Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for Loan Losses
Beginning balance	$ 251,475	$ 184,701	$ 112,507
Total provision	244,584	185,595	159,397
Charge-offs	(175,526)	(131,596)	(91,551)
Recoveries	21,804	17,646	10,382
Net charge-offs	(153,722)	(113,950)	(81,169)
Loan sales	(1,216)	(4,871)	(6,034)
Ending Balance	341,121	251,475	184,701
Allowance:
Ending balance: individually evaluated for impairment	120,110	94,682	86,930
Ending balance: collectively evaluated for impairment	221,011	156,793	97,771
Loans:
Ending balance: individually evaluated for impairment	1,257,856	990,351	612,606
Ending balance: collectively evaluated for impairment	21,283,187	17,769,756	14,662,871
FFELP Loans
Allowance for Loan Losses
Beginning balance	1,132	2,171	3,691
Total provision	980	(85)	(172)
Charge-offs	(1,135)	(954)	(1,348)
Recoveries	0	0	0
Net charge-offs	(1,135)	(954)	(1,348)
Loan sales	0	0	0
Ending Balance	977	1,132	2,171
Allowance:
Ending balance: individually evaluated for impairment	0	0	0
Ending balance: collectively evaluated for impairment	977	1,132	2,171
Loans:
Ending balance: individually evaluated for impairment	0	0	0
Ending balance: collectively evaluated for impairment	$ 846,487	$ 927,660	$ 1,010,908
Net charge-offs as a percentage of average loans in repayment	0.16%	0.13%	0.17%
Allowance as a percentage of the ending total loan balance	0.12%	0.12%	0.21%
Allowance as a percentage of the ending loans in repayment	0.15%	0.15%	0.28%
Allowance coverage of net charge-offs	0.86	1.19	1.61
Ending total loans, gross	$ 846,487	$ 927,660	$ 1,010,908
Average loans in repayment	691,406	745,039	793,203
Ending loans in repayment	665,807	746,456	786,332
Private Education Loans
Allowance for Loan Losses
Beginning balance	243,715	182,472	108,816
Total provision	169,287	178,542	159,511
Charge-offs	(154,701)	(130,063)	(90,203)
Recoveries	20,858	17,635	10,382
Net charge-offs	(133,843)	(112,428)	(79,821)
Loan sales	(1,216)	(4,871)	(6,034)
Ending Balance	277,943	243,715	182,472
Allowance:
Ending balance: individually evaluated for impairment	120,110	94,682	86,930
Ending balance: collectively evaluated for impairment	157,833	149,033	95,542
Loans:
Ending balance: individually evaluated for impairment	1,257,856	990,351	612,606
Ending balance: collectively evaluated for impairment	$ 19,246,609	$ 16,441,816	$ 13,639,069
Net charge-offs as a percentage of average loans in repayment	1.01%	1.03%	0.96%
Allowance as a percentage of the ending total loan balance	1.36%	1.40%	1.28%
Allowance as a percentage of the ending loans in repayment	1.90%	2.00%	1.88%
Allowance coverage of net charge-offs	2.08	2.17	2.29
Ending total loans, gross	$ 20,504,465	$ 17,432,167	$ 14,251,675
Average loans in repayment	13,303,801	10,881,058	8,283,036
Ending loans in repayment	14,666,856	12,206,033	9,709,758
Personal Loans
Allowance for Loan Losses
Beginning balance	6,628	58	0
Total provision	74,317	7,138	58
Charge-offs	(19,690)	(579)	0
Recoveries	946	11	0
Net charge-offs	(18,744)	(568)	0
Loan sales	0	0	0
Ending Balance	62,201	6,628	58
Allowance:
Ending balance: individually evaluated for impairment	0	0	0
Ending balance: collectively evaluated for impairment	62,201	6,628	58
Loans:
Ending balance: individually evaluated for impairment	0	0	0
Ending balance: collectively evaluated for impairment	$ 1,190,091	$ 400,280	$ 12,894
Net charge-offs as a percentage of average loans in repayment	2.11%	0.47%	0.00%
Allowance as a percentage of the ending total loan balance	5.23%	1.66%	0.00%
Allowance as a percentage of the ending loans in repayment	5.23%	1.66%	0.00%
Allowance coverage of net charge-offs	3.32	11.67	0
Ending total loans, gross	$ 1,190,091	$ 400,280	$ 0
Average loans in repayment	889,348	119,606	0
Ending loans in repayment	$ 1,190,091	$ 400,280	$ 0