Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance for Loan Losses
Beginning balance	$ 341,121	$ 251,475	$ 184,701
Total provision	353,934	244,584	185,595
Charge-offs	(284,114)	(175,526)	(131,596)
Recoveries	30,971	21,804	17,646
Net charge-offs	(253,143)	(153,722)	(113,950)
Loan sales		(1,216)	(4,871)
Ending Balance	441,912	341,121	251,475
Allowance:
Ending balance: individually evaluated for impairment	186,697	120,110	94,682
Ending balance: collectively evaluated for impairment	255,215	221,011	156,793
Loans:
Ending balance: individually evaluated for impairment	1,581,966	1,257,856	990,351
Ending balance: collectively evaluated for impairment	23,443,822	21,283,187	17,769,756
FFELP Loans
Allowance for Loan Losses
Beginning balance	977	1,132	2,171
Total provision	1,478	980	(85)
Charge-offs	(822)	(1,135)	(954)
Recoveries	0	0	0
Net charge-offs	(822)	(1,135)	(954)
Loan sales		0	0
Ending Balance	1,633	977	1,132
Allowance:
Ending balance: individually evaluated for impairment	0	0	0
Ending balance: collectively evaluated for impairment	1,633	977	1,132
Loans:
Ending balance: individually evaluated for impairment	0	0	0
Ending balance: collectively evaluated for impairment	$ 783,306	$ 846,487	$ 927,660
Net charge-offs as a percentage of average loans in repayment	0.13%	0.16%	0.13%
Allowance as a percentage of the ending total loan balance	0.21%	0.12%	0.12%
Allowance as a percentage of the ending loans in repayment	0.26%	0.15%	0.15%
Allowance coverage of net charge-offs	1.99	0.86	1.19
Ending total loans, gross	$ 783,306	$ 846,487	$ 927,660
Average loans in repayment	631,029	691,406	745,039
Ending loans in repayment	617,646	665,807	746,456
Private Education Loans
Allowance for Loan Losses
Beginning balance	277,943	243,715	182,472
Total provision	279,570	169,287	178,542
Charge-offs	(208,978)	(154,701)	(130,063)
Recoveries	25,765	20,858	17,635
Net charge-offs	(183,213)	(133,843)	(112,428)
Loan sales		(1,216)	(4,871)
Ending Balance	374,300	277,943	243,715
Allowance:
Ending balance: individually evaluated for impairment	186,697	120,110	94,682
Ending balance: collectively evaluated for impairment	187,603	157,833	149,033
Loans:
Ending balance: individually evaluated for impairment	1,581,966	1,257,856	990,351
Ending balance: collectively evaluated for impairment	$ 21,607,625	$ 19,246,609	$ 16,441,816
Net charge-offs as a percentage of average loans in repayment	1.17%	1.01%	1.03%
Allowance as a percentage of the ending total loan balance	1.61%	1.36%	1.40%
Allowance as a percentage of the ending loans in repayment	2.23%	1.90%	2.00%
Allowance coverage of net charge-offs	2.04	2.08	2.17
Ending total loans, gross	$ 23,189,591	$ 20,504,465	$ 17,432,167
Average loans in repayment	15,605,927	13,303,801	10,881,058
Ending loans in repayment	16,787,670	14,666,856	12,206,033
Personal Loans
Allowance for Loan Losses
Beginning balance	62,201	6,628	58
Total provision	72,783	74,317	7,138
Charge-offs	(74,313)	(19,690)	(579)
Recoveries	5,206	946	11
Net charge-offs	(69,107)	(18,744)	(568)
Loan sales		0	0
Ending Balance	65,877	62,201	6,628
Allowance:
Ending balance: individually evaluated for impairment	0	0	0
Ending balance: collectively evaluated for impairment	65,877	62,201	6,628
Loans:
Ending balance: individually evaluated for impairment	0	0	0
Ending balance: collectively evaluated for impairment	$ 1,049,007	$ 1,190,091	$ 400,280
Net charge-offs as a percentage of average loans in repayment	6.07%	2.11%	0.47%
Allowance as a percentage of the ending total loan balance	6.28%	5.23%	1.66%
Allowance as a percentage of the ending loans in repayment	6.28%	5.23%	1.66%
Allowance coverage of net charge-offs	0.95	3.32	11.67
Ending total loans, gross	$ 1,049,007	$ 1,190,091	$ 400,280
Average loans in repayment	1,138,887	889,348	119,606
Ending loans in repayment	1,049,007	1,190,091	$ 400,280
Credit Cards
Allowance for Loan Losses
Beginning balance	0
Total provision	103
Charge-offs	(1)
Recoveries	0
Net charge-offs	(1)
Ending Balance	102	$ 0
Allowance:
Ending balance: individually evaluated for impairment	0
Ending balance: collectively evaluated for impairment	102
Loans:
Ending balance: individually evaluated for impairment	0
Ending balance: collectively evaluated for impairment	$ 3,884
Net charge-offs as a percentage of average loans in repayment	0.13%
Allowance as a percentage of the ending total loan balance	2.63%
Allowance as a percentage of the ending loans in repayment	2.63%
Allowance coverage of net charge-offs	102.00
Ending total loans, gross	$ 3,884
Average loans in repayment	786
Ending loans in repayment	$ 3,884