Allowance for Loan Losses - Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) - Consumer Portfolio Segment - Student Loan - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Private Education Loans \| With Cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 18,147,180	$ 15,658,539
Private Education Loans with cosigner (in percentage)	90.00%	90.00%
Private Education Loans \| Without Cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,092,811	$ 1,773,628
Private Education Loans without cosigner (in percentage)	10.00%	10.00%
Private Education Loans \| Cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 20,239,991	$ 17,432,167
Total in percent	100.00%	100.00%
Private Education Loans \| FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 20,239,991	$ 17,432,167
Total in percent	100.00%	100.00%
Private Education Loans \| Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 20,239,991	$ 17,432,167
Total in percent	100.00%	100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments (in percentage)	27.00%	24.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments (in percentage)	16.00%	19.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments (in percentage)	13.00%	14.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments (in percentage)	8.00%	9.00%
Seasoning based on monthly scheduled payments due from more than 48 payments (in percentage)	7.00%	7.00%
Seasoning based on monthly scheduled payments due from not yet in repayment (in percentage)	29.00%	27.00%
Private Education Loans \| FICO score less than 670 \| FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,373,155	$ 1,153,591
Private Education Loans At Origination	7.00%	6.00%
Private Education Loans \| FICO score 670-699 \| FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 3,042,228	$ 2,596,959
Private Education Loans At Origination	15.00%	15.00%
Private Education Loans \| FICO score 700-749 \| FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 6,655,405	$ 5,714,554
Private Education Loans At Origination	33.00%	33.00%
Private Education Loans \| FICO score greater than or equal to 750 \| FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 9,169,203	$ 7,967,063
Private Education Loans At Origination	45.00%	46.00%
Private Education Loans \| 1-12 payments \| Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 5,419,487	$ 4,256,592
Private Education Loans \| 13-24 payments \| Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	3,238,910	3,229,465
Private Education Loans \| 25-36 payments \| Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	2,533,308	2,429,238
Private Education Loans \| 37-48 payments \| Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	1,638,309	1,502,327
Private Education Loans \| More than 48 payments \| Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	1,473,944	1,256,813
Private Education Loans \| Not yet in repayment \| Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	5,936,033	4,757,732
Personal Loans \| FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,133,005	$ 400,280
Total in percent	100.00%	100.00%
Personal Loans \| Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,133,005	$ 400,280
Total in percent	100.00%	100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments (in percentage)	96.00%	100.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments (in percentage)	4.00%	0.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments (in percentage)	0.00%	0.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments (in percentage)	0.00%	0.00%
Seasoning based on monthly scheduled payments due from more than 48 payments (in percentage)	0.00%	0.00%
Personal Loans \| FICO score less than 670 \| FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 79,769	$ 32,156
Private Education Loans At Origination	7.00%	8.00%
Personal Loans \| FICO score 670-699 \| FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 327,364	$ 114,731
Private Education Loans At Origination	29.00%	29.00%
Personal Loans \| FICO score 700-749 \| FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 520,370	$ 182,025
Private Education Loans At Origination	46.00%	45.00%
Personal Loans \| FICO score greater than or equal to 750 \| FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 205,502	$ 71,368
Private Education Loans At Origination	18.00%	18.00%
Personal Loans \| 1-12 payments \| Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,084,706	$ 400,280
Personal Loans \| 13-24 payments \| Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	48,299	0
Personal Loans \| 25-36 payments \| Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	0	0
Personal Loans \| 37-48 payments \| Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	0	0
Personal Loans \| More than 48 payments \| Seasoning
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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Seasoning based on monthly scheduled payments due from thirteen to twenty four payments in percent

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- Definition

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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