Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Earning assets:
Trading investments	$ 53,262	$ 54,481
Accrued interest receivable, difference	116,884	68,862
Derivative instruments	0	0
Total earning assets, difference	3,325,107	2,534,325
Interest-bearing liabilities:
Long-term borrowings, difference	116,961	353,822
Derivative instruments	40	370
Total interest-bearing liabilities, difference	86,316	489,912
Excess of net asset fair value over carrying value	3,411,423	3,024,237
Money-market and savings accounts
Interest-bearing liabilities:
Deposits, difference	22,593	68,409
Certificates of deposit
Interest-bearing liabilities:
Deposits, difference	(53,238)	67,681
Fair Value
Earning assets:
Cash and cash equivalents	4,700,366	4,149,838
Trading investments	53,262	54,481
Available-for-sale investments	1,933,226	2,411,622
Accrued interest receivable	1,663,474	1,448,766
Derivative instruments	0	0
Total	32,460,709	30,836,527
Interest-bearing liabilities:
Long-term borrowings	6,323,384	4,873,690
Accrued interest payable	108,488	105,066
Derivative instruments	40	370
Total interest-bearing liabilities	27,529,309	26,494,693
Fair Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	10,503,731	11,134,883
Fair Value | Certificates of deposit
Interest-bearing liabilities:
Deposits	10,593,666	10,380,684
Carrying Value
Earning assets:
Cash and cash equivalents	4,700,366	4,149,838
Trading investments	53,262	54,481
Available-for-sale investments	1,933,226	2,411,622
Accrued interest receivable	1,546,590	1,379,904
Derivative instruments	0	0
Total	29,135,602	28,302,202
Interest-bearing liabilities:
Long-term borrowings	6,440,345	5,227,512
Accrued interest payable	108,488	105,066
Derivative instruments	40	370
Total interest-bearing liabilities	27,615,625	26,984,605
Carrying Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	10,526,324	11,203,292
Carrying Value | Certificates of deposit
Interest-bearing liabilities:
Deposits	10,540,428	10,448,365
Private Education Loans
Earning assets:
Loans held for investment, net, difference	3,208,223	2,456,752
Private Education Loans | Fair Value
Earning assets:
Private Education Loans	24,110,381	22,229,045
Private Education Loans | Carrying Value
Earning assets:
Private Education Loans	20,902,158	19,772,293
FFELP Loans
Earning assets:
Loans held for investment, net, difference	0	8,711
FFELP Loans | Fair Value
Earning assets:
Private Education Loans	0	542,775
FFELP Loans | Carrying Value
Earning assets:
Private Education Loans	$ 0	$ 534,064