Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Allowance for Loan Losses
Beginning balance	$ 60,527		$ 55,484		$ 68,081		$ 69,189
Total provision	14,898		20,404		55,071		40,081
Charge-offs	(5,139)	[1]	(671)	[1]	(6,436)	[1]	(1,425)	[1]
Student loan sales	(4,571)	[2]	(15,632)	[3]	(51,001)	[4]	(48,260)	[3]
Ending Balance	65,715		59,585		65,715		59,585
Allowance:
Ending balance: individually evaluated for impairment	2,966		0		2,966		0
Ending balance: collectively evaluated for impairment	62,749		59,585		62,749		59,585
Loans:
Ending balance: individually evaluated for impairment	13,115		0		13,115		0
Ending balance: collectively evaluated for impairment	9,134,268		7,432,244		9,134,268		7,432,244
FFELP Loans
Allowance for Loan Losses
Beginning balance	6,212		4,616		6,318		3,971
Total provision	291		1,403		1,482		2,802
Charge-offs	(761)	[1]	(671)	[1]	(2,058)	[1]	(1,425)	[1]
Student loan sales	0	[2]	0	[3]	0	[4]	0	[3]
Ending Balance	5,742		5,348		5,742		5,348
Allowance:
Ending balance: individually evaluated for impairment	0		0		0		0
Ending balance: collectively evaluated for impairment	5,742		5,348		5,742		5,348
Loans:
Ending balance: individually evaluated for impairment	0		0		0		0
Ending balance: collectively evaluated for impairment	1,317,963		1,217,404		1,317,963		1,217,404
Charge-offs as a percentage of average loans in repayment (annualized)	0.32%		0.30%		0.28%		0.23%
Allowance as a percentage of the ending total loan balance	0.44%		0.44%		0.44%		0.44%
Allowance as a percentage of the ending loans in repayment	0.61%		0.59%		0.61%		0.59%
Allowance coverage of charge-offs (annualized)	1.89		1.99		2.09		2.81
Ending total loans, gross	1,317,963		1,217,404		1,317,963		1,217,404
Average loans in repayment	953,620		896,801		980,733		839,085
Ending loans in repayment	945,230		902,766		945,230		902,766
Private Education Loans
Allowance for Loan Losses
Beginning balance	54,315		50,868		61,763		65,218
Total provision	14,607		19,001		53,589		37,279
Charge-offs	(4,378)	[1]	0	[1]	(4,378)	[1]	0	[1]
Student loan sales	(4,571)	[2]	(15,632)	[3]	(51,001)	[4]	(48,260)	[3]
Ending Balance	59,973		54,237		59,973		54,237
Allowance:
Ending balance: individually evaluated for impairment	2,966		0		2,966		0
Ending balance: collectively evaluated for impairment	57,007		54,237		57,007		54,237
Loans:
Ending balance: individually evaluated for impairment	13,115		0		13,115		0
Ending balance: collectively evaluated for impairment	7,816,305		6,214,840		7,816,305		6,214,840
Charge-offs as a percentage of average loans in repayment (annualized)	0.39%		0.00%		0.13%		0.00%
Allowance as a percentage of the ending total loan balance	0.77%		0.87%		0.77%		0.87%
Allowance as a percentage of the ending loans in repayment	1.31%		1.54%		1.31%		1.54%
Allowance coverage of charge-offs (annualized)	3.42		0.00		10.27		0
Ending total loans, gross	7,829,420		6,214,840		7,829,420		6,214,840
Average loans in repayment	4,453,775		3,400,620		4,408,852		3,580,401
Ending loans in repayment	$ 4,575,143		$ 3,518,997		$ 4,575,143		$ 3,518,997

[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 loans sold.

[3] 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.

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