Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) charge_off	Sep. 30, 2016 USD ($) charge_off	Sep. 30, 2017 USD ($) charge_off	Sep. 30, 2016 USD ($) charge_off
Allowance for Loan Losses
Allowance at beginning of period	$ 207,448	$ 144,925	$ 184,701	$ 112,507
Total provision	53,847	40,770	130,524	116,307
Net charge-offs:
Charge-offs	(34,681)	(22,428)	(94,133)	(66,065)
Recoveries	4,562	2,973	12,218	7,098
Net charge-offs	(30,119)	(19,455)	(81,915)	(58,967)
Loan sales	(1,257)	(1,401)	(3,391)	(5,008)
Allowance at end of period	229,919	164,839	229,919	164,839
Allowance:
Ending balance: individually evaluated for impairment	100,999	77,521	100,999	77,521
Ending balance: collectively evaluated for impairment	128,920	87,318	128,920	87,318
Loans:
Ending balance: individually evaluated for impairment	942,561	503,632	942,561	503,632
Ending balance: collectively evaluated for impairment	17,271,626	14,378,559	17,271,626	14,378,559
FFELP Loans
Allowance for Loan Losses
Allowance at beginning of period	1,606	2,297	2,171	3,691
Total provision	(73)	268	(161)	(396)
Net charge-offs:
Charge-offs	(181)	(356)	(658)	(1,086)
Recoveries	0	0	0	0
Net charge-offs	(181)	(356)	(658)	(1,086)
Loan sales	0	0	0	0
Allowance at end of period	1,352	2,209	1,352	2,209
Allowance:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	1,352	2,209	1,352	2,209
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 949,180	$ 1,033,929	$ 949,180	$ 1,033,929
Net charge-offs as a percentage of average loans in repayment (annualized)	0.10%	0.18%	0.12%	0.18%
Allowance as a percentage of the ending total loan balance	0.14%	0.21%	0.14%	0.21%
Allowance as a percentage of the ending loans in repayment	0.20%	0.28%	0.20%	0.28%
Allowance coverage of net charge-offs (annualized) | charge_off	1.87	1.55	1.54	1.53
Ending total loans, gross	$ 949,180	$ 1,033,929	$ 949,180	$ 1,033,929
Average loans in repayment	734,613	791,296	752,990	795,452
Ending loans in repayment	690,849	795,665	690,849	795,665
Private Education Loans
Allowance for Loan Losses
Allowance at beginning of period	205,024	142,628	182,472	108,816
Total provision	53,120	40,502	129,105	116,703
Net charge-offs:
Charge-offs	(34,280)	(22,072)	(93,235)	(64,979)
Recoveries	4,560	2,973	12,216	7,098
Net charge-offs	(29,720)	(19,099)	(81,019)	(57,881)
Loan sales	(1,257)	(1,401)	(3,391)	(5,008)
Allowance at end of period	227,167	162,630	227,167	162,630
Allowance:
Ending balance: individually evaluated for impairment	100,999	77,521	100,999	77,521
Ending balance: collectively evaluated for impairment	126,168	85,109	126,168	85,109
Loans:
Ending balance: individually evaluated for impairment	942,561	503,632	942,561	503,632
Ending balance: collectively evaluated for impairment	$ 16,190,346	$ 13,344,630	$ 16,190,346	$ 13,344,630
Net charge-offs as a percentage of average loans in repayment (annualized)	1.08%	0.91%	1.02%	0.97%
Allowance as a percentage of the ending total loan balance	1.33%	1.17%	1.33%	1.17%
Allowance as a percentage of the ending loans in repayment	1.99%	1.83%	1.99%	1.83%
Allowance coverage of net charge-offs (annualized) | charge_off	1.91	2.13	2.10	2.11
Ending total loans, gross	$ 17,132,907	$ 13,848,262	$ 17,132,907	$ 13,848,262
Average loans in repayment	10,971,028	8,420,625	10,589,725	7,952,469
Ending loans in repayment	11,406,581	$ 8,905,812	11,406,581	$ 8,905,812
Personal Loans
Allowance for Loan Losses
Allowance at beginning of period	818		58
Total provision	800		1,580
Net charge-offs:
Charge-offs	(220)		(240)
Recoveries	2		2
Net charge-offs	(218)		(238)
Loan sales	0		0
Allowance at end of period	1,400		1,400
Allowance:
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	1,400		1,400
Loans:
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	$ 132,100		$ 132,100
Net charge-offs as a percentage of average loans in repayment (annualized)	0.96%		0.51%
Allowance as a percentage of the ending total loan balance	1.06%		1.06%
Allowance as a percentage of the ending loans in repayment	1.06%		1.06%
Allowance coverage of net charge-offs (annualized) | charge_off	1.60		4.41
Ending total loans, gross	$ 132,100		$ 132,100
Average loans in repayment	90,850		62,747
Ending loans in repayment	$ 132,100		$ 132,100