Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 13,848,262	$ 10,596,437
Private Education Loans | Cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans with cosigner	12,456,310	9,515,136
Private Education Loans without cosigner	1,391,952	1,081,301
Total	$ 13,848,262	$ 10,596,437
Private Education Loans with cosigner in percent	90.00%	90.00%
Private Education Loans without cosigner in percent	10.00%	10.00%
Total in percent	100.00%	100.00%
Private Education Loans | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 13,848,262	$ 10,596,437
Total in percent	100.00%	100.00%
Private Education Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 13,848,262	$ 10,596,437
Total in percent	100.00%	100.00%
Seasoning - based on monthly scheduled payments due from 1-12 payments	$ 4,307,106	$ 3,059,901
Seasoning - based on monthly scheduled payments due from 13-24 payments	2,398,396	2,096,412
Seasoning - based on monthly scheduled payments due from 25-36 payments	1,357,242	1,084,818
Seasoning - based on monthly scheduled payments due from 37-48 payments	630,420	513,125
Seasoning - based on monthly scheduled payments due from more than 48 payments	492,157	414,217
Seasoning - based on monthly scheduled payments due from not yet in repayment	$ 4,662,941	$ 3,427,964
Seasoning based on monthly scheduled payments due from 1-12 payments, in percent	31.00%	29.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments, in percent	17.00%	20.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments, in percent	10.00%	10.00%
Seasoning based on monthly scheduled payments due from 37 - 48 payments, in percent	4.00%	5.00%
Seasoning based on monthly scheduled payments due from more than 48 payments, in percent	4.00%	4.00%
Seasoning - based on monthly scheduled payments due from not yet in repayment, in percent	34.00%	32.00%
FICO score less than 670 | Private Education Loans | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 889,151	$ 700,779
Total in percent	6.00%	7.00%
FICO score 670-699 | Private Education Loans | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 2,025,444	$ 1,554,959
Total in percent	15.00%	15.00%
FICO score 700-749 | Private Education Loans | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 4,492,235	$ 3,403,823
Total in percent	32.00%	32.00%
FICO score greater than or equal to 750 | Private Education Loans | FICO at Original Approval
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 6,441,432	$ 4,936,876
Total in percent	47.00%	46.00%