Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 9,312,937	$ 8,311,376
Private Education Loans | Cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans with cosigner	8,341,122	7,465,339
Private Education Loans without cosigner	971,815	846,037
Total	$ 9,312,937	$ 8,311,376
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 Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 9,312,937	$ 8,311,376
Total in percent	100.00%	100.00%
Private Education Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 9,312,937	$ 8,311,376
Total in percent	100.00%	100.00%
Seasoning - based on monthly scheduled payments due from 1-12 payments	$ 3,052,670	$ 2,373,117
Seasoning - based on monthly scheduled payments due from 13-24 payments	1,545,880	1,532,042
Seasoning - based on monthly scheduled payments due from 25-36 payments	773,371	755,143
Seasoning - based on monthly scheduled payments due from 37-48 payments	380,153	411,493
Seasoning - based on monthly scheduled payments due from more than 48 payments	256,692	212,438
Seasoning - based on monthly scheduled payments due from not yet in repayment	$ 3,304,171	$ 3,027,143
Seasoning based on monthly scheduled payments due from 1-12 payments, in percent	33.00%	29.00%
Seasoning based on monthly scheduled payments due from 13 - 24 payments, in percent	17.00%	18.00%
Seasoning based on monthly scheduled payments due from 25 - 36 payments, in percent	8.00%	9.00%
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	3.00%	3.00%
Seasoning - based on monthly scheduled payments due from not yet in repayment, in percent	35.00%	36.00%
FICO score less than 670 | Private Education Loans | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 614,918	$ 558,801
Total in percent	6.00%	7.00%
FICO score 670-699 | Private Education Loans | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 1,371,546	$ 1,227,860
Total in percent	15.00%	15.00%
FICO score 700-749 | Private Education Loans | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 2,970,077	$ 2,626,238
Total in percent	32.00%	32.00%
FICO score greater than or equal to 750 | Private Education Loans | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 4,356,396	$ 3,898,477
Total in percent	47.00%	46.00%