Allowance for Loan Losses - Allowance for Credit Losses and Recorded Investments in Loans (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) charge_off	Jun. 30, 2017 USD ($) charge_off	Jun. 30, 2018 USD ($) charge_off	Jun. 30, 2017 USD ($) charge_off
Allowance for Loan Losses
Allowance at beginning of period	$ 272,123	$ 187,086	$ 251,475	$ 184,701
Total provision	62,894	49,886	118,443	76,678
Net charge-offs:
Charge-offs	(45,434)	(33,007)	(84,237)	(59,452)
Recoveries	5,694	4,396	10,812	7,655
Net charge-offs	(39,740)	(28,611)	(73,425)	(51,797)
Loan sales	0	(913)	(1,216)	(2,134)
Allowance at end of period	295,277	207,448	295,277	207,448
Allowance:
Ending balance: individually evaluated for impairment	113,343	95,177	113,343	95,177
Ending balance: collectively evaluated for impairment	181,934	112,271	181,934	112,271
Loans:
Ending balance: individually evaluated for impairment	1,121,816	803,456	1,121,816	803,456
Ending balance: collectively evaluated for impairment	19,417,648	15,912,746	19,417,648	15,912,746
FFELP Loans
Allowance for Loan Losses
Allowance at beginning of period	1,113	1,637	1,132	2,171
Total provision	252	228	483	(88)
Net charge-offs:
Charge-offs	(292)	(259)	(542)	(477)
Recoveries	0	0	0	0
Net charge-offs	(292)	(259)	(542)	(477)
Loan sales	0	0	0	0
Allowance at end of period	1,073	1,606	1,073	1,606
Allowance:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	1,073	1,606	1,073	1,606
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 885,357	$ 967,237	$ 885,357	$ 967,237
Net charge-offs as a percentage of average loans in repayment (annualized)	0.17%	0.14%	0.15%	0.12%
Allowance as a percentage of the ending total loan balance	0.12%	0.17%	0.12%	0.17%
Allowance as a percentage of the ending loans in repayment	0.16%	0.21%	0.16%	0.21%
Allowance coverage of net charge-offs (annualized) | charge_off	0.92	1.55	0.99	1.68
Ending total loans, gross	$ 885,357	$ 967,237	$ 885,357	$ 967,237
Average loans in repayment	698,197	757,186	709,010	765,347
Ending loans in repayment	680,802	765,980	680,802	765,980
Private Education Loans
Allowance for Loan Losses
Allowance at beginning of period	252,103	185,103	243,715	182,472
Total provision	46,264	49,166	88,134	75,986
Net charge-offs:
Charge-offs	(42,270)	(32,728)	(79,623)	(58,955)
Recoveries	5,598	4,396	10,685	7,655
Net charge-offs	(36,672)	(28,332)	(68,938)	(51,300)
Loan sales	0	(913)	(1,216)	(2,134)
Allowance at end of period	261,695	205,024	261,695	205,024
Allowance:
Ending balance: individually evaluated for impairment	113,343	95,177	113,343	95,177
Ending balance: collectively evaluated for impairment	148,352	109,847	148,352	109,847
Loans:
Ending balance: individually evaluated for impairment	1,121,816	803,456	1,121,816	803,456
Ending balance: collectively evaluated for impairment	$ 17,566,211	$ 14,876,001	$ 17,566,211	$ 14,876,001
Net charge-offs as a percentage of average loans in repayment (annualized)	1.14%	1.08%	1.08%	0.99%
Allowance as a percentage of the ending total loan balance	1.40%	1.31%	1.40%	1.31%
Allowance as a percentage of the ending loans in repayment	2.02%	1.93%	2.02%	1.93%
Allowance coverage of net charge-offs (annualized) | charge_off	1.78	1.81	1.90	2.00
Ending total loans, gross	$ 18,688,027	$ 15,679,457	$ 18,688,027	$ 15,679,457
Average loans in repayment	12,909,623	10,523,225	12,810,072	10,375,463
Ending loans in repayment	12,979,523	10,615,105	12,979,523	10,615,105
Personal Loans
Allowance for Loan Losses
Allowance at beginning of period	18,907	346	6,628	58
Total provision	16,378	492	29,826	780
Net charge-offs:
Charge-offs	(2,872)	(20)	(4,072)	(20)
Recoveries	96	0	127	0
Net charge-offs	(2,776)	(20)	(3,945)	(20)
Loan sales	0	0	0	0
Allowance at end of period	32,509	818	32,509	818
Allowance:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	32,509	818	32,509	818
Loans:
Ending balance: individually evaluated for impairment	0	0	0	0
Ending balance: collectively evaluated for impairment	$ 966,080	$ 69,508	$ 966,080	$ 69,508
Net charge-offs as a percentage of average loans in repayment (annualized)	1.36%	0.13%	1.17%	0.08%
Allowance as a percentage of the ending total loan balance	3.37%	1.18%	3.37%	1.18%
Allowance as a percentage of the ending loans in repayment	3.37%	1.18%	3.37%	1.18%
Allowance coverage of net charge-offs (annualized) | charge_off	2.93	10.23	4.12	20.45
Ending total loans, gross	$ 966,080	$ 69,508	$ 966,080	$ 69,508
Average loans in repayment	815,741	61,439	673,552	47,654
Ending loans in repayment	$ 966,080	$ 69,508	$ 966,080	$ 69,508