Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) charge_off	Jun. 30, 2016 USD ($) charge_off	Jun. 30, 2017 USD ($) charge_off	Jun. 30, 2016 USD ($) charge_off
Allowance for Loan Losses
Allowance at beginning of period	$ 187,086	$ 126,249	$ 184,701	$ 112,507
Total provision	49,886	41,377	76,678	75,537
Net charge-offs:
Charge-offs	(33,007)	(24,250)	(59,452)	(43,637)
Recoveries	4,396	3,082	7,655	4,125
Net charge-offs	(28,611)	(21,168)	(51,797)	(39,512)
Loan sales	(913)	(1,533)	(2,134)	(3,607)
Allowance at end of period	207,448	144,925	207,448	144,925
Allowance:
Ending balance: individually evaluated for impairment	95,177	63,370	95,177	63,370
Ending balance: collectively evaluated for impairment	112,271	81,555	112,271	81,555
Loans:
Ending balance: individually evaluated for impairment	803,456	400,969	803,456	400,969
Ending balance: collectively evaluated for impairment	15,912,746	12,951,257	15,912,746	12,951,257
FFELP Loans
Allowance for Loan Losses
Allowance at beginning of period	1,637	3,629	2,171	3,691
Total provision	228	(985)	(88)	(664)
Net charge-offs:
Charge-offs	(259)	(347)	(477)	(730)
Recoveries	0	0	0	0
Net charge-offs	(259)	(347)	(477)	(730)
Loan sales	0	0	0	0
Allowance at end of period	1,606	2,297	1,606	2,297
Allowance:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	1,606	2,297	1,606	2,297
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 967,237	$ 1,061,517	$ 967,237	$ 1,061,517
Net charge-offs as a percentage of average loans in repayment (annualized)	0.14%	0.18%	0.12%	0.18%
Allowance as a percentage of the ending total loan balance	0.17%	0.22%	0.17%	0.22%
Allowance as a percentage of the ending loans in repayment	0.21%	0.30%	0.21%	0.30%
Allowance coverage of net charge-offs (annualized) | charge_off	1.55	1.65	1.68	1.57
Ending total loans, gross	$ 967,237	$ 1,061,517	$ 967,237	$ 1,061,517
Average loans in repayment	757,186	786,818	765,347	794,665
Ending loans in repayment	765,980	773,321	765,980	773,321
Private Education Loans
Allowance for Loan Losses
Allowance at beginning of period	185,103	122,620	182,472	108,816
Total provision	49,166	42,362	75,986	76,201
Net charge-offs:
Charge-offs	(32,728)	(23,903)	(58,955)	(42,907)
Recoveries	4,396	3,082	7,655	4,125
Net charge-offs	(28,332)	(20,821)	(51,300)	(38,782)
Loan sales	(913)	(1,533)	(2,134)	(3,607)
Allowance at end of period	205,024	142,628	205,024	142,628
Allowance:
Ending balance: individually evaluated for impairment	95,177	63,370	95,177	63,370
Ending balance: collectively evaluated for impairment	109,847	79,258	109,847	79,258
Loans:
Ending balance: individually evaluated for impairment	803,456	400,969	803,456	400,969
Ending balance: collectively evaluated for impairment	$ 14,876,001	$ 11,889,740	$ 14,876,001	$ 11,889,740
Net charge-offs as a percentage of average loans in repayment (annualized)	1.08%	1.05%	0.99%	1.01%
Allowance as a percentage of the ending total loan balance	1.31%	1.16%	1.31%	1.16%
Allowance as a percentage of the ending loans in repayment	1.93%	1.78%	1.93%	1.78%
Allowance coverage of net charge-offs (annualized) | charge_off	1.81	1.71	2.00	1.84
Ending total loans, gross	$ 15,679,457	$ 12,290,709	$ 15,679,457	$ 12,290,709
Average loans in repayment	10,523,225	7,894,340	10,375,463	7,695,889
Ending loans in repayment	10,615,105	$ 8,029,034	10,615,105	$ 8,029,034
Personal Loans
Allowance for Loan Losses
Allowance at beginning of period	346		58
Total provision	492		780
Net charge-offs:
Charge-offs	(20)		(20)
Recoveries	0		0
Net charge-offs	(20)		(20)
Loan sales	0		0
Allowance at end of period	818		818
Allowance:
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	818		818
Loans:
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	$ 69,508		$ 69,508
Net charge-offs as a percentage of average loans in repayment (annualized)	0.13%		0.08%
Allowance as a percentage of the ending total loan balance	1.18%		1.18%
Allowance as a percentage of the ending loans in repayment	1.18%		1.18%
Allowance coverage of net charge-offs (annualized) | charge_off	10.23		20.45
Ending total loans, gross	$ 69,508		$ 69,508
Average loans in repayment	61,439		47,654
Ending loans in repayment	$ 69,508		$ 69,508