Allowance for Loan Losses - Age Analysis of Past Due Loan Delinquencies (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
TDR loans in repayment and percentage of each status:
Private Education Loans allowance for losses	$ (251,475)	$ (184,701)	$ (112,507)	$ (83,842)
Student Loan | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in in-school/grace/deferment	4,757,732	4,189,955	3,427,964
Loans in forbearance	468,402	351,962	241,207
TDR loans in repayment and percentage of each status:
Loans current	11,911,128	9,509,394	6,773,095
Total TDR loans in repayment	$ 12,206,033	$ 9,709,758	$ 6,927,266
Loans current, in percentage	97.60%	97.90%	97.80%
Loans in repayment, in percentage	100.00%	100.00%	100.00%
Total TDR loans, gross	$ 17,432,167	$ 14,251,675	$ 10,596,437
Private Education Loans deferred origination costs	56,378	44,206	27,884
Total Private Education Loans	17,488,545	14,295,881	10,624,321
Private Education Loans allowance for losses	(243,715)	(182,472)	(108,816)
Percentage of Private Education Loans in repayment	$ 17,244,830	$ 14,113,409	$ 10,515,505
Percentage of Private Education Loans in repayment	70.00%	68.10%	65.40%
Delinquencies as a percentage of Private Education Loans in repayment	2.40%	2.10%	2.20%
Loans in forbearance as a percentage of Private Education Loans in repayment and forbearance	3.70%	3.50%	3.40%
Student Loan | Consumer Portfolio Segment | Loans delinquent 31-60 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 179,002	$ 124,773	$ 91,129
Loans delinquent (in percentage)	1.50%	1.30%	1.30%
Student Loan | Consumer Portfolio Segment | Loans delinquent 61-90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 78,292	$ 51,423	$ 42,048
Loans delinquent (in percentage)	0.60%	0.50%	0.60%
Student Loan | Consumer Portfolio Segment | Loans delinquent greater than 90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 37,611	$ 24,168	$ 20,994
Loans delinquent (in percentage)	0.30%	0.30%	0.30%
Troubled Debt Restructured Loans | Student Loan | Consumer Portfolio Segment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in in-school/grace/deferment	$ 51,745	$ 24,185
Loans in forbearance	69,652	71,851
TDR loans in repayment and percentage of each status:
Loans current	774,222	462,187
Total TDR loans in repayment	$ 868,954	$ 516,570
Loans current, in percentage	89.10%	89.50%
Loans in repayment, in percentage	100.00%	100.00%
Total TDR loans, gross	$ 990,351	$ 612,606
Troubled Debt Restructured Loans | Student Loan | Consumer Portfolio Segment | Loans delinquent 31-60 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 48,377	$ 28,452
Loans delinquent (in percentage)	5.60%	5.50%
Troubled Debt Restructured Loans | Student Loan | Consumer Portfolio Segment | Loans delinquent 61-90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 28,778	$ 17,326
Loans delinquent (in percentage)	3.30%	3.40%
Troubled Debt Restructured Loans | Student Loan | Consumer Portfolio Segment | Loans delinquent greater than 90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 17,577	$ 8,605
Loans delinquent (in percentage)	2.00%	1.60%