Significant Accounting Policies - Additional Information (Detail)		3 Months Ended	4 Months Ended		8 Months Ended	12 Months Ended
	Apr. 29, 2014	Jun. 30, 2014 USD ($)	Apr. 30, 2014 USD ($)	Apr. 29, 2014	Dec. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) payment	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
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
Period after grace period for forbearance allowance for loans						9 months
Private education loans					$ 8,246,647,000	$ 10,515,505,000	$ 8,246,647,000
Loss confirmation period	2 years					1 year
Interest rate reduction and forbearance usage (greater than)						3 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					10.00%	23.00%	10.00%
Percentage of Private Education Loans related to borrowers inschool, grace or deferment					36.00%	32.00%	36.00%
Dollar amount of change in allowance due to policy change		$ 14,000,000
Period after last payment contractually due private education loan considered to be delinquent						31 days
Percentage reimbursement on all qualifying default claims period two						97.00%
Percentage reimbursement on all qualifying default claims period one						98.00%
Percentage reimbursement on all qualifying default claims period three						100.00%
Period of notification for withdrawal of deposits (less than or equal to)						7 days
Consolidation loan rebate fee						1.05%
Loans sold							$ 805,000,000	$ 2,400,000,000
Gains on sales of loans, net			$ 36,000,000		$ 85,000,000	$ 135,358,000	121,359,000	$ 196,593,000
Asset balance related to securitization trust (Percent)						10.00%
Tax indemnification receivable					240,311,000	$ 186,076,000	240,311,000
Remaining amount of indemnification						170,000,000
Deferred tax asset discount						16,000,000
Deferred taxes			283,000,000		$ 158,216,000	$ 81,588,000	158,216,000
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan delinquent period	150 days					30 days
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan delinquent period	212 days					120 days
FFELP Loans Rehabilitated
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payment period for rehabilitation qualification | payment						9
Period of Loan Rehabilitation						10 months
Loans purchased						$ 0	$ 7,464,000
Percent of loan par value					101.67%		101.67%
Split Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private education loans						89,000,000
Navient
Accounts, Notes, Loans and Financing Receivable [Line Items]
Tax indemnification receivable			291,000,000			186,076,000
Remaining amount of indemnification						170,000,000
Deferred tax asset discount			27,000,000			16,000,000
Deferred tax liability			310,000,000			207,000,000
Deferred taxes			283,000,000			$ 191,000,000
Deferred tax assets			310,000,000
Deferred tax asset discount			$ 19,000,000
ABCP borrowings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Contractual maturity related to ABCP facility						2 years