Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) charge_off	Dec. 31, 2016 USD ($) charge_off	Dec. 31, 2015 USD ($) charge_off
Allowance for Loan Losses
Beginning balance	$ 184,701	$ 112,507	$ 83,842
Total provision	185,595	159,397	88,349
Charge-offs	(131,596)	(91,551)	(57,939)
Recoveries	17,646	10,382	5,820
Net charge-offs	(113,950)	(81,169)	(52,119)
Student loan sales	(4,871)	(6,034)	(7,565)
Ending Balance	251,475	184,701	112,507
Allowance:
Ending balance: individually evaluated for impairment	94,682	86,930	43,480
Ending balance: collectively evaluated for impairment	156,793	97,771	69,027
Loans:
Ending balance: individually evaluated for impairment	990,351	612,606	265,831
Ending balance: collectively evaluated for impairment	17,769,756	14,662,871	11,446,269
FFELP Loans
Allowance for Loan Losses
Beginning balance	2,171	3,691	5,268
Total provision	(85)	(172)	1,005
Charge-offs	(954)	(1,348)	(2,582)
Recoveries	0	0	0
Net charge-offs	(954)	(1,348)	(2,582)
Student loan sales	0	0	0
Ending Balance	1,132	2,171	3,691
Allowance:
Ending balance: individually evaluated for impairment	0	0	0
Ending balance: collectively evaluated for impairment	1,132	2,171	3,691
Loans:
Ending balance: individually evaluated for impairment	0	0	0
Ending balance: collectively evaluated for impairment	$ 927,660	$ 1,010,908	$ 1,115,663
Net charge-offs as a percentage of average loans in repayment	0.13%	0.17%	0.30%
Allowance as a percentage of the ending total loan balance	0.12%	0.21%	0.33%
Allowance as a percentage of the ending loans in repayment	0.15%	0.28%	0.45%
Allowance coverage of net charge-offs | charge_off	1.19	1.61	1.43
Ending total loans, gross	$ 927,660	$ 1,010,908	$ 1,115,663
Average loans in repayment	745,039	793,203	857,359
Ending loans in repayment	746,456	786,332	813,815
Private Education Loans
Allowance for Loan Losses
Beginning balance	182,472	108,816	78,574
Total provision	178,542	159,511	87,344
Charge-offs	(130,063)	(90,203)	(55,357)
Recoveries	17,635	10,382	5,820
Net charge-offs	(112,428)	(79,821)	(49,537)
Student loan sales	(4,871)	(6,034)	(7,565)
Ending Balance	243,715	182,472	108,816
Allowance:
Ending balance: individually evaluated for impairment	94,682	86,930	43,480
Ending balance: collectively evaluated for impairment	149,033	95,542	65,336
Loans:
Ending balance: individually evaluated for impairment	990,351	612,606	265,831
Ending balance: collectively evaluated for impairment	$ 16,441,816	$ 13,639,069	$ 10,330,606
Net charge-offs as a percentage of average loans in repayment	1.03%	0.96%	0.82%
Allowance as a percentage of the ending total loan balance	1.40%	1.28%	1.03%
Allowance as a percentage of the ending loans in repayment	2.00%	1.88%	1.57%
Allowance coverage of net charge-offs | charge_off	2.17	2.29	2.20
Ending total loans, gross	$ 17,432,167	$ 14,251,675	$ 10,596,437
Average loans in repayment	10,881,058	8,283,036	6,031,741
Ending loans in repayment	12,206,033	9,709,758	6,927,266
Personal Loans
Allowance for Loan Losses
Beginning balance	58	0
Total provision	7,138	58
Charge-offs	(579)	0
Recoveries	11	0
Net charge-offs	(568)	0
Student loan sales	0	0
Ending Balance	6,628	58	$ 0
Allowance:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	6,628	58
Loans:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	$ 400,280	$ 12,894
Net charge-offs as a percentage of average loans in repayment	0.47%	0.00%
Allowance as a percentage of the ending total loan balance	1.66%	0.00%
Allowance as a percentage of the ending loans in repayment	1.66%	0.00%
Allowance coverage of net charge-offs | charge_off	11.67	0.00
Ending total loans, gross	$ 400,280	$ 0
Average loans in repayment	119,606	0
Ending loans in repayment	$ 400,280	$ 0