Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 9,768,761		$ 8,311,376
Private Education Loans | Cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans with cosigner	8,804,812	[1]	7,465,339	[1]
Private Education Loans without cosigner	963,949	[1]	846,037	[1]
Private Education Loans with cosigner in percent	90.00%		90.00%
Private Education Loans without cosigner in percent	10.00%		10.00%
Total	9,768,761	[1]	8,311,376	[1]
Total in percent	100.00%		100.00%
Private Education Loans | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	9,768,761	[1]	8,311,376	[1]
Total in percent	100.00%		100.00%
Private Education Loans | Seasoning
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	9,768,761	[1],[2]	8,311,376	[1],[2]
Seasoning - based on monthly scheduled payments due from 1-12 payments	3,031,655	[1],[2]	2,373,117	[1],[2]
Seasoning - based on monthly scheduled payments due from 13-24 payments	1,650,103	[1],[2]	1,532,042	[1],[2]
Seasoning - based on monthly scheduled payments due from 25-36 payments	812,674	[1],[2]	755,143	[1],[2]
Seasoning - based on monthly scheduled payments due from 37-48 payments	412,865	[1],[2]	411,493	[1],[2]
Seasoning - based on monthly scheduled payments due from more than 48 payments	257,986	[1],[2]	212,438	[1],[2]
Seasoning - based on monthly scheduled payments due from not yet in repayment	3,603,478	[1],[2]	3,027,143	[1],[2]
Seasoning based on monthly scheduled payments due from 1-12 payments, in percent	31.00%	[2]	29.00%	[2]
Seasoning based on monthly scheduled payments due from 13 - 24 payments, in percent	17.00%	[2]	18.00%	[2]
Seasoning based on monthly scheduled payments due from 25 - 36 payments, in percent	8.00%	[2]	9.00%	[2]
Seasoning based on monthly scheduled payments due from 37 - 48 payments, in percent	4.00%	[2]	5.00%	[2]
Seasoning based on monthly scheduled payments due from more than 48 payments, in percent	3.00%	[2]	3.00%	[2]
Seasoning - based on monthly scheduled payments due from not yet in repayment, in percent	37.00%	[2]	36.00%	[2]
Total in percent	100.00%	[2]	100.00%	[2]
FICO score less than 670 | Private Education Loans | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	638,195	[1]	558,801	[1]
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	1,431,436	[1]	1,227,860	[1]
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	3,103,824	[1]	2,626,238	[1]
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	$ 4,595,306	[1]	$ 3,898,477	[1]
Total in percent	47.00%		46.00%

[1] Balance represents gross Private Education Loans.

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