Allowance for Loan Losses - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Jan. 01, 2020
Allowance for Loan Losses
Beginning balance	$ 441,912	$ 341,121	$ 341,121
Provision for current period	169,508
Loan sale reduction to provision	(161,793)
Total provision	7,715	65,660
Net charge-offs:
Charge-offs	(70,949)	(55,062)
Recoveries	9,518	6,606
Current period net charge-offs	61,431	48,456
Ending Balance	1,673,324	358,325	441,912
Allowance:
Ending balance: individually evaluated for impairment	150,822	132,442
Ending balance: collectively evaluated for impairment	1,522,502	225,883
Loans:
Ending balance: individually evaluated for impairment	1,518,763	1,327,668
Ending balance: collectively evaluated for impairment	21,781,876	22,455,468
Provisions for credit losses for new loan commitments made during the quarter	53,543
Total provision for allowance for loan losses	7,715
Provisions for credit losses reported in consolidated statements of income	61,258	63,790
Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan Losses
Day 1 adjustment for the adoption of CECL				$ 1,143,053
Transfer from unfunded commitment liability	142,075
FFELP Loans
Allowance for Loan Losses
Beginning balance	1,633	977	977
Provision for current period	37
Loan sale reduction to provision	0
Total provision	37	1,017
Net charge-offs:
Charge-offs	(226)	(234)
Recoveries	0	0
Current period net charge-offs	226	234
Ending Balance	4,296	1,760	1,633
Allowance:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	4,296	1,760
Loans:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	$ 766,954	$ 828,640
Net charge-offs as a percentage of average loans in repayment (annualized)	0.15%	0.14%
Allowance as a percentage of the ending total loan balance	0.56%	0.21%
Allowance as a percentage of the ending loans in repayment	0.74%	0.27%
Allowance coverage of net charge-offs (annualized)	4.75	1.88
Ending total loans, gross	$ 766,954	$ 828,640
Average loans in repayment	600,534	650,196
Ending loans in repayment	581,997	641,658
FFELP Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan Losses
Day 1 adjustment for the adoption of CECL				2,852
Transfer from unfunded commitment liability	0
Private Education Loans
Allowance for Loan Losses
Beginning balance	374,300	277,943	277,943
Provision for current period	143,862
Loan sale reduction to provision	(161,793)
Total provision	(17,931)	41,883
Net charge-offs:
Charge-offs	(51,469)	(39,577)
Recoveries	7,976	5,697
Current period net charge-offs	43,493	33,880
Ending Balance	1,515,781	285,946	374,300
Allowance:
Ending balance: individually evaluated for impairment	150,822	132,442
Ending balance: collectively evaluated for impairment	1,364,959	153,504
Loans:
Ending balance: individually evaluated for impairment	1,518,763	1,327,668
Ending balance: collectively evaluated for impairment	$ 20,107,984	$ 20,463,954
Net charge-offs as a percentage of average loans in repayment (annualized)	1.05%	0.89%
Allowance as a percentage of the ending total loan balance	7.01%	1.31%
Allowance as a percentage of the ending loans in repayment	10.11%	1.87%
Allowance coverage of net charge-offs (annualized)	8.71	2.11
Ending total loans, gross	$ 21,626,747	$ 21,791,622
Average loans in repayment	16,521,356	15,165,072
Ending loans in repayment	14,988,345	15,310,560
Private Education Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan Losses
Day 1 adjustment for the adoption of CECL				1,060,830
Transfer from unfunded commitment liability	142,075
Personal Loans
Allowance for Loan Losses
Beginning balance	65,877	62,201	62,201
Provision for current period	25,318
Loan sale reduction to provision	0
Total provision	25,318	22,760
Net charge-offs:
Charge-offs	(19,247)	(15,251)
Recoveries	1,542	909
Current period net charge-offs	17,705	14,342
Ending Balance	152,673	70,619	65,877
Allowance:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	152,673	70,619
Loans:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	$ 899,704	$ 1,162,874
Net charge-offs as a percentage of average loans in repayment (annualized)	7.27%	4.88%
Allowance as a percentage of the ending total loan balance	16.97%	6.07%
Allowance as a percentage of the ending loans in repayment	16.97%	6.07%
Allowance coverage of net charge-offs (annualized)	2.16	1.23
Ending total loans, gross	$ 899,704	$ 1,162,874
Average loans in repayment	973,772	1,175,356
Ending loans in repayment	899,704	$ 1,162,874
Personal Loans | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan Losses
Day 1 adjustment for the adoption of CECL				79,183
Transfer from unfunded commitment liability	0
Credit Cards
Allowance for Loan Losses
Beginning balance	102
Provision for current period	291
Loan sale reduction to provision	0
Total provision	291
Net charge-offs:
Charge-offs	(7)
Recoveries	0
Current period net charge-offs	7
Ending Balance	574		$ 102
Allowance:
Ending balance: individually evaluated for impairment	0
Ending balance: collectively evaluated for impairment	574
Loans:
Ending balance: individually evaluated for impairment	0
Ending balance: collectively evaluated for impairment	$ 7,234
Net charge-offs as a percentage of average loans in repayment (annualized)	0.52%
Allowance as a percentage of the ending total loan balance	7.93%
Allowance as a percentage of the ending loans in repayment	7.93%
Allowance coverage of net charge-offs (annualized)	20.50
Ending total loans, gross	$ 7,234
Average loans in repayment	5,364
Ending loans in repayment	7,234
Credit Cards | Cumulative Effect, Period Of Adoption, Adjustment
Allowance for Loan Losses
Day 1 adjustment for the adoption of CECL				$ 188
Transfer from unfunded commitment liability	$ 0