Significant Accounting Policies - Effect of CECL Adoption (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	$ 1,165,335	$ 1,361,723		$ 441,912	$ 341,121
Off-balance sheet exposures	72,713	110,044		2,481	$ 2,165
Retained earnings	2,817,134	1,722,365		1,850,512
Deferred tax asset
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred tax asset				109,369
Private Education Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	1,158,977	1,355,844		374,300
FFELP Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	4,077	4,378		1,633
Personal Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				65,877
Credit Cards
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	$ 2,281	$ 1,501		102
Cumulative Effect, Period Of Adoption, Adjusted Balance
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			$ 1,584,965
Off-balance sheet exposures			118,239	118,239
Retained earnings			897,873
Cumulative Effect, Period Of Adoption, Adjusted Balance | Deferred tax asset
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred tax asset			415,540
Cumulative Effect, Period Of Adoption, Adjusted Balance | Private Education Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			1,435,130
Cumulative Effect, Period Of Adoption, Adjusted Balance | FFELP Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			4,485
Cumulative Effect, Period Of Adoption, Adjusted Balance | Personal Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			145,060
Cumulative Effect, Period Of Adoption, Adjusted Balance | Credit Cards
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			290
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			1,143,053	1,584,965
Off-balance sheet exposures			115,758	$ 115,758
Retained earnings			(952,639)
Cumulative Effect, Period of Adoption, Adjustment | Deferred tax asset
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred tax asset			306,171
Cumulative Effect, Period of Adoption, Adjustment | Private Education Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			1,060,830
Cumulative Effect, Period of Adoption, Adjustment | FFELP Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			2,852
Cumulative Effect, Period of Adoption, Adjustment | Personal Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			79,183
Cumulative Effect, Period of Adoption, Adjustment | Credit Cards
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			$ 188