Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Allowance for Loan Losses
Beginning balance	$ 77,634		$ 69,580		$ 68,081		$ 69,189
Total provision	1,014		(1,015)		40,173		19,677
Charge-offs	(654)	[1]	(534)	[1]	(1,297)	[1]	(754)	[1]
Student loan sales	(17,467)	[2]	(12,546)	[2]	(46,430)	[2]	(32,627)	[2]
Ending Balance	60,527		55,485		60,527		55,485
Allowance:
Ending balance: individually evaluated for impairment	1,037		0		1,037		0
Ending balance: collectively evaluated for impairment	59,490		55,485		59,490		55,485
Loans:
Ending balance: individually evaluated for impairment	4,508		0		4,508		0
Ending balance: collectively evaluated for impairment	8,838,393		6,545,604		8,838,393		6,545,604
Ffelp Loans
Allowance for Loan Losses
Beginning balance	6,181		4,199		6,318		3,971
Total provision	685		951		1,191		1,399
Charge-offs	(654)	[1]	(534)	[1]	(1,297)	[1]	(754)	[1]
Student loan sales	0	[2]	0	[2]	0	[2]	0	[2]
Ending Balance	6,212		4,616		6,212		4,616
Allowance:
Ending balance: individually evaluated for impairment	0		0		0		0
Ending balance: collectively evaluated for impairment	6,212		4,616		6,212		4,616
Loans:
Ending balance: individually evaluated for impairment	0		0		0		0
Ending balance: collectively evaluated for impairment	1,360,107		1,162,476		1,360,107		1,162,476
Charge-offs as a percentage of average loans in repayment	0.07%		0.06%		0.13%		0.09%
Allowance as a percentage of the ending total loan balance	0.46%		0.40%		0.46%		0.40%
Allowance as a percentage of the ending loans in repayment	0.66%		0.56%		0.66%		0.56%
Allowance coverage of charge-offs	2.40		2.16		2.40		3.00
Ending total loans, gross	1,360,107		1,162,476		1,360,107		1,162,476
Average loans in repayment	973,894		825,038		994,290		825,038
Ending loans in repayment	947,972		824,523		947,972		824,523
Private Education Loans
Allowance for Loan Losses
Beginning balance	71,453		65,381		61,763		65,218
Total provision	329		(1,966)		38,982		18,278
Charge-offs	0	[1]	0	[1]	0	[1]	0	[1]
Student loan sales	(17,467)	[2]	(12,546)	[2]	(46,430)	[2]	(32,627)	[2]
Ending Balance	54,315		50,869		54,315		50,869
Allowance:
Ending balance: individually evaluated for impairment	1,037		0		1,037		0
Ending balance: collectively evaluated for impairment	53,278		50,869		53,278		50,869
Loans:
Ending balance: individually evaluated for impairment	4,508		0		4,508		0
Ending balance: collectively evaluated for impairment	7,478,286		5,383,128		7,478,286		5,383,128
Charge-offs as a percentage of average loans in repayment	0.00%		0.00%		0.00%		0.00%
Allowance as a percentage of the ending total loan balance	0.73%		0.94%		0.73%		0.94%
Allowance as a percentage of the ending loans in repayment	1.23%		1.65%		1.23%		1.65%
Allowance coverage of charge-offs	0		0		0		0.0000
Ending total loans, gross	7,482,794		5,383,128		7,482,794		5,383,128
Average loans in repayment	4,322,356		3,243,513		4,354,878		3,670,291
Ending loans in repayment	$ 4,425,573		$ 3,081,929		$ 4,425,573		$ 3,081,929

[1] Prior to the Spin-Off, Private Education Loans were sold to an entity that is now a subsidiary of Navient prior to being charged-off.

[2] Represents fair value write-downs on delinquent loans sold prior to the Spin-Off to an entity that is now a subsidiary of Navient, recorded at the time of sale.