Deposits	9,370,957	8,470,209
Fair Value \| Certificates of Deposit
Interest-bearing liabilities:
Deposits	9,513,194	7,044,208
Carrying Value
Earning assets:
Cash and cash equivalents	2,559,106	1,534,339
Available-for-sale investments	176,245	244,088
Accrued interest receivable	1,191,981	967,482
Income taxes receivable, net	39,207	168,011
Derivative instruments	2,090	812
Total	26,239,548	21,482,373
Interest-bearing liabilities:
Short-term borrowings	0	0
Long-term borrowings	4,284,304	3,275,270
Accrued interest payable	61,341	35,363
Derivative instruments	2,032	2,584
Total interest-bearing liabilities	23,289,759	18,817,547
Carrying Value \| Money-market and savings accounts
Interest-bearing liabilities:
Deposits	9,390,108	8,470,209
Certificates of deposits, difference	19,151
Carrying Value \| Certificates of Deposit
Interest-bearing liabilities:
Deposits	9,551,974	7,034,121
Private Education Loans
Earning assets:
Loans held for investment, net, difference	2,018,576	2,082,214
Private Education Loans \| Fair Value
Earning assets:
Private Education Loans	22,313,419	19,327,044
Private Education Loans \| Carrying Value
Earning assets:
Private Education Loans	20,294,843	17,244,830
FFELP Loans
Earning assets:
Loans held for investment, net, difference	11,296	17,054
FFELP Loans \| Fair Value
Earning assets:
Private Education Loans	859,185	946,213
FFELP Loans \| Carrying Value
Earning assets:
Private Education Loans	847,889	929,159
Personal Loans
Earning assets:
Loans held for investment, net, difference	28,344	6,227
Personal Loans \| Fair Value
Earning assets:
Private Education Loans	1,156,531	399,879
Personal Loans \| Carrying Value
Earning assets:
Private Education Loans	$ 1,128,187	$ 393,652

Accrued Interest Receivable, Fair Value Disclosure

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Accrued Interest Receivable, Fair Value Disclosure, Different Between Carrying Value And Fair Value

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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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