Allowance for Loan Losses - Loan Status and Aging of Past Due TDR Loans (Details) - Student Loan - Consumer Portfolio Segment - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2015	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR loans in in-school/grace/deferment	$ 4,778,295		$ 4,189,955
TDR loans in forbearance	349,777		351,962
TDR loans in repayment and percentage of each status:
Loans current	10,327,843		9,509,394
Total TDR loans in repayment	10,526,782		9,709,758
Total TDR loans, gross	$ 15,654,854		14,251,675
Loans current (as a percentage)	98.10%	97.90%
Total TDR loans in repayment	100.00%	100.00%
Troubled Debt Restructured Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
TDR loans in in-school/grace/deferment	$ 32,121		24,185
TDR loans in forbearance	78,791		71,851
TDR loans in repayment and percentage of each status:
Loans current	532,459		462,187
Total TDR loans in repayment	590,948		516,570
Total TDR loans, gross	$ 701,860		612,606
Loans current (as a percentage)	90.20%	89.50%
Total TDR loans in repayment	100.00%	100.00%
Loans delinquent 31-60 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 112,167		124,773
Loans delinquent (as a percentage)	1.10%	1.30%
Loans delinquent 31-60 days | Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 29,733		28,452
Loans delinquent (as a percentage)	5.00%	5.50%
Loans delinquent 61-90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 54,128		51,423
Loans delinquent (as a percentage)	0.50%	0.50%
Loans delinquent 61-90 days | Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 16,837		17,326
Loans delinquent (as a percentage)	2.80%	3.40%
Loans delinquent, greater than 90 days
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 32,644		24,168
Loans delinquent (as a percentage)	0.30%	0.30%
Loans delinquent, greater than 90 days | Troubled Debt Restructured Loans
TDR loans in repayment and percentage of each status:
Loans delinquent	$ 11,919		$ 8,605
Loans delinquent (as a percentage)	2.00%	1.60%