Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) $ in Thousands		12 Months Ended
	Apr. 29, 2014	Dec. 31, 2015 USD ($) SecurityLoan	Dec. 31, 2014 USD ($) SecurityLoan	Dec. 31, 2013 USD ($)
Allowance for Loan Losses
Beginning balance		$ 83,842	$ 68,081	$ 69,189
Total provision		88,349	85,529	69,339
Charge-offs		(57,939)	(17,438)	(2,037)
Recoveries		5,820	1,155	0
Net charge-offs		(52,119)	(16,283)	(2,037)
Student loan sales		(7,565)	(53,485)	(68,410)
Ending Balance		112,507	83,842	68,081
Allowance:
Ending balance: individually evaluated for impairment		43,480	9,815	0
Ending balance: collectively evaluated for impairment		69,027	74,027	68,081
Loans:
Ending balance: individually evaluated for impairment		265,831	59,402	0
Ending balance: collectively evaluated for impairment		11,446,269	9,516,781	7,990,314
Period of delinquent loans sold to subsidiary (greater than)	90 days
FFELP Loans
Allowance for Loan Losses
Beginning balance		5,268	6,318	3,971
Total provision		1,005	1,946	4,384
Charge-offs		(2,582)	(2,996)	(2,037)
Recoveries		0	0	0
Net charge-offs		(2,582)	(2,996)	(2,037)
Student loan sales		0	0	0
Ending Balance		3,691	5,268	6,318
Allowance:
Ending balance: individually evaluated for impairment		0	0	0
Ending balance: collectively evaluated for impairment		3,691	5,268	6,318
Loans:
Ending balance: individually evaluated for impairment		0	0	0
Ending balance: collectively evaluated for impairment		$ 1,115,663	$ 1,264,807	$ 1,426,972
Net charge-offs as a percentage of average loans in repayment		0.30%	0.31%	0.23%
Allowance as a percentage of the ending total loan balance		0.33%	0.42%	0.44%
Allowance as a percentage of the ending loans in repayment		0.45%	0.57%	0.62%
Allowance coverage of net charge-offs		1.43	1.76	3.10
Ending total loans, gross		$ 1,115,663	$ 1,264,807	$ 1,426,972
Average loans in repayment		857,359	972,390	870,460
Ending loans in repayment		813,815	926,891	1,023,471
Private Education Loans
Allowance for Loan Losses
Beginning balance		78,574	61,763	65,218
Total provision		87,344	83,583	64,955
Charge-offs		(55,357)	(14,442)	0
Recoveries		5,820	1,155	0
Net charge-offs		(49,537)	(13,287)	0
Student loan sales		(7,565)	(53,485)	(68,410)
Ending Balance		108,816	78,574	61,763
Allowance:
Ending balance: individually evaluated for impairment		43,480	9,815	0
Ending balance: collectively evaluated for impairment		65,336	68,759	61,763
Loans:
Ending balance: individually evaluated for impairment		265,831	59,402	0
Ending balance: collectively evaluated for impairment		$ 10,330,606	$ 8,251,974	$ 6,563,342
Net charge-offs as a percentage of average loans in repayment		0.82%	0.30%	0.00%
Allowance as a percentage of the ending total loan balance		1.03%	0.95%	0.94%
Allowance as a percentage of the ending loans in repayment		1.57%	1.53%	1.55%
Allowance coverage of net charge-offs		2.20	5.91	0.00
Ending total loans, gross		$ 10,596,437	$ 8,311,376	$ 6,563,342
Average loans in repayment		6,031,741	4,495,709	3,509,502
Ending loans in repayment		$ 6,927,266	$ 5,149,215	$ 3,972,317