Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 8,311,376		$ 6,563,342
Private Education Loans | Cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans with cosigner	7,465,339	[1]	5,898,751	[1]
Private Education Loans without cosigner	846,037	[1]	664,591	[1]
Private Education Loans with cosigner in percent	90.00%		90.00%
Private Education Loans without cosigner in percent	10.00%		10.00%
Total	8,311,376	[1]	6,563,342	[1]
Total in percent	100.00%		100.00%
Private Education Loans | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	8,311,376	[1]	6,563,342	[1]
Total in percent	100.00%		100.00%
Private Education Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	8,311,376	[1],[2]	6,563,342	[1],[2]
Seasoning - based on monthly scheduled payments due from 1-12 payments	2,373,117	[1],[2]	1,840,538	[1],[2]
Seasoning - based on monthly scheduled payments due from 13-24 payments	1,532,042	[1],[2]	1,085,393	[1],[2]
Seasoning - based on monthly scheduled payments due from 25-36 payments	755,143	[1],[2]	669,685	[1],[2]
Seasoning - based on monthly scheduled payments due from 37-48 payments	411,493	[1],[2]	362,124	[1],[2]
Seasoning - based on monthly scheduled payments due from more than 48 payments	212,438	[1],[2]	30,891	[1],[2]
Seasoning - based on monthly scheduled payments due from not yet in repayment	3,027,143	[1],[2]	2,574,711	[1],[2]
Seasoning based on monthly scheduled payments due from 1-12 payments, in percent	29.00%	[2]	28.00%	[2]
Seasoning based on monthly scheduled payments due from 13 - 24 payments, in percent	18.00%	[2]	17.00%	[2]
Seasoning based on monthly scheduled payments due from 25 - 36 payments, in percent	9.00%	[2]	10.00%	[2]
Seasoning based on monthly scheduled payments due from 37 - 48 payments, in percent	5.00%	[2]	6.00%	[2]
Seasoning based on monthly scheduled payments due from more than 48 payments, in percent	3.00%	[2]	0.00%	[2]
Seasoning - based on monthly scheduled payments due from not yet in repayment, in percent	36.00%	[2]	39.00%	[2]
Total in percent	100.00%	[2]	100.00%	[2]
FICO score greater than 670 | Private Education Loans | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	558,801	[1]	461,412	[1]
Total in percent	7.00%		7.00%
FICO score 670-709 | Private Education Loans | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,227,860	[1]	1,364,286	[1]
Total in percent	15.00%		21.00%
FICO score 710-749 | Private Education Loans | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2,626,238	[1]	1,649,192	[1]
Total in percent	32.00%		25.00%
FICO score greater than 750 | Private Education Loans | School Type/FICO Scores
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 3,898,477	[1]	$ 3,088,452	[1]
Total in percent	46.00%		47.00%

[1] Balance represents gross Private Education Loans.

[2] Number of months in active repayment for which a scheduled payment was due.