Significant Accounting Policies - Additional Information (Detail) - USD ($)		3 Months Ended	4 Months Ended		8 Months Ended	12 Months Ended
	Apr. 29, 2014	Jun. 30, 2014	Apr. 30, 2014	Apr. 29, 2014	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans purchased						$ 0
Private Education Loan loss confirmation period (in years)						1 year
FFELP loan loss confirmation period (in years)						2 years
Loan delinquent period				90 days
Minimum days past due for spin off loan purchase						90 days
Period of hardship forbearance						6 months
Private education loans						$ 14,113,409,000	$ 10,515,505,000
Loss confirmation period	2 years					1 year
Personal loans held at end of year						$ 12.8
Interest rate reduction and forbearance usage (greater than)						3 months
Period after grace period for forbearance allowance for loans						9 months
Forbearance period after grace period for loans						6 months
Period of forbearance period to be classified as TDR						3 months
Percentage of loans granted forbearance (as a percentage)						26.00%	23.00%
Dollar amount of change in allowance due to policy change		$ 14,000,000
Percentage of Private Education Loans related to borrowers inschool, grace or deferment (as a percentage)						29.00%	32.00%
Period after last payment contractually due private education loan considered to be delinquent						31 days
Percentage reimbursement on all qualifying default claims period two (as a percentage)						97.00%
Percentage reimbursement on all qualifying default claims period one (as a percentage)						98.00%
Percentage reimbursement on all qualifying default claims period three (as a percentage)						100.00%
Period of notification for withdrawal of deposits (less than or equal to)						7 days
Constant prepayment rate for Private Education Loans (in percentage)						5.00%	5.12%
Loans sold								$ 805,000,000
Gains on sales of loans, net			$ 36,000,000		$ 85,000,000	$ 230,000	$ 135,358,000	$ 121,359,000
Asset balance related to securitization trust (as a percentage)						10.00%
Deferred taxes			283,000,000			$ 9,180,000	81,588,000
Tax indemnification receivable						259,532,000	186,076,000
Deferred tax asset discount			27,000,000			116,000,000
Remaining amount of indemnification						$ 116,000,000
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan delinquent period	212 days					120 days
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan delinquent period	150 days					30 days
FFELP Loans Rehabilitated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans purchased						$ 0
Split Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private education loans						67,000,000
Navient
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred taxes			283,000,000				131,000,000
Tax indemnification receivable			291,000,000				144,000,000
Deferred tax asset discount			27,000,000			$ 0	28,000,000
Deferred tax liability			310,000,000				159,000,000
Deferred tax assets			310,000,000
Deferred tax asset discount			$ 19,000,000
Remaining amount of indemnification							$ 116,000,000
ABCP borrowings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractual maturity related to ABCP facility						2 years
Additional Tax Positions [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase (decrease) in tax receivable and income tax payable						$ 120,000,000
Increase (decrease) in other income and income tax expense						$ 9,000,000