Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) - Consumer Portfolio Segment - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
With cosigner \| Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 20,709,636	$ 18,378,398
Private Education Loans with cosigner in percent	89.00%	90.00%
Without cosigner \| Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,479,955	$ 2,126,067
Private Education Loans without cosigner in percent	11.00%	10.00%
Cosigners \| Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 23,189,591	$ 20,504,465
Total in percent	100.00%	100.00%
School Type/FICO Scores \| Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 23,189,591	$ 20,504,465
Total in percent	100.00%	100.00%
School Type/FICO Scores \| Student Loan \| FICO score less than 670
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,665,589	$ 1,409,789
Private Education Loans at origination	7.00%	7.00%
School Type/FICO Scores \| Student Loan \| FICO score 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 3,570,025	$ 3,106,983
Private Education Loans at origination	16.00%	15.00%
School Type/FICO Scores \| Student Loan \| FICO score 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 7,670,748	$ 6,759,721
Private Education Loans at origination	33.00%	33.00%
School Type/FICO Scores \| Student Loan \| FICO score greater than 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 10,283,229	$ 9,227,972
Private Education Loans at origination	44.00%	45.00%
School Type/FICO Scores \| Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,049,007	$ 1,190,091
Total in percent	100.00%	100.00%
School Type/FICO Scores \| Personal Loans \| FICO score less than 670
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 47,367	$ 77,702
Private Education Loans at origination	4.00%	7.00%
School Type/FICO Scores \| Personal Loans \| FICO score 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 259,098	$ 339,053
Private Education Loans at origination	25.00%	28.00%
School Type/FICO Scores \| Personal Loans \| FICO score 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 521,856	$ 554,700
Private Education Loans at origination	50.00%	47.00%
School Type/FICO Scores \| Personal Loans \| FICO score greater than 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 220,686	$ 218,636
Private Education Loans at origination	21.00%	18.00%
School FICO, Refreshed Amounts \| Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 23,189,591	$ 20,504,465
Total in percent	100.00%	100.00%
School FICO, Refreshed Amounts \| Student Loan \| FICO score less than 670
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,979,437	$ 2,416,979
Private Education Loans at origination	13.00%	12.00%
School FICO, Refreshed Amounts \| Student Loan \| FICO score 670-699
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,883,122	$ 2,504,467
Private Education Loans at origination	13.00%	12.00%
School FICO, Refreshed Amounts \| Student Loan \| FICO score 700-749
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 6,806,602	$ 6,144,489
Private Education Loans at origination	29.00%	30.00%
School FICO, Refreshed Amounts \| Student Loan \| FICO score greater than 750
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 10,520,430	$ 9,438,530
Private Education Loans at origination	45.00%	46.00%
Seasoning \| Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 23,189,591	$ 20,504,465
Total in percent	100.00%	100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments, in percent	23.00%	24.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments, in percent	17.00%	17.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments, in percent	12.00%	13.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments, in percent	10.00%	10.00%
Seasoning based on monthly scheduled payments due from more than 48 payments, in percent	13.00%	10.00%
Seasoning - based on monthly scheduled payments due from not yet in repayment, in percent	25.00%	26.00%
Seasoning \| Student Loan \| 1-12 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 5,351,702	$ 4,969,334
Seasoning \| Student Loan \| 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	4,004,151	3,481,235
Seasoning \| Student Loan \| 25-36 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	2,902,365	2,741,954
Seasoning \| Student Loan \| 37-48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	2,213,944	1,990,049
Seasoning \| Student Loan \| More than 48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	3,030,024	2,061,448
Seasoning \| Student Loan \| Not yet in repayment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	5,687,405	5,260,445
Seasoning \| Personal Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,049,007	$ 1,190,091
Total in percent	100.00%	100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments, in percent	45.00%	85.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments, in percent	48.00%	15.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments, in percent	7.00%	0.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments, in percent	0.00%	0.00%
Seasoning based on monthly scheduled payments due from more than 48 payments, in percent	0.00%	0.00%
Seasoning \| Personal Loans \| 1-12 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 469,940	$ 1,008,758
Seasoning \| Personal Loans \| 13-24 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	505,318	181,333
Seasoning \| Personal Loans \| 25-36 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	73,749	0
Seasoning \| Personal Loans \| 37-48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	0	0
Seasoning \| Personal Loans \| More than 48 payments
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 0	$ 0

Percent of private loans with cosigner.

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Percent of private loans without cosigner.

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Private Education Loans At Origination

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Private education loans gross.

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Private education loans gross in percent.

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Seasoning based on monthly scheduled payments due from more than forty eight payments in percent

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Seasoning based on monthly scheduled payments due from not yet in repayment in percent

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Seasoning based on monthly scheduled payments due from one to twelve payments in percent.

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Seasoning based on monthly scheduled payments due from thirteen to twenty four payments in percent.

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Seasoning based on monthly scheduled payments due from thirty seven to forty eight payments in percent.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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