Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) - Consumer Portfolio Segment - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Student Loan | With cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 18,378,398	$ 15,658,539
Private Education Loans with cosigner in percent	90.00%	90.00%
Student Loan | Without cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,126,067	$ 1,773,628
Private Education Loans without cosigner in percent	10.00%	10.00%
Student Loan | Cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 20,504,465	$ 17,432,167
Total in percent	100.00%	100.00%
Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 20,504,465	$ 17,432,167
Total in percent	100.00%	100.00%
Student Loan | School FICO, Refreshed Amounts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 20,504,465	$ 17,432,167
Total in percent	100.00%	100.00%
Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 20,504,465	$ 17,432,167
Total in percent	100.00%	100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments, in percent	24.00%	24.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments, in percent	17.00%	19.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments, in percent	13.00%	14.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments, in percent	10.00%	9.00%
Seasoning based on monthly scheduled payments due from more than 48 payments, in percent	10.00%	7.00%
Seasoning - based on monthly scheduled payments due from not yet in repayment, in percent	26.00%	27.00%
Personal Loans | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,190,091	$ 400,280
Total in percent	100.00%	100.00%
Personal Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,190,091	$ 400,280
Total in percent	100.00%	100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments, in percent	85.00%	100.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments, in percent	15.00%	0.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments, in percent	0.00%	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%
FICO score less than 670 | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,409,789	$ 1,153,591
Private Education Loans at origination	7.00%	6.00%
FICO score less than 670 | Student Loan | School FICO, Refreshed Amounts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,416,979	$ 1,964,962
Private Education Loans at origination	12.00%	11.00%
FICO score less than 670 | Personal Loans | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 77,702	$ 32,156
Private Education Loans at origination	7.00%	8.00%
FICO score 670-699 | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 3,106,983	$ 2,596,959
Private Education Loans at origination	15.00%	15.00%
FICO score 670-699 | Student Loan | School FICO, Refreshed Amounts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,504,467	$ 2,138,379
Private Education Loans at origination	12.00%	12.00%
FICO score 670-699 | Personal Loans | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 339,053	$ 114,731
Private Education Loans at origination	28.00%	29.00%
FICO score 700-749 | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 6,759,721	$ 5,714,554
Private Education Loans at origination	33.00%	33.00%
FICO score 700-749 | Student Loan | School FICO, Refreshed Amounts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 6,144,489	$ 5,220,245
Private Education Loans at origination	30.00%	30.00%
FICO score 700-749 | Personal Loans | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 554,700	$ 182,025
Private Education Loans at origination	47.00%	45.00%
FICO score greater than 750 | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 9,227,972	$ 7,967,063
Private Education Loans at origination	45.00%	46.00%
FICO score greater than 750 | Student Loan | School FICO, Refreshed Amounts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 9,438,530	$ 8,108,581
Private Education Loans at origination	46.00%	47.00%
FICO score greater than 750 | Personal Loans | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 218,636	$ 71,368
Private Education Loans at origination	18.00%	18.00%
1-12 payments | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 4,969,334	$ 4,256,592
1-12 payments | Personal Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	1,008,758	400,280
13-24 payments | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	3,481,235	3,229,465
13-24 payments | Personal Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	181,333	0
25-36 payments | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	2,741,954	2,429,238
25-36 payments | Personal Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	0	0
37-48 payments | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	1,990,049	1,502,327
37-48 payments | Personal Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	0	0
More than 48 payments | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	2,061,448	1,256,813
More than 48 payments | Personal Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	0	0
Not yet in repayment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 5,260,445	$ 4,757,732