Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 14,251,675	$ 10,596,437
Consumer Portfolio Segment | Student Loan | With cosigner
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 12,816,512	$ 9,515,136
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	$ 1,435,163	$ 1,081,301
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	$ 14,251,675	$ 10,596,437
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	$ 14,251,675	$ 10,596,437
Total in percent	100.00%	100.00%
Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 14,251,675	$ 10,596,437
Total in percent	100.00%	100.00%
Seasoning based on monthly scheduled payments due from 1-12 payments, in percent	26.00%	29.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments, in percent	20.00%	20.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments, in percent	13.00%	10.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments, in percent	7.00%	5.00%
Seasoning based on monthly scheduled payments due from more than 48 payments, in percent	5.00%	4.00%
Seasoning - based on monthly scheduled payments due from not yet in repayment, in percent	29.00%	32.00%
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	$ 920,132	$ 700,779
Private Education Loans at origination	6.00%	7.00%
FICO score 670-699 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 2,092,722	$ 1,554,959
Private Education Loans at origination	15.00%	15.00%
FICO score 700-749 | Consumer Portfolio Segment | Student Loan | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 4,639,958	$ 3,403,823
Private Education Loans at origination	33.00%	32.00%
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	$ 6,598,863	$ 4,936,876
Private Education Loans at origination	46.00%	46.00%
1-12 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 3,737,110	$ 3,059,901
13-24 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	2,841,107	2,096,412
25-36 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	1,839,764	1,084,818
37-48 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	917,633	513,125
More than 48 payments | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	726,106	414,217
Not yet in repayment | Consumer Portfolio Segment | Student Loan | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans	$ 4,189,955	$ 3,427,964