Allowance for Credit Losses - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Jan. 01, 2020
Allowance for Credit Losses
Beginning balance	$ 1,929,323	$ 383,997	$ 441,912	$ 341,121	$ 341,121
Provision for current period	(90,014)		338,120
Loan sale reduction to provision	(42,916)		(204,709)
Total provision	(132,930)	98,314	133,411	256,889
Net charge-offs:
Charge-offs	(60,666)	(76,227)	(178,120)	(205,916)
Recoveries	6,896	8,322	23,282	22,312
Net charge-offs	53,770	67,905	154,838	183,604
Loan sales	(108,534)		(108,534)
Ending Balance	1,734,559	414,406	1,734,559	414,406	441,912
Allowance:
Ending balance: individually evaluated for impairment	138,663	171,884	138,663	171,884
Ending balance: collectively evaluated for impairment	1,595,896	242,522	1,595,896	242,522
Loans:
Ending balance: individually evaluated for impairment	1,495,161	1,474,819	1,495,161	1,474,819
Ending balance: collectively evaluated for impairment	21,876,262	23,575,351	21,876,262	23,575,351
Provision for loan losses	(81,710)		134,374
Provision for unfunded commitments	129,290		276,094
Total Private Education Loan provision for credit losses	47,580		410,468
Private Education Loan provision for credit losses, other impacts	(51,220)		(963)
Less: provisions for credit losses	(3,640)	99,526	409,505	256,691
Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Transfer from unfunded commitment liability	100,470		279,555
Net charge-offs:
Day 1 adjustment for the adoption of CECL						$ 1,143,053
FFELP Loans
Allowance for Credit Losses
Beginning balance	4,385	1,734	1,633	977	977
Provision for current period	67		277
Loan sale reduction to provision	0		0
Total provision	67	158	277	1,320
Net charge-offs:
Charge-offs	(89)	(203)	(399)	(608)
Recoveries	0	0	0	0
Net charge-offs	89	203	399	608
Loan sales	0		0
Ending Balance	4,363	1,689	4,363	1,689	1,633
Allowance:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	4,363	1,689	4,363	1,689
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 745,556	$ 798,168	$ 745,556	$ 798,168
Net charge-offs as a percentage of average loans in repayment (annualized)	0.07%	0.13%	0.10%	0.13%
Allowance as a percentage of the ending total loan balance	0.59%	0.21%	0.59%	0.21%
Allowance as a percentage of the ending loans in repayment	0.77%	0.27%	0.77%	0.27%
Allowance coverage of net charge-offs (annualized)	12.26	2.08	8.20	2.08
Ending total loans, gross	$ 745,556	$ 798,168	$ 745,556	$ 798,168
Average loans in repayment	510,809	621,706	546,443	636,538
Ending loans in repayment	564,442	631,626	564,442	631,626
Private Education Loan provision for credit losses, other impacts	67		277
FFELP Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit 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 Credit Losses
Beginning balance	1,760,559	307,968	374,300	277,943	277,943
Provision for current period	(81,710)		296,167
Loan sale reduction to provision	0		(161,793)
Total provision	(81,710)	84,110	134,374	197,289
Net charge-offs:
Charge-offs	(55,298)	(56,398)	(138,546)	(151,357)
Recoveries	4,790	6,864	18,298	18,669
Net charge-offs	50,508	49,534	120,248	132,688
Loan sales	0		0
Ending Balance	1,728,811	342,544	1,728,811	342,544	374,300
Allowance:
Ending balance: individually evaluated for impairment	138,663	171,884	138,663	171,884
Ending balance: collectively evaluated for impairment	1,590,148	170,660	1,590,148	170,660
Loans:
Ending balance: individually evaluated for impairment	1,495,161	1,474,819	1,495,161	1,474,819
Ending balance: collectively evaluated for impairment	$ 21,119,166	$ 21,645,350	$ 21,119,166	$ 21,645,350
Net charge-offs as a percentage of average loans in repayment (annualized)	1.33%	1.27%	1.05%	1.15%
Allowance as a percentage of the ending total loan balance	7.64%	1.48%	7.64%	1.48%
Allowance as a percentage of the ending loans in repayment	10.91%	2.13%	10.91%	2.13%
Allowance coverage of net charge-offs (annualized)	8.56	1.73	10.78	1.94
Ending total loans, gross	$ 22,614,327	$ 23,120,169	$ 22,614,327	$ 23,120,169
Average loans in repayment	15,182,703	15,632,028	15,336,253	15,351,188
Ending loans in repayment	15,853,309	16,072,979	15,853,309	16,072,979
Private Education Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Transfer from unfunded commitment liability	100,470		279,555
Net charge-offs:
Day 1 adjustment for the adoption of CECL						1,060,830
Personal Loans
Allowance for Credit Losses
Beginning balance	163,337	74,295	65,877	62,201	62,201
Provision for current period	(8,762)		40,485
Loan sale reduction to provision	(42,916)		(42,916)
Total provision	(51,678)	14,046	(2,431)	58,280
Net charge-offs:
Charge-offs	(5,231)	(19,626)	(39,079)	(53,951)
Recoveries	2,106	1,458	4,984	3,643
Net charge-offs	3,125	18,168	34,095	50,308
Loan sales	(108,534)		(108,534)
Ending Balance	0	70,173	0	70,173	65,877
Allowance:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	0	70,173	0	70,173
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 0	$ 1,131,833	$ 0	$ 1,131,833
Net charge-offs as a percentage of average loans in repayment (annualized)	0.00%	6.42%	0.00%	5.82%
Allowance as a percentage of the ending total loan balance	0.00%	6.20%	0.00%	6.20%
Allowance as a percentage of the ending loans in repayment	0.00%	6.20%	0.00%	6.20%
Allowance coverage of net charge-offs (annualized)	0	0.97	0	1.05
Ending total loans, gross	$ 0	$ 1,131,833	$ 0	$ 1,131,833
Average loans in repayment	0	1,131,965	0	1,152,555
Ending loans in repayment	0	$ 1,131,833	0	$ 1,131,833
Private Education Loan provision for credit losses, other impacts	(51,678)		(2,431)
Personal Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit 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 Credit Losses
Beginning balance	1,042		102
Provision for current period	391		1,191
Loan sale reduction to provision	0		0
Total provision	391		1,191
Net charge-offs:
Charge-offs	(48)		(96)
Recoveries	0		0
Net charge-offs	48		96
Loan sales	0		0
Ending Balance	1,385		1,385		$ 102
Allowance:
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	1,385		1,385
Loans:
Ending balance: individually evaluated for impairment	0		0
Ending balance: collectively evaluated for impairment	$ 11,540		$ 11,540
Net charge-offs as a percentage of average loans in repayment (annualized)	1.73%		1.51%
Allowance as a percentage of the ending total loan balance	12.00%		12.00%
Allowance as a percentage of the ending loans in repayment	12.00%		12.00%
Allowance coverage of net charge-offs (annualized)	7.21		10.82
Ending total loans, gross	$ 11,540		$ 11,540
Average loans in repayment	11,103		8,499
Ending loans in repayment	11,540		11,540
Private Education Loan provision for credit losses, other impacts	391		1,191
Credit Cards | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Credit Losses
Transfer from unfunded commitment liability	$ 0		$ 0
Net charge-offs:
Day 1 adjustment for the adoption of CECL						$ 188