Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Allowance for Loan Losses
Beginning balance	$ 83,842		$ 68,081
Total provision	16,618		39,159
Charge-offs	(9,861)		(643)	[1]
Recoveries	1,387
Net charge-offs	(8,474)
Loan sales	(2,181)	[2]	(28,963)	[2]
Ending Balance	89,805		77,634
Allowance:
Ending balance: individually evaluated for impairment	20,105		0
Ending balance: collectively evaluated for impairment	69,700		77,634
Loans:
Ending balance: individually evaluated for impairment	122,120		0
Ending balance: collectively evaluated for impairment	10,855,618		8,671,494
FFELP Loans
Allowance for Loan Losses
Beginning balance	5,268		6,318
Total provision	435		506
Charge-offs	(1,134)		(643)	[1]
Recoveries	0
Net charge-offs	(1,134)
Loan sales	0	[2]	0	[2]
Ending Balance	4,569		6,181
Allowance:
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	4,569		6,181
Loans:
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	1,208,977		1,396,776
Net charge-offs as a percentage of average loans in repayment (annualized)	0.50%	[3]	0.25%	[3]
Allowance as a percentage of the ending total loan balance	0.38%		0.44%
Allowance as a percentage of the ending loans in repayment	0.52%	[3]	0.62%	[3]
Allowance coverage of net charge-offs (annualized)	1.01		2.40
Ending total loans, gross	1,208,977		1,396,776
Average loans in repayment	898,360	[3]	1,023,329	[3]
Ending loans in repayment	872,579	[3]	997,538	[3]
Private Education Loans
Allowance for Loan Losses
Beginning balance	78,574		61,763
Total provision	16,183		38,653
Charge-offs	(8,727)		0	[1]
Recoveries	1,387
Net charge-offs	(7,340)
Loan sales	(2,181)	[2]	(28,963)	[2]
Ending Balance	85,236		71,453
Allowance:
Ending balance: individually evaluated for impairment	20,105		0
Ending balance: collectively evaluated for impairment	65,131		71,453
Loans:
Ending balance: individually evaluated for impairment	122,120		0
Ending balance: collectively evaluated for impairment	9,646,641		7,274,718
Net charge-offs as a percentage of average loans in repayment (annualized)	0.51%	[3]	0.00%	[3]
Allowance as a percentage of the ending total loan balance	0.87%		0.98%
Allowance as a percentage of the ending loans in repayment	1.42%	[3]	1.67%	[3]
Allowance coverage of net charge-offs (annualized)	2.90		0.00
Ending total loans, gross	9,768,761		7,274,718
Average loans in repayment	5,705,067	[3]	4,329,157	[3]
Ending loans in repayment	$ 5,995,121	[3]	$ 4,278,969	[3]

[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] Loans in repayment includes in-school loans making interest only and fixed payments as well as loans that have entered full principal and interest repayment status after any applicable grace period.