Allowance for Credit Losses - Payment Status (Amortized Cost Basis) (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2024	Dec. 31, 2023
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, modified in period, amount	$ 941,410	$ 1,020,863	$ 373,683
Current
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, modified in period, amount	766,273	837,473	334,967
30-59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, modified in period, amount	74,279	77,145	17,205
60-89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, modified in period, amount	43,208	44,846	7,689
90 Days or Greater Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, modified in period, amount	57,650	61,399	13,822
Private Education Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, modified in period, amount	$ 966,356	$ 1,049,190	$ 380,526