Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Allowance for Loan Losses
Beginning balance	$ 112,507	$ 83,842
Total provision	34,160	16,618
Net charge-offs:
Charge-offs	(19,387)	(9,861)
Recoveries	1,044	1,387
Net charge-offs	(18,343)	(8,474)
Loan sales	(2,075)	(2,181)
Ending Balance	126,249	89,805
Allowance:
Ending balance: individually evaluated for impairment	49,212	20,105
Ending balance: collectively evaluated for impairment	77,037	69,700
Loans:
Ending balance: individually evaluated for impairment	318,094	122,120
Ending balance: collectively evaluated for impairment	12,881,802	10,855,618
FFELP Loans
Allowance for Loan Losses
Beginning balance	3,691	5,268
Total provision	321	435
Net charge-offs:
Charge-offs	(383)	(1,134)
Recoveries	0	0
Net charge-offs	(383)	(1,134)
Loan sales	0	0
Ending Balance	3,629	4,569
Allowance:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	3,629	4,569
Loans:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	$ 1,088,026	$ 1,208,977
Net charge-offs as a percentage of average loans in repayment (annualized)	0.19%	0.50%
Allowance as a percentage of the ending total loan balance	0.33%	0.38%
Allowance as a percentage of the ending loans in repayment	0.45%	0.52%
Allowance coverage of net charge-offs (annualized)	2.37	1.01
Ending total loans, gross	$ 1,088,026	$ 1,208,977
Average loans in repayment	804,690	898,360
Ending loans in repayment	803,378	872,579
Private Education Loans
Allowance for Loan Losses
Beginning balance	108,816	78,574
Total provision	33,839	16,183
Net charge-offs:
Charge-offs	(19,004)	(8,727)
Recoveries	1,044	1,387
Net charge-offs	(17,960)	(7,340)
Loan sales	(2,075)	(2,181)
Ending Balance	122,620	85,236
Allowance:
Ending balance: individually evaluated for impairment	49,212	20,105
Ending balance: collectively evaluated for impairment	73,408	65,131
Loans:
Ending balance: individually evaluated for impairment	318,094	122,120
Ending balance: collectively evaluated for impairment	$ 11,793,776	$ 9,646,641
Net charge-offs as a percentage of average loans in repayment (annualized)	0.95%	0.51%
Allowance as a percentage of the ending total loan balance	1.01%	0.87%
Allowance as a percentage of the ending loans in repayment	1.56%	1.42%
Allowance coverage of net charge-offs (annualized)	1.71	2.90
Ending total loans, gross	$ 12,111,870	$ 9,768,761
Average loans in repayment	7,534,234	5,705,067
Ending loans in repayment	$ 7,843,076	$ 5,995,121