Significant Accounting Policies - Additional Information (Detail)		3 Months Ended								4 Months Ended	12 Months Ended
	Jul. 01, 2018 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Apr. 29, 2014	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Apr. 30, 2014 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Discrete tax expense												$ 15
Private Education Loan loss confirmation period (in years)											1 year
FFELP loan loss confirmation period (in years)											2 years
Loan delinquent period										90 days
Personal loans held at end of year		$ 1,100,000,000				$ 393,700,000					$ 1,100,000,000	$ 393,700,000
Interest rate reduction and forbearance usage (greater than)											3 months
Loans And Leases Receivable, Temporary Interest Rate Reduction, Financial Difficulty Status		2.00%									2.00%
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)		57.00%				66.00%					57.00%	66.00%
Percentage of Private Education Loans related to borrowers inschool, grace or deferment (as a percentage)		26.00%				27.00%					26.00%	27.00%
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%									97.00%
Percentage reimbursement on all qualifying default claims period one (as a percentage)		98.00%									98.00%
Percentage reimbursement on all qualifying default claims period three (as a percentage)		100.00%									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)											6.83%	5.94%
Variable rate		1.05									1.05
Gains on sales of loans, net											$ 2,060,000	$ 0	$ 230,000
Asset balance related to securitization trust (as a percentage)											10.00%
Total amount of notional derivative contracts		$ 6,000,000,000									$ 6,000,000,000
Income tax expense		44,449,000	$ (53,667,000)	$ 40,074,000	$ 40,997,000	$ 66,190,000	$ 40,617,000	$ 44,713,000	$ 51,011,000		71,853,000	202,531,000	164,109,000
Income taxes receivable, net		41,570,000				0					41,570,000	0
Net positive exposure		27,000,000				20,000,000					27,000,000	20,000,000
Reclassification resulting from the adoption of ASU No. 2017-12						(165,000)						(165,000)
Losses on derivatives and hedging activities, net											$ 87,000	8,266,000	$ 958,000
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		43,000,000									$ 43,000,000
Net gain											2,000,000
Navient
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred tax asset discount														$ 27,000,000
Deferred tax liability		24,000,000									24,000,000			310,000,000
Deferred taxes														283,000,000
Income taxes receivable, net						24,000,000						24,000,000		291,000,000
Deferred tax assets														310,000,000
Deferred tax asset discount														$ 19,000,000
CME
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total amount of notional derivative contracts		5,400,000,000									$ 5,400,000,000
Percent of total notional derivative contracts											90.20%
Amount of variation margin included as settlement											$ (16,700,000)
LCH
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total amount of notional derivative contracts		600,000,000									$ 600,000,000
Percent of total notional derivative contracts											9.80%
Amount of variation margin included as settlement											$ (6,100,000)
Retained Earnings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification resulting from the adoption of ASU No. 2017-12						$ 264,000						$ 264,000
Accumulated Other Comprehensive Income (Loss)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification resulting from the adoption of ASU No. 2018-02					$ 600,000						592,000
Discontinued Operations, Disposed of by Sale
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal Amount Outstanding of Loans Held-in-portfolio				43,000,000
Gains on sales of loans, net				$ 2,000,000
Accounting Standards Update 2017-12 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification resulting from the adoption of ASU No. 2017-12		(861,000)									(861,000)
Accounting Standards Update 2017-12 [Member] | Retained Earnings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification resulting from the adoption of ASU No. 2017-12		(591,000)									(591,000)
Accounting Standards Update 2017-12 [Member] | Accumulated Other Comprehensive Income (Loss)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification resulting from the adoption of ASU No. 2017-12		$ (270,000)									$ (270,000)
Fair Value | Accounting Standards Update 2017-12 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net positive exposure	$ 2,000,000
Losses on derivatives and hedging activities, net	3,000,000
Fair Value | Accounting Standards Update 2017-12 [Member] | Retained Earnings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification resulting from the adoption of ASU No. 2017-12	(800,000)
Cash Flow | Accounting Standards Update 2017-12 [Member] | Retained Earnings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification resulting from the adoption of ASU No. 2017-12	200,000
Cash Flow | Accounting Standards Update 2017-12 [Member] | Accumulated Other Comprehensive Income (Loss)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification resulting from the adoption of ASU No. 2017-12	$ (300,000)
Accounting Standards Update 2016-09 | Adjustments for New Accounting Pronouncement [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Income tax expense									8,500,000
Reclassification resulting from the adoption of ASU No. 2017-12									$ (200,000)