Significant Accounting Policies (Details) - USD ($) $ in Thousands			3 Months Ended									12 Months Ended
	Jul. 01, 2018	Jan. 01, 2018	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2020	Jan. 01, 2019	Jan. 01, 2017	Dec. 31, 2016	Apr. 30, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private Education Loan loss confirmation period												1 year
FFELP loan loss confirmation period												2 years
Percentage of Private Education Loans related to borrowers inschool, grace or deferment			25.00%				26.00%					25.00%	26.00%
Interest rate reduction and forbearance usage (greater than)												3 months
Interest rate offered to borrowers facing financial difficulty			4.00%									4.00%
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)			50.00%				57.00%					50.00%	57.00%
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)												6.92%	6.83%
Variable rate			1.05									1.05
Gains on sales of loans, net												$ 0	$ 2,060	$ 0
Asset balance related to securitization trust (as a percentage)												10.00%
Notional derivative contracts			$ 9,900,000									$ 9,900,000
Basis increase to our hedged deposit balances			52,000				$ 27,000					52,000	27,000
Reclassification resulting from the adoption of ASU No. 2017-12																	$ 165
Gains (losses) on derivatives and hedging activities, net												17,825	(87)	(8,266)
Income tax expense			34,666	$ 40,701	$ 33,801	$ 56,296	44,449	$ (53,667)	$ 40,074	$ 40,997		165,464	71,853	202,531
Discrete tax expense														15,000
Reclassification resulting from the adoption of ASU No. 2018-02		$ 600
Income taxes receivable, net			88,844				41,570					88,844	41,570
Increase in allowance for loan losses			441,912				341,121					441,912	341,121	$ 251,475				$ 184,701
Increase in exposure for unfunded commitments			254,731				224,951					254,731	224,951
Increase in deferred tax asset			113,900				95,804					113,900	95,804
Navient
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred tax asset discount																			$ 27,000
Deferred tax liability			15,000									15,000							310,000
Deferred taxes																			283,000
Income taxes receivable, net							$ 15,000						15,000						291,000
Deferred tax assets																			310,000
Deferred tax asset discount																			$ 19,000
Retained Earnings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification resulting from the adoption of ASU No. 2017-12																	$ (264)
Reclassification resulting from the adoption of ASU No. 2018-02													(592)
Accumulated Other Comprehensive Income (Loss)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification resulting from the adoption of ASU No. 2018-02													$ 592
Accounting Standards Update 2017-12
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification resulting from the adoption of ASU No. 2017-12		861
Accounting Standards Update 2017-12 | Retained Earnings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification resulting from the adoption of ASU No. 2017-12		591
Accounting Standards Update 2017-12 | Accumulated Other Comprehensive Income (Loss)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification resulting from the adoption of ASU No. 2017-12		$ 270
Accounting Standards Update 2017-12 | Fair Value
Accounts, Notes, Loans and Financing Receivable [Line Items]
Basis increase to our hedged deposit balances	$ 2,000
Accounting Standards Update 2017-12 | Fair Value | Retained Earnings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification resulting from the adoption of ASU No. 2017-12	800
Accounting Standards Update 2017-12 | Fair Value | Accumulated Other Comprehensive Income (Loss)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gains (losses) on derivatives and hedging activities, net	3,000
Accounting Standards Update 2017-12 | Cash Flow | Retained Earnings
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification resulting from the adoption of ASU No. 2017-12	200
Accounting Standards Update 2017-12 | Cash Flow | Accumulated Other Comprehensive Income (Loss)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification resulting from the adoption of ASU No. 2017-12	$ 300
Adjustments for New Accounting Pronouncement | Accounting Standards Update 2016-09
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassification resulting from the adoption of ASU No. 2017-12											$ 200
Income tax expense											$ 8,500
Accounting Standards Update 2016-02
Accounts, Notes, Loans and Financing Receivable [Line Items]
Operating lease, right-of-use asset																$ 34,000
Finance lease, liability																$ 34,000
Accounting Standards Update 2016-13 | Forecast
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase in allowance for loan losses															$ 1,100,000
Increase in deferred tax asset															300,000
Cumulative effect adjustment estimated reduction to retained earnings															950,000
Accounting Standards Update 2016-13 | Forecast | Unfunded Loan Commitment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Increase in exposure for unfunded commitments															$ 115,000
CME
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notional derivative contracts			9,400,000									$ 9,400,000
Percent of total notional derivative contracts												95.00%
Amount of variation margin included as settlement												$ (107,000)
LCH
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notional derivative contracts			500,000									$ 500,000
Percent of total notional derivative contracts												5.00%
Amount of variation margin included as settlement												$ 8,000
Discontinued Operations, Disposed of by Sale
Accounts, Notes, Loans and Financing Receivable [Line Items]
Split Loans portfolio sales price									43,000
Gains on sales of loans, net									$ 2,000
Split Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Private education loans			$ 43,000									43,000
Net gain												$ 2,000
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan delinquent period												120 days
Interest rate offered to borrowers facing financial difficulty				4.00%