Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Dec. 31, 2024
Earning assets:
Trading investments	$ 53,732	$ 53,262
Accrued interest receivable, difference	121,056	116,884
Derivative instruments	0	0
Total earning assets, difference	3,397,947	3,325,107
Interest-bearing liabilities:
Long-term borrowings, difference	69,250	116,961
Derivative instruments	23	40
Total interest-bearing liabilities, difference	60,858	86,316
Excess of net asset fair value over carrying value	3,458,805	3,411,423
Money-market and savings accounts
Interest-bearing liabilities:
Deposits, difference	20,201	22,593
Certificates of deposit
Interest-bearing liabilities:
Deposits, difference	(28,593)	(53,238)
Fair Value
Earning assets:
Cash and cash equivalents	3,695,076	4,700,366
Trading investments	53,732	53,262
Available-for-sale investments	1,656,977	1,933,226
Accrued interest receivable	1,675,035	1,663,474
Derivative instruments	0	0
Total earning assets	31,448,915	32,460,709
Interest-bearing liabilities:
Long-term borrowings	6,078,223	6,323,384
Accrued interest payable	81,504	108,488
Derivative instruments	23	40
Total interest-bearing liabilities	26,238,694	27,529,309
Fair Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	10,396,823	10,503,731
Fair Value | Certificates of deposit
Interest-bearing liabilities:
Deposits	9,682,121	10,593,666
Carrying Value
Earning assets:
Cash and cash equivalents	3,695,076	4,700,366
Trading investments	53,732	53,262
Available-for-sale investments	1,656,977	1,933,226
Accrued interest receivable	1,553,979	1,546,590
Derivative instruments	0	0
Total earning assets	28,050,968	29,135,602
Interest-bearing liabilities:
Long-term borrowings	6,147,473	6,440,345
Accrued interest payable	81,504	108,488
Derivative instruments	23	40
Total interest-bearing liabilities	26,299,552	27,615,625
Carrying Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	10,417,024	10,526,324
Carrying Value | Certificates of deposit
Interest-bearing liabilities:
Deposits	9,653,528	10,540,428
Private Education Loans
Earning assets:
Loans held for investment, net, difference	3,276,891	3,208,223
Private Education Loans | Fair Value
Earning assets:
Loans held for investment, net	24,368,095	24,110,381
Private Education Loans | Carrying Value
Earning assets:
Loans held for investment, net	$ 21,091,204	$ 20,902,158