Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 17,432,167	$ 14,251,675
Consumer Portfolio Segment | Student Loan | With cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 15,658,539	$ 12,816,512
Private Education Loans with cosigner in percent	90.00%	90.00%
Consumer Portfolio Segment | Student Loan | Without cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,773,628	$ 1,435,163
Private Education Loans without cosigner in percent	10.00%	10.00%
Consumer Portfolio Segment | Student Loan | Cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 17,432,167	$ 14,251,675
Total in percent	100.00%	100.00%
Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 17,432,167	$ 14,251,675
Total in percent	100.00%	100.00%
Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 17,432,167	$ 14,251,675
Total in percent	100.00%	100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments, in percent	24.00%	26.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments, in percent	19.00%	20.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments, in percent	14.00%	13.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments, in percent	9.00%	7.00%
Seasoning based on monthly scheduled payments due from more than 48 payments, in percent	7.00%	5.00%
Seasoning - based on monthly scheduled payments due from not yet in repayment, in percent	27.00%	29.00%
Consumer Portfolio Segment | Personal Loans | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 400,280	$ 12,894
Total in percent	100.00%	100.00%
Consumer Portfolio Segment | Personal Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 400,280	$ 12,894
Total in percent	100.00%	100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments, in percent	100.00%	100.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments, in percent	0.00%	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 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 1,153,591	$ 920,132
Private Education Loans at origination	6.00%	6.00%
FICO score less than 670 | Consumer Portfolio Segment | Personal Loans | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 32,156	$ 1,189
Private Education Loans at origination	8.00%	9.00%
FICO score 670-699 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,596,959	$ 2,092,722
Private Education Loans at origination	15.00%	15.00%
FICO score 670-699 | Consumer Portfolio Segment | Personal Loans | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 114,731	$ 3,139
Private Education Loans at origination	29.00%	24.00%
FICO score 700-749 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 5,714,554	$ 4,639,958
Private Education Loans at origination	33.00%	33.00%
FICO score 700-749 | Consumer Portfolio Segment | Personal Loans | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 182,025	$ 5,678
Private Education Loans at origination	45.00%	44.00%
FICO score greater than 750 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 7,967,063	$ 6,598,863
Private Education Loans at origination	46.00%	46.00%
FICO score greater than 750 | Consumer Portfolio Segment | Personal Loans | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 71,368	$ 2,888
Private Education Loans at origination	18.00%	23.00%
1-12 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 4,256,592	$ 3,737,110
1-12 payments | Consumer Portfolio Segment | Personal Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	400,280	12,894
13-24 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	3,229,465	2,841,107
13-24 payments | Consumer Portfolio Segment | Personal Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	0	0
25-36 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	2,429,238	1,839,764
25-36 payments | Consumer Portfolio Segment | Personal Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	0	0
37-48 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	1,502,327	917,633
37-48 payments | Consumer Portfolio Segment | Personal Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	0	0
More than 48 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	1,256,813	726,106
More than 48 payments | Consumer Portfolio Segment | Personal Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	0	0
Not yet in repayment | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 4,757,732	$ 4,189,955