Allowance for Loan Losses - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Allowance for Loan Losses
Allowance at beginning of period	$ 383,997	$ 295,277	$ 341,121	$ 251,475
Total provision	98,314	68,896	256,889	187,339
Net charge-offs:
Charge-offs	(76,227)	(40,221)	(205,916)	(124,458)
Recoveries	8,322	5,022	22,312	15,834
Net charge-offs	(67,905)	(35,199)	(183,604)	(108,624)
Loan sales				(1,216)
Allowance at end of period	414,406	328,974	414,406	328,974
Allowance:
Ending balance: individually evaluated for impairment	171,884	119,643	171,884	119,643
Ending balance: collectively evaluated for impairment	242,522	209,331	242,522	209,331
Loans:
Ending balance: individually evaluated for impairment	1,474,819	1,199,493	1,474,819	1,199,493
Ending balance: collectively evaluated for impairment	23,575,351	21,040,289	23,575,351	21,040,289
FFELP Loans
Allowance for Loan Losses
Allowance at beginning of period	1,734	1,073	977	1,132
Total provision	158	259	1,320	742
Net charge-offs:
Charge-offs	(203)	(252)	(608)	(794)
Recoveries	0	0	0	0
Net charge-offs	(203)	(252)	(608)	(794)
Loan sales				0
Allowance at end of period	1,689	1,080	1,689	1,080
Allowance:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	1,689	1,080	1,689	1,080
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 798,168	$ 866,786	$ 798,168	$ 866,786
Net charge-offs as a percentage of average loans in repayment (annualized)	0.13%	0.15%	0.13%	0.15%
Allowance as a percentage of the ending total loan balance	0.21%	0.12%	0.21%	0.12%
Allowance as a percentage of the ending loans in repayment	0.27%	0.16%	0.27%	0.16%
Allowance coverage of net charge-offs (annualized)	2.08	1.07	2.08	1.02
Ending total loans, gross	$ 798,168	$ 866,786	$ 798,168	$ 866,786
Average loans in repayment	621,706	681,131	636,538	700,679
Ending loans in repayment	631,626	679,110	631,626	679,110
Private Education Loans
Allowance for Loan Losses
Allowance at beginning of period	307,968	261,695	277,943	243,715
Total provision	84,110	42,482	197,289	130,616
Net charge-offs:
Charge-offs	(56,398)	(34,229)	(151,357)	(113,852)
Recoveries	6,864	4,736	18,669	15,421
Net charge-offs	(49,534)	(29,493)	(132,688)	(98,431)
Loan sales				(1,216)
Allowance at end of period	342,544	274,684	342,544	274,684
Allowance:
Ending balance: individually evaluated for impairment	171,884	119,643	171,884	119,643
Ending balance: collectively evaluated for impairment	170,660	155,041	170,660	155,041
Loans:
Ending balance: individually evaluated for impairment	1,474,819	1,199,493	1,474,819	1,199,493
Ending balance: collectively evaluated for impairment	$ 21,645,350	$ 19,040,498	$ 21,645,350	$ 19,040,498
Net charge-offs as a percentage of average loans in repayment (annualized)	1.27%	0.88%	1.15%	1.01%
Allowance as a percentage of the ending total loan balance	1.48%	1.36%	1.48%	1.36%
Allowance as a percentage of the ending loans in repayment	2.13%	1.99%	2.13%	1.99%
Allowance coverage of net charge-offs (annualized)	1.73	2.33	1.94	2.09
Ending total loans, gross	$ 23,120,169	$ 20,239,991	$ 23,120,169	$ 20,239,991
Average loans in repayment	15,632,028	13,351,517	15,351,188	13,009,704
Ending loans in repayment	16,072,979	13,815,415	16,072,979	13,815,415
Personal Loans
Allowance for Loan Losses
Allowance at beginning of period	74,295	32,509	62,201	6,628
Total provision	14,046	26,155	58,280	55,981
Net charge-offs:
Charge-offs	(19,626)	(5,740)	(53,951)	(9,812)
Recoveries	1,458	286	3,643	413
Net charge-offs	(18,168)	(5,454)	(50,308)	(9,399)
Loan sales				0
Allowance at end of period	70,173	53,210	70,173	53,210
Allowance:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	70,173	53,210	70,173	53,210
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 1,131,833	$ 1,133,005	$ 1,131,833	$ 1,133,005
Net charge-offs as a percentage of average loans in repayment (annualized)	6.42%	2.03%	5.82%	1.56%
Allowance as a percentage of the ending total loan balance	6.20%	4.70%	6.20%	4.70%
Allowance as a percentage of the ending loans in repayment	6.20%	4.70%	6.20%	4.70%
Allowance coverage of net charge-offs (annualized)	0.97	2.44	1.05	4.25
Ending total loans, gross	$ 1,131,833	$ 1,133,005	$ 1,131,833	$ 1,133,005
Average loans in repayment	1,131,965	1,072,624	1,152,555	803,928
Ending loans in repayment	$ 1,131,833	$ 1,133,005	$ 1,131,833	$ 1,133,005