Allowance for Loan Losses - Age Analysis of Past Due Loan Delinquencies (Details) - Consumer Portfolio Segment - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Personal Loans
TDR loans in repayment and percentage of each status:
Loans current	$ 1,172,776	$ 398,988
Total TDR loans in repayment	$ 1,190,091	$ 400,280
Loans current, in percentage	98.50%	99.70%
Loans in repayment, in percentage	100.00%	100.00%
Personal Loans | Financing Receivables, 30 to 59 Days Past Due
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 6,722	$ 761
Loans delinquent (in percentage)	0.60%	0.20%
Personal Loans | Financing Receivables, 60 to 89 Days Past Due
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 5,416	$ 340
Loans delinquent (in percentage)	0.50%	0.10%
Personal Loans | Financing Receivables, Equal to Greater than 90 Days Past Due
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 5,177	$ 191
Loans delinquent (in percentage)	0.40%	0.00%
Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in in-school/grace/deferment	$ 5,260,445	$ 4,757,732	$ 4,189,955
Loans in forbearance	577,164	468,402	351,962
TDR loans in repayment and percentage of each status:
Loans current	14,289,705	11,911,128	9,509,394
Total TDR loans in repayment	$ 14,666,856	$ 12,206,033	$ 9,709,758
Loans current, in percentage	97.40%	97.60%	97.90%
Loans in repayment, in percentage	100.00%	100.00%	100.00%
Total TDR loans, gross	$ 20,504,465	$ 17,432,167	$ 14,251,675
Student Loan | Loans delinquent 31-60 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 231,216	$ 179,002	$ 124,773
Loans delinquent (in percentage)	1.60%	1.50%	1.30%
Student Loan | Loans delinquent 61-90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 95,105	$ 78,292	$ 51,423
Loans delinquent (in percentage)	0.70%	0.60%	0.50%
Student Loan | Loans delinquent greater than 90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 50,830	$ 37,611	$ 24,168
Loans delinquent (in percentage)	0.30%	0.30%	0.30%
Troubled Debt Restructured Loans | Student Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans in in-school/grace/deferment	$ 69,212	$ 51,745
Loans in forbearance	69,796	69,652
TDR loans in repayment and percentage of each status:
Loans current	994,411	774,222
Total TDR loans in repayment	$ 1,118,848	$ 868,954
Loans current, in percentage	88.90%	89.10%
Loans in repayment, in percentage	100.00%	100.00%
Total TDR loans, gross	$ 1,257,856	$ 990,351
Troubled Debt Restructured Loans | Student Loan | Loans delinquent 31-60 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 63,074	$ 48,377
Loans delinquent (in percentage)	5.60%	5.60%
Troubled Debt Restructured Loans | Student Loan | Loans delinquent 61-90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 36,804	$ 28,778
Loans delinquent (in percentage)	3.30%	3.30%
Troubled Debt Restructured Loans | Student Loan | Loans delinquent greater than 90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 24,559	$ 17,577
Loans delinquent (in percentage)	2.20%	2.00%