Allowance for Loan Losses - Allowance and Recorded Investments in Loans (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Allowance for Loan Losses
Allowance at beginning of period	$ 341,121	$ 251,475
Total provision	65,660	55,549
Net charge-offs:
Charge-offs	(55,062)	(38,803)
Recoveries	6,606	5,118
Net charge-offs	(48,456)	(33,685)
Loan Sales		(1,216)
Allowance at end of period	358,325	272,123
Allowance:
Ending balance: individually evaluated for impairment	132,442	101,824
Ending balance: collectively evaluated for impairment	225,883	170,299
Loans:
Ending balance: individually evaluated for impairment	1,327,668	1,043,103
Ending balance: collectively evaluated for impairment	22,455,468	19,334,407
FFELP Loans
Allowance for Loan Losses
Allowance at beginning of period	977	1,132
Total provision	1,017	231
Net charge-offs:
Charge-offs	(234)	(250)
Recoveries	0	0
Net charge-offs	(234)	(250)
Loan Sales		0
Allowance at end of period	1,760	1,113
Allowance:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	1,760	1,113
Loans:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	$ 828,640	$ 907,842
Net charge-offs as a percentage of average loans in repayment (annualized)	0.14%	0.14%
Allowance as a percentage of the ending total loan balance	0.21%	0.12%
Allowance as a percentage of the ending loans in repayment	0.27%	0.16%
Allowance coverage of net charge-offs (annualized)	1.88	1.11
Ending total loans, gross	$ 828,640	$ 907,842
Average loans in repayment	650,196	718,311
Ending loans in repayment	641,658	702,965
Private Education Loans
Allowance for Loan Losses
Allowance at beginning of period	277,943	243,715
Total provision	41,883	41,870
Net charge-offs:
Charge-offs	(39,577)	(37,353)
Recoveries	5,697	5,087
Net charge-offs	(33,880)	(32,266)
Loan Sales		(1,216)
Allowance at end of period	285,946	252,103
Allowance:
Ending balance: individually evaluated for impairment	132,442	101,824
Ending balance: collectively evaluated for impairment	153,504	150,279
Loans:
Ending balance: individually evaluated for impairment	1,327,668	1,043,103
Ending balance: collectively evaluated for impairment	$ 20,463,954	$ 17,750,909
Net charge-offs as a percentage of average loans in repayment (annualized)	0.89%	1.01%
Allowance as a percentage of the ending total loan balance	1.31%	1.34%
Allowance as a percentage of the ending loans in repayment	1.87%	1.95%
Allowance coverage of net charge-offs (annualized)	2.11	1.95
Ending total loans, gross	$ 21,791,622	$ 18,794,012
Average loans in repayment	15,165,072	12,747,929
Ending loans in repayment	15,310,560	12,958,742
Personal Loans
Allowance for Loan Losses
Allowance at beginning of period	62,201	6,628
Total provision	22,760	13,448
Net charge-offs:
Charge-offs	(15,251)	(1,200)
Recoveries	909	31
Net charge-offs	(14,342)	(1,169)
Loan Sales		0
Allowance at end of period	70,619	18,907
Allowance:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	70,619	18,907
Loans:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	$ 1,162,874	$ 675,656
Net charge-offs as a percentage of average loans in repayment (annualized)	4.88%	0.88%
Allowance as a percentage of the ending total loan balance	6.07%	2.80%
Allowance as a percentage of the ending loans in repayment	6.07%	2.80%
Allowance coverage of net charge-offs (annualized)	1.23	4.04
Ending total loans, gross	$ 1,162,874	$ 675,656
Average loans in repayment	1,175,356	531,889
Ending loans in repayment	$ 1,162,874	$ 675,656