Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Deferred tax assets:
Loan reserves	$ 354,412	$ 362,368
Net unrealized losses	24,176	30,160
Accrued expenses not currently deductible	16,297	12,949
Unrecorded tax benefits	11,568	12,916
Research and development costs	26,519	10,929
Stock-based compensation plans	10,847	9,624
Acquired intangible assets	14,536	781
Operating loss carryovers	26	300
Other	1,426	3,837
Total deferred tax assets	459,807	443,864
Deferred tax liabilities:
Student loan premiums and discounts, net	15,908	14,065
Fixed assets	8,533	9,347
Federal deferred for state receivable	1,171	2,111
Other	614	397
Total deferred tax liabilities	26,226	25,920
Net deferred tax assets	$ 433,581	$ 417,944