Significant Accounting Policies - Effect of CECL Adoption (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	$ 1,357,075	$ 1,165,335	$ 1,361,723	$ 1,143,053	$ 441,912
Deferred tax asset				306,171
Off-balance sheet exposures	124,924	72,713	110,044	115,758	2,481
Retained earnings	3,163,640	2,817,134		952,639	1,850,512
Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				1,100,000
Deferred tax asset				306,000
Off-balance sheet exposures				$ 116,000
Amount deferred				25.00%
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	1,353,631	1,158,977			374,300
FFELP Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	3,444	4,077			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	0	2,281			102
Remaining Adjusted Transition Amounts to be Phased-In
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				$ 1,584,965	1,584,965
Off-balance sheet exposures		72,713	110,044	118,239	118,239
Retained earnings				897,873
Remaining Adjusted Transition Amounts to be Phased-In | Deferred tax asset
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred tax asset				415,540
Remaining Adjusted Transition Amounts to be Phased-In | Private Education Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				1,435,130
Remaining Adjusted Transition Amounts to be Phased-In | FFELP Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				4,485
Remaining Adjusted Transition Amounts to be Phased-In | Personal Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				145,060
Remaining Adjusted Transition Amounts to be Phased-In | Credit Cards
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				290
Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			1,143,053	1,143,053
Off-balance sheet exposures			0	115,758	$ 115,758
Retained earnings				(952,639)
Adjustments | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	(259,536)	(49,097)	(55,811)
Deferred tax asset	(76,542)	0	0
Off-balance sheet exposures	(26,094)	(9,333)	(2,048)
Retained earnings	$ (209,088)	$ (58,429)	$ (57,859)
Adjustments | Deferred tax asset
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred tax asset				306,171
Adjustments | Private Education Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				1,060,830
Adjustments | FFELP Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				2,852
Adjustments | Personal Loans
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				79,183
Adjustments | Credit Cards
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses				$ 188