Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) charge_off	Mar. 31, 2017 USD ($) charge_off
Allowance for Loan Losses
Allowance at beginning of period	$ 251,475	$ 184,701
Total provision	55,549	26,792
Net charge-offs:
Charge-offs	(38,803)	(26,445)
Recoveries	5,118	3,259
Net charge-offs	(33,685)	(23,186)
Loan sales	(1,216)	(1,221)
Allowance at end of period	272,123	187,086
Allowance:
Ending balance: individually evaluated for impairment	101,824	87,150
Ending balance: collectively evaluated for impairment	170,299	99,936
Loans:
Ending balance: individually evaluated for impairment	1,043,103	701,860
Ending balance: collectively evaluated for impairment	19,334,407	15,997,889
FFELP Loans
Allowance for Loan Losses
Allowance at beginning of period	1,132	2,171
Total provision	231	(316)
Net charge-offs:
Charge-offs	(250)	(218)
Recoveries	0	0
Net charge-offs	(250)	(218)
Loan sales	0	0
Allowance at end of period	1,113	1,637
Allowance:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	1,113	1,637
Loans:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	$ 907,842	$ 989,393
Net charge-offs as a percentage of average loans in repayment (annualized)	0.14%	0.11%
Allowance as a percentage of the ending total loan balance	0.12%	0.17%
Allowance as a percentage of the ending loans in repayment	0.16%	0.22%
Allowance coverage of net charge-offs (annualized) | charge_off	1.11	1.88
Ending total loans, gross	$ 907,842	$ 989,393
Average loans in repayment	718,311	771,435
Ending loans in repayment	702,965	757,052
Private Education Loans
Allowance for Loan Losses
Allowance at beginning of period	243,715	182,472
Total provision	41,870	26,820
Net charge-offs:
Charge-offs	(37,353)	(26,227)
Recoveries	5,087	3,259
Net charge-offs	(32,266)	(22,968)
Loan sales	(1,216)	(1,221)
Allowance at end of period	252,103	185,103
Allowance:
Ending balance: individually evaluated for impairment	101,824	87,150
Ending balance: collectively evaluated for impairment	150,279	97,953
Loans:
Ending balance: individually evaluated for impairment	1,043,103	701,860
Ending balance: collectively evaluated for impairment	$ 17,750,909	$ 14,952,994
Net charge-offs as a percentage of average loans in repayment (annualized)	1.01%	0.89%
Allowance as a percentage of the ending total loan balance	1.34%	1.18%
Allowance as a percentage of the ending loans in repayment	1.95%	1.76%
Allowance coverage of net charge-offs (annualized) | charge_off	1.95	2.01
Ending total loans, gross	$ 18,794,012	$ 15,654,854
Average loans in repayment	12,747,929	10,265,530
Ending loans in repayment	12,958,742	10,526,782
Personal Loans
Allowance for Loan Losses
Allowance at beginning of period	6,628	58
Total provision	13,448	288
Net charge-offs:
Charge-offs	(1,200)	0
Recoveries	31	0
Net charge-offs	(1,169)
Loan sales	0	0
Allowance at end of period	18,907	346
Allowance:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	18,907	346
Loans:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	$ 675,656	$ 55,502
Net charge-offs as a percentage of average loans in repayment (annualized)	0.88%	0.00%
Allowance as a percentage of the ending total loan balance	2.80%	0.62%
Allowance as a percentage of the ending loans in repayment	2.80%	0.62%
Allowance coverage of net charge-offs (annualized) | charge_off	4.04	0.00
Ending total loans, gross	$ 675,656	$ 55,502
Average loans in repayment	531,889	35,830
Ending loans in repayment	$ 675,656	$ 55,502