Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) charge_off	Jun. 30, 2015 USD ($) charge_off	Jun. 30, 2016 USD ($) charge_off	Jun. 30, 2015 USD ($) charge_off
Allowance for Loan Losses
Beginning balance	$ 126,249	$ 89,805	$ 112,507	$ 83,842
Total provision	41,377	15,558	75,537	32,176
Net charge-offs:
Charge-offs	(24,250)	(13,757)	(43,637)	(23,618)
Recoveries	3,082	1,780	4,125	3,168
Net charge-offs	(21,168)	(11,977)	(39,512)	(20,450)
Loan sales	(1,533)	(1,520)	(3,607)	(3,702)
Ending Balance	144,925	91,866	144,925	91,866
Allowance:
Ending balance: individually evaluated for impairment	63,370	32,446	63,370	32,446
Ending balance: collectively evaluated for impairment	81,555	59,420	81,555	59,420
Loans:
Ending balance: individually evaluated for impairment	400,969	187,143	400,969	187,143
Ending balance: collectively evaluated for impairment	12,951,257	10,304,670	12,951,257	10,304,670
FFELP Loans
Allowance for Loan Losses
Beginning balance	3,629	4,569	3,691	5,268
Total provision	(985)	466	(664)	901
Net charge-offs:
Charge-offs	(347)	(479)	(730)	(1,613)
Recoveries	0	0	0	0
Net charge-offs	(347)	(479)	(730)	(1,613)
Loan sales	0	0	0	0
Ending Balance	2,297	4,556	2,297	4,556
Allowance:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	2,297	4,556	2,297	4,556
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 1,061,517	$ 1,178,876	$ 1,061,517	$ 1,178,876
Net charge-offs as a percentage of average loans in repayment (annualized)	0.18%	0.22%	0.18%	0.37%
Allowance as a percentage of the ending total loan balance	0.22%	0.39%	0.22%	0.39%
Allowance as a percentage of the ending loans in repayment	0.30%	0.54%	0.30%	0.54%
Allowance coverage of net charge-offs (annualized) | charge_off	1.65	2.38	1.57	1.41
Ending total loans, gross	$ 1,061,517	$ 1,178,876	$ 1,061,517	$ 1,178,876
Average loans in repayment	786,818	861,453	794,665	880,953
Ending loans in repayment	773,321	836,545	773,321	836,545
Private Education Loans
Allowance for Loan Losses
Beginning balance	122,620	85,236	108,816	78,574
Total provision	42,362	15,092	76,201	31,275
Net charge-offs:
Charge-offs	(23,903)	(13,278)	(42,907)	(22,005)
Recoveries	3,082	1,780	4,125	3,168
Net charge-offs	(20,821)	(11,498)	(38,782)	(18,837)
Loan sales	(1,533)	(1,520)	(3,607)	(3,702)
Ending Balance	142,628	87,310	142,628	87,310
Allowance:
Ending balance: individually evaluated for impairment	63,370	32,446	63,370	32,446
Ending balance: collectively evaluated for impairment	79,258	54,864	79,258	54,864
Loans:
Ending balance: individually evaluated for impairment	400,969	187,143	400,969	187,143
Ending balance: collectively evaluated for impairment	$ 11,889,740	$ 9,125,794	$ 11,889,740	$ 9,125,794
Net charge-offs as a percentage of average loans in repayment (annualized)	1.05%	0.81%	1.01%	0.66%
Allowance as a percentage of the ending total loan balance	1.16%	0.94%	1.16%	0.94%
Allowance as a percentage of the ending loans in repayment	1.78%	1.54%	1.78%	1.54%
Allowance coverage of net charge-offs (annualized) | charge_off	1.71	1.90	1.84	2.32
Ending total loans, gross	$ 12,290,709	$ 9,312,937	$ 12,290,709	$ 9,312,937
Average loans in repayment	7,894,340	5,712,559	7,695,889	5,667,912
Ending loans in repayment	$ 8,029,034	$ 5,666,645	$ 8,029,034	$ 5,666,645