Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Earning assets:
Loans held for sale	$ 0	$ 0
Trading investments	54,481	55,903
Accrued interest receivable, difference	68,862	35,015
Tax indemnification receivable	366,247	380,058
Derivative instruments	0	405
Total earning assets, difference	2,534,325	2,088,881
Interest-bearing liabilities:
Long-term borrowings, difference	353,822	421,881
Derivative instruments	370	0
Total interest-bearing liabilities, difference	489,912	838,340
Excess of net asset fair value over carrying value	3,024,237	2,927,221
Money-market and savings accounts
Interest-bearing liabilities:
Certificates of deposits, difference	68,409	104,979
Certificates of Deposit
Interest-bearing liabilities:
Certificates of deposits, difference	67,681	311,480
Fair Value
Earning assets:
Loans held for sale	0	29,448
Cash and cash equivalents	4,149,838	4,616,117
Trading investments	54,481	55,903
Available-for-sale investments	2,411,622	2,342,089
Accrued interest receivable	1,448,766	1,237,074
Tax indemnification receivable	0	2,816
Derivative instruments	0	972
Total	30,836,527	29,965,153
Interest-bearing liabilities:
Long-term borrowings	4,873,690	4,813,233
Accrued interest payable	105,066	71,586
Derivative instruments	370	567
Total interest-bearing liabilities	26,494,693	25,915,574
Fair Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	11,134,883	11,854,849
Fair Value | Certificates of Deposit
Interest-bearing liabilities:
Deposits	10,380,684	9,175,339
Carrying Value
Earning assets:
Loans held for sale	0	29,448
Cash and cash equivalents	4,149,838	4,616,117
Trading investments	54,481	55,903
Available-for-sale investments	2,411,622	2,342,089
Accrued interest receivable	1,379,904	1,202,059
Tax indemnification receivable	0	2,816
Derivative instruments	0	972
Total	28,302,202	27,876,272
Interest-bearing liabilities:
Long-term borrowings	5,227,512	5,235,114
Accrued interest payable	105,066	71,586
Derivative instruments	370	567
Total interest-bearing liabilities	26,984,605	26,753,914
Carrying Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	11,203,292	11,959,828
Carrying Value | Certificates of Deposit
Interest-bearing liabilities:
Deposits	10,448,365	9,486,819
Private Education Loans
Earning assets:
Loans held for investment, net, difference	2,456,752	2,042,835
Private Education Loans | Fair Value
Earning assets:
Private Education Loans	22,229,045	21,062,548
Private Education Loans | Carrying Value
Earning assets:
Private Education Loans	19,772,293	19,019,713
FFELP Loans
Earning assets:
Loans held for investment, net, difference	8,711	11,031
FFELP Loans | Fair Value
Earning assets:
Private Education Loans	542,775	618,186
FFELP Loans | Carrying Value
Earning assets:
Private Education Loans	$ 534,064	$ 607,155