Allowance for Credit Losses - Amortized Cost Basis of Financing Receivables that Subsequently Defaulted (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified loans		$ 513,371				$ 380,526
60 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified loans	$ 27,322		$ 12,360	$ 39,634	$ 19,374
Payment Default	26,760		12,099	40,128	20,304
Charge-Offs	4,304		3,348	6,371	5,279
Combination - Interest Rate Reduction and Term Extension
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified loans	25,700		11,000	37,500	17,000
Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified loans	1,600		1,400	2,100	2,400
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified loans		$ 513,371				$ 380,526
Private Education Loans | 60 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Modified loans	27,322		12,360	39,634	19,374
Payment Default	26,760		12,099	40,128	20,304
Charge-Offs	$ 4,304		$ 3,348	$ 6,371	$ 5,279