Allowance for Loan Losses - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Jan. 01, 2020
Allowance for Loan Losses
Beginning balance	$ 1,673,324	$ 441,912	$ 358,325	$ 441,912	$ 341,121	$ 341,121
Provision for current period	253,146			428,134
Loan sale reduction to provision	0			(161,793)
Total provision	253,146		92,915	266,341	158,575
Net charge-offs:
Charge-offs	(46,505)		(74,627)	(117,454)	(129,689)
Recoveries	6,868		7,384	16,386	13,990
Net charge-offs	39,637		67,243	101,068	115,699
Ending Balance	1,929,323	1,673,324	383,997	1,929,323	383,997	441,912
Allowance:
Ending balance: individually evaluated for impairment	160,234		146,403	160,234	146,403
Ending balance: collectively evaluated for impairment	1,769,089		237,594	1,769,089	237,594
Loans:
Ending balance: individually evaluated for impairment	1,520,240		1,411,189	1,520,240	1,411,189
Ending balance: collectively evaluated for impairment	21,502,357		22,164,795	21,502,357	22,164,795
Provisions for credit losses for new loan commitments made during the quarter	98,741			146,804
Total provision for allowance for loan losses	253,146			266,341
Provisions for credit losses reported in consolidated statements of operations	351,887		93,375	413,145	157,165
Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan Losses
Transfer from unfunded commitment liability	42,490			179,085
Net charge-offs:
Day 1 adjustment for the adoption of CECL							$ 1,143,053
FFELP Loans
Allowance for Loan Losses
Beginning balance	4,296	1,633	1,760	1,633	977	977
Provision for current period	173			210
Loan sale reduction to provision	0			0
Total provision	173		145	210	1,162
Net charge-offs:
Charge-offs	(84)		(171)	(310)	(405)
Recoveries	0		0	0	0
Net charge-offs	84		171	310	405
Ending Balance	4,385	4,296	1,734	4,385	1,734	1,633
Allowance:
Ending balance: individually evaluated for impairment	0		0	0	0
Ending balance: collectively evaluated for impairment	4,385		1,734	4,385	1,734
Loans:
Ending balance: individually evaluated for impairment	0		0	0	0
Ending balance: collectively evaluated for impairment	$ 754,340		$ 812,500	$ 754,340	$ 812,500
Net charge-offs as a percentage of average loans in repayment (annualized)	0.07%		0.11%	0.11%	0.13%
Allowance as a percentage of the ending total loan balance	0.58%		0.21%	0.58%	0.21%
Allowance as a percentage of the ending loans in repayment	0.97%		0.28%	0.97%	0.28%
Allowance coverage of net charge-offs (annualized)	13.05		2.54	7.07	2.14
Ending total loans, gross	$ 754,340		$ 812,500	$ 754,340	$ 812,500
Average loans in repayment	513,418		634,932	553,402	642,693
Ending loans in repayment	452,617		620,292	452,617	620,292
FFELP Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan Losses
Transfer from unfunded commitment liability	0			0
Net charge-offs:
Day 1 adjustment for the adoption of CECL							2,852
Private Education Loans
Allowance for Loan Losses
Beginning balance	1,515,781	374,300	285,946	374,300	277,943	277,943
Provision for current period	228,535			377,877
Loan sale reduction to provision	0			(161,793)
Total provision	228,535		71,296	216,084	113,179
Net charge-offs:
Charge-offs	(31,779)		(55,382)	(83,248)	(94,959)
Recoveries	5,532		6,108	13,508	11,805
Net charge-offs	26,247		49,274	69,740	83,154
Ending Balance	1,760,559	1,515,781	307,968	1,760,559	307,968	374,300
Allowance:
Ending balance: individually evaluated for impairment	160,234		146,403	160,234	146,403
Ending balance: collectively evaluated for impairment	1,600,325		161,565	1,600,325	161,565
Loans:
Ending balance: individually evaluated for impairment	1,520,240		1,411,189	1,520,240	1,411,189
