Allowance for Loan Losses - Allowance and Recorded Investments in Loans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Allowance for Loan Losses
Allowance at beginning of period	$ 358,325	$ 272,123	$ 341,121	$ 251,475
Total provision	92,915	62,894	158,575	118,443
Net charge-offs:
Charge-offs	(74,627)	(45,434)	(129,689)	(84,237)
Recoveries	7,384	5,694	13,990	10,812
Net charge-offs	(67,243)	(39,740)	(115,699)	(73,425)
Loan sales				(1,216)
Allowance at end of period	383,997	295,277	383,997	295,277
Allowance:
Ending balance: individually evaluated for impairment	146,403	113,343	146,403	113,343
Ending balance: collectively evaluated for impairment	237,594	181,934	237,594	181,934
Loans:
Ending balance: individually evaluated for impairment	1,411,189	1,121,816	1,411,189	1,121,816
Ending balance: collectively evaluated for impairment	22,164,795	19,417,648	22,164,795	19,417,648
FFELP Loans
Allowance for Loan Losses
Allowance at beginning of period	1,760	1,113	977	1,132
Total provision	145	252	1,162	483
Net charge-offs:
Charge-offs	(171)	(292)	(405)	(542)
Recoveries	0	0	0	0
Net charge-offs	(171)	(292)	(405)	(542)
Loan sales				0
Allowance at end of period	1,734	1,073	1,734	1,073
Allowance:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	1,734	1,073	1,734	1,073
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 812,500	$ 885,357	$ 812,500	$ 885,357
Net charge-offs as a percentage of average loans in repayment (annualized)	0.11%	0.17%	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	0.28%	0.16%	0.28%	0.16%
Allowance coverage of net charge-offs (annualized)	2.54	0.92	2.14	0.99
Ending total loans, gross	$ 812,500	$ 885,357	$ 812,500	$ 885,357
Average loans in repayment	634,932	698,197	642,693	709,010
Ending loans in repayment	620,292	680,802	620,292	680,802
Private Education Loans
Allowance for Loan Losses
Allowance at beginning of period	285,946	252,103	277,943	243,715
Total provision	71,296	46,264	113,179	88,134
Net charge-offs:
Charge-offs	(55,382)	(42,270)	(94,959)	(79,623)
Recoveries	6,108	5,598	11,805	10,685
Net charge-offs	(49,274)	(36,672)	(83,154)	(68,938)
Loan sales				(1,216)
Allowance at end of period	307,968	261,695	307,968	261,695
Allowance:
Ending balance: individually evaluated for impairment	146,403	113,343	146,403	113,343
Ending balance: collectively evaluated for impairment	161,565	148,352	161,565	148,352
Loans:
Ending balance: individually evaluated for impairment	1,411,189	1,121,816	1,411,189	1,121,816
Ending balance: collectively evaluated for impairment	$ 20,217,658	$ 17,566,211	$ 20,217,658	$ 17,566,211
Net charge-offs as a percentage of average loans in repayment (annualized)	1.29%	1.14%	1.09%	1.08%
Allowance as a percentage of the ending total loan balance	1.42%	1.40%	1.42%	1.40%
Allowance as a percentage of the ending loans in repayment	2.01%	2.02%	2.01%	2.02%
Allowance coverage of net charge-offs (annualized)	1.56	1.78	1.85	1.90
Ending total loans, gross	$ 21,628,847	$ 18,688,027	$ 21,628,847	$ 18,688,027
Average loans in repayment	15,241,574	12,909,623	15,188,003	12,810,072
Ending loans in repayment	15,332,251	12,979,523	15,332,251	12,979,523
Personal Loans
Allowance for Loan Losses
Allowance at beginning of period	70,619	18,907	62,201	6,628
Total provision	21,474	16,378	44,234	29,826
Net charge-offs:
Charge-offs	(19,074)	(2,872)	(34,325)	(4,072)
Recoveries	1,276	96	2,185	127
Net charge-offs	(17,798)	(2,776)	(32,140)	(3,945)
Loan sales				0
Allowance at end of period	74,295	32,509	74,295	32,509
Allowance:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	74,295	32,509	74,295	32,509
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 1,134,637	$ 966,080	$ 1,134,637	$ 966,080
Net charge-offs as a percentage of average loans in repayment (annualized)	6.20%	1.36%	5.53%	1.17%
Allowance as a percentage of the ending total loan balance	6.55%	3.37%	6.55%	3.37%
Allowance as a percentage of the ending loans in repayment	6.55%	3.37%	6.55%	3.37%
Allowance coverage of net charge-offs (annualized)	1.04	2.93	1.16	4.12
Ending total loans, gross	$ 1,134,637	$ 966,080	$ 1,134,637	$ 966,080
Average loans in repayment	1,148,444	815,741	1,161,761	673,552
Ending loans in repayment	$ 1,134,637	$ 966,080	$ 1,134,637	$ 966,080