Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2017 USD ($) charge_off	Mar. 31, 2016 USD ($) charge_off
Allowance for Loan Losses
Allowance at beginning of period	$ 184,701	$ 112,507
Total provision	26,792	34,160
Net charge-offs:
Charge-offs	(26,445)	(19,387)
Recoveries	3,259	1,044
Net charge-offs	(23,186)	(18,343)
Loan sales	(1,221)	(2,075)
Allowance at end of period	187,086	126,249
Allowance:
Ending balance: individually evaluated for impairment	87,150	49,212
Ending balance: collectively evaluated for impairment	99,936	77,037
Loans:
Ending balance: individually evaluated for impairment	701,860	318,094
Ending balance: collectively evaluated for impairment	15,997,889	12,881,802
FFELP Loans
Allowance for Loan Losses
Allowance at beginning of period	2,171	3,691
Total provision	(316)	321
Net charge-offs:
Charge-offs	(218)	(383)
Recoveries	0	0
Net charge-offs	(218)	(383)
Loan sales	0	0
Allowance at end of period	1,637	3,629
Allowance:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	1,637	3,629
Loans:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	$ 989,393	$ 1,088,026
Net charge-offs as a percentage of average loans in repayment (annualized)	0.11%	0.19%
Allowance as a percentage of the ending total loan balance	0.17%	0.33%
Allowance as a percentage of the ending loans in repayment	0.22%	0.45%
Allowance coverage of net charge-offs (annualized) | charge_off	1.88	2.37
Ending total loans, gross	$ 989,393	$ 1,088,026
Average loans in repayment	771,435	804,690
Ending loans in repayment	757,052	803,378
Private Education Loans
Allowance for Loan Losses
Allowance at beginning of period	182,472	108,816
Total provision	26,820	33,839
Net charge-offs:
Charge-offs	(26,227)	(19,004)
Recoveries	3,259	1,044
Net charge-offs	(22,968)	(17,960)
Loan sales	(1,221)	(2,075)
Allowance at end of period	185,103	122,620
Allowance:
Ending balance: individually evaluated for impairment	87,150	49,212
Ending balance: collectively evaluated for impairment	97,953	73,408
Loans:
Ending balance: individually evaluated for impairment	701,860	318,094
Ending balance: collectively evaluated for impairment	$ 14,952,994	$ 11,793,776
Net charge-offs as a percentage of average loans in repayment (annualized)	0.89%	0.95%
Allowance as a percentage of the ending total loan balance	1.18%	1.01%
Allowance as a percentage of the ending loans in repayment	1.76%	1.56%
Allowance coverage of net charge-offs (annualized) | charge_off	2.01	1.71
Ending total loans, gross	$ 15,654,854	$ 12,111,870
Average loans in repayment	10,265,530	7,534,234
Ending loans in repayment	10,526,782	$ 7,843,076
Personal Loans
Allowance for Loan Losses
Allowance at beginning of period	58
Total provision	288
Net charge-offs:
Charge-offs	0
Recoveries	0
Net charge-offs
Loan sales	0
Allowance at end of period	346
Allowance:
Ending balance: individually evaluated for impairment	0
Ending balance: collectively evaluated for impairment	346
Loans:
Ending balance: individually evaluated for impairment	0
Ending balance: collectively evaluated for impairment	$ 55,502
Net charge-offs as a percentage of average loans in repayment (annualized)	0.00%
Allowance as a percentage of the ending total loan balance	0.62%
Allowance as a percentage of the ending loans in repayment	0.62%
Allowance coverage of net charge-offs (annualized) | charge_off	0.00
Ending total loans, gross	$ 55,502
Average loans in repayment	35,830
Ending loans in repayment	$ 55,502