Ending balance: collectively evaluated for impairment	$ 19,965,225		$ 20,217,658	$ 19,965,225	$ 20,217,658
Net charge-offs as a percentage of average loans in repayment (annualized)	0.75%		1.29%	0.91%	1.09%
Allowance as a percentage of the ending total loan balance	8.19%		1.42%	8.19%	1.42%
Allowance as a percentage of the ending loans in repayment	12.13%		2.01%	12.13%	2.01%
Allowance coverage of net charge-offs (annualized)	16.77		1.56	12.62	1.85
Ending total loans, gross	$ 21,485,465		$ 21,628,847	$ 21,485,465	$ 21,628,847
Average loans in repayment	14,011,841		15,241,574	15,306,349	15,188,003
Ending loans in repayment	14,512,723		15,332,251	14,512,723	15,332,251
Private Education Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan Losses
Transfer from unfunded commitment liability	42,490			179,085
Net charge-offs:
Day 1 adjustment for the adoption of CECL							1,060,830
Personal Loans
Allowance for Loan Losses
Beginning balance	152,673	65,877	70,619	65,877	62,201	62,201
Provision for current period	23,929			49,247
Loan sale reduction to provision	0			0
Total provision	23,929		21,474	49,247	44,234
Net charge-offs:
Charge-offs	(14,601)		(19,074)	(33,848)	(34,325)
Recoveries	1,336		1,276	2,878	2,185
Net charge-offs	13,265		17,798	30,970	32,140
Ending Balance	163,337	152,673	74,295	163,337	74,295	65,877
Allowance:
Ending balance: individually evaluated for impairment	0		0	0	0
Ending balance: collectively evaluated for impairment	163,337		74,295	163,337	74,295
Loans:
Ending balance: individually evaluated for impairment	0		0	0	0
Ending balance: collectively evaluated for impairment	$ 772,086		$ 1,134,637	$ 772,086	$ 1,134,637
Net charge-offs as a percentage of average loans in repayment (annualized)	6.87%		6.20%	7.13%	5.53%
Allowance as a percentage of the ending total loan balance	21.16%		6.55%	21.16%	6.55%
Allowance as a percentage of the ending loans in repayment	22.46%		6.55%	22.46%	6.55%
Allowance coverage of net charge-offs (annualized)	3.08		1.04	2.64	1.16
Ending total loans, gross	$ 772,086		$ 1,134,637	$ 772,086	$ 1,134,637
Average loans in repayment	772,137		1,148,444	869,133	1,161,761
Ending loans in repayment	727,214		$ 1,134,637	727,214	$ 1,134,637
Personal Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan Losses
Transfer from unfunded commitment liability	0			0
Net charge-offs:
Day 1 adjustment for the adoption of CECL							79,183
Credit Cards
Allowance for Loan Losses
Beginning balance	574	102		102
Provision for current period	509			800
Loan sale reduction to provision	0			0
Total provision	509			800
Net charge-offs:
Charge-offs	(41)			(48)
Recoveries	0			0
Net charge-offs	41			48
Ending Balance	1,042	$ 574		1,042		$ 102
Allowance:
Ending balance: individually evaluated for impairment	0			0
Ending balance: collectively evaluated for impairment	1,042			1,042
Loans:
Ending balance: individually evaluated for impairment	0			0
Ending balance: collectively evaluated for impairment	$ 10,706			$ 10,706
Net charge-offs as a percentage of average loans in repayment (annualized)	1.77%			1.31%
Allowance as a percentage of the ending total loan balance	9.73%			9.73%
Allowance as a percentage of the ending loans in repayment	9.73%			9.73%
Allowance coverage of net charge-offs (annualized)	6.35			10.85
Ending total loans, gross	$ 10,706			$ 10,706
Average loans in repayment	9,265			7,326
Ending loans in repayment	10,706			10,706
Credit Cards | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan Losses
Transfer from unfunded commitment liability	$ 0			$ 0
Net charge-offs:
Day 1 adjustment for the adoption of CECL							$ 188