Allowance for Loan Losses - Private Education Loan Portfolio Stratified by Key Credit Quality Indicators (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 7,829,420		$ 6,563,342
Private Education Loans | Cosigners
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loans with cosigner	7,032,973	[1]	5,898,751	[1]
Private Education Loans without cosigner	796,447	[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	7,829,420	[1]	6,563,342	[1]
Total in percent	100.00%		100.00%
Private Education Loans | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	7,829,420	[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	7,829,420	[1],[2]	6,563,342	[1],[2]
Seasoning - based on monthly scheduled payments due from 1-12 payments	2,683,899	[1],[2]	1,840,538	[1],[2]
Seasoning - based on monthly scheduled payments due from 13-24 payments	1,084,549	[1],[2]	1,085,393	[1],[2]
Seasoning - based on monthly scheduled payments due from 25-36 payments	515,960	[1],[2]	669,685	[1],[2]
Seasoning - based on monthly scheduled payments due from 37-48 payments	308,387	[1],[2]	362,124	[1],[2]
Seasoning - based on monthly scheduled payments due from more than 48 payments	58,130	[1],[2]	30,891	[1],[2]
Seasoning - based on monthly scheduled payments due from not yet in repayment	3,178,495	[1],[2]	2,574,711	[1],[2]
Seasoning based on monthly scheduled payments due from 1-12 payments, in percent	34.00%	[2]	28.00%	[2]
Seasoning based on monthly scheduled payments due from 13 - 24 payments, in percent	14.00%	[2]	17.00%	[2]
Seasoning based on monthly scheduled payments due from 25 - 36 payments, in percent	6.00%	[2]	10.00%	[2]
Seasoning based on monthly scheduled payments due from 37 - 48 payments, in percent	4.00%	[2]	6.00%	[2]
Seasoning based on monthly scheduled payments due from more than 48 payments, in percent	0.00%	[2]	0.00%	[2]
Seasoning - based on monthly scheduled payments due from not yet in repayment, in percent	42.00%	[2]	39.00%	[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	529,649	[1]	461,412	[1]
Total in percent	7.00%		7.00%
FICO score 670-699 | Private Education Loans | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	1,154,088	[1]	1,364,286	[1]
Total in percent	15.00%		21.00%
FICO score 700-749 | Private Education Loans | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	2,465,490	[1]	1,649,192	[1]
Total in percent	31.00%		25.00%
FICO score greater than or equal to 750 | Private Education Loans | FICO at Origination
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 3,680,193	[1]	$ 3,088,452	[1]
Total in percent	47.00%		47.00%

[1] Balance represents gross Private Education Loans.

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