Significant Accounting Policies - Additional Information (Detail)	3 Months Ended	4 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($)	Apr. 29, 2014	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Apr. 30, 2014 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
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			$ 17,244,830,000	$ 14,113,409,000
Personal loans held at end of year			$ 393,700,000	$ 12,800,000.0
Interest rate reduction and forbearance usage (greater than)			3 months
Period of forbearance period to be classified as TDR			3 months
Period of forbearance			24 months
Period after grace period for forbearance allowance for loans			9 months
Forbearance period after grace period for loans			6 months
Percentage of loans granted forbearance (as a percentage)			66.00%	69.00%
Percentage of Private Education Loans related to borrowers inschool, grace or deferment (as a percentage)			27.00%	29.00%
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 loss emergence to estimate credit losses incurred			2 years
Period of notification for withdrawal of deposits (less than or equal to)			7 days
Constant prepayment rate for Private Education Loans (in percentage)			5.94%	5.00%
Variable rate			1.05
Gains on sales of loans, net			$ 0	$ 230,000	$ 135,358,000
Asset balance related to securitization trust (as a percentage)			10.00%
Total amount of notional derivative contracts			$ 5,500,000,000
Excess tax benefit, amount	$ 200,000		8,500,000
Deferred taxes				9,180,000		$ 283,000,000
Tax indemnification receivable			168,011,000	259,532,000
Deferred tax asset discount			108,000,000
Remaining amount of indemnification			$ 35,000,000
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan delinquent period			120 days
Split Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private education loans			$ 48,000,000
Navient
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred taxes			37,000,000			283,000,000
Tax indemnification receivable				60,000,000		291,000,000
Deferred tax asset discount			25,000,000	25,000,000		27,000,000
Deferred tax liability			62,000,000			310,000,000
Deferred tax assets						310,000,000
Deferred tax asset discount						$ 19,000,000
Remaining amount of indemnification				35,000,000
Remaining balance of liability for uncertain tax positions			108
Additional Tax Positions
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase in tax receivable and income tax payable				120,000,000
Increase in other income and income tax expense				$ 9,000,000
CME
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total amount of notional derivative contracts			$ 4,800,000,000
Percent of total notional derivative contracts			87.60%
LCH
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total amount of notional derivative contracts			$ 700,000,000
Percent of total notional derivative contracts			12.40%