Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Earning assets:
Trading investments	$ 58,166	$ 54,481
Accrued interest receivable, difference	80,762	68,862
Tax indemnification receivable	277,733	366,247
Derivative instruments	125	0
Total earning assets, difference	2,743,938	2,534,325
Interest-bearing liabilities:
Long-term borrowings, difference	273,686	353,822
Derivative instruments	33	370
Total interest-bearing liabilities, difference	474,975	489,912
Excess of net asset fair value over carrying value	3,218,913	3,024,237
Money-market and savings accounts
Interest-bearing liabilities:
Deposits, difference	149,051	68,409
Certificates of deposit
Interest-bearing liabilities:
Deposits, difference	52,238	67,681
Fair Value
Earning assets:
Cash and cash equivalents	3,584,013	4,149,838
Trading investments	58,166	54,481
Available-for-sale investments	2,271,108	2,411,622
Accrued interest receivable	1,467,249	1,448,766
Tax indemnification receivable	0	0
Derivative instruments	208	0
Total earning assets	30,244,709	30,836,527
Interest-bearing liabilities:
Long-term borrowings	4,703,196	4,873,690
Accrued interest payable	89,328	105,066
Derivative instruments	116	370
Total interest-bearing liabilities	25,492,560	26,494,693
Fair Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	10,268,578	11,134,883
Fair Value | Certificates of deposit
Interest-bearing liabilities:
Deposits	10,431,342	10,380,684
Carrying Value
Earning assets:
Cash and cash equivalents	3,584,013	4,149,838
Trading investments	58,166	54,481
Available-for-sale investments	2,271,108	2,411,622
Accrued interest receivable	1,386,487	1,379,904
Tax indemnification receivable	0	0
Derivative instruments	208	0
Total earning assets	27,500,771	28,302,202
Interest-bearing liabilities:
Long-term borrowings	4,976,882	5,227,512
Accrued interest payable	89,328	105,066
Derivative instruments	116	370
Total interest-bearing liabilities	25,967,535	26,984,605
Carrying Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	10,417,629	11,203,292
Carrying Value | Certificates of deposit
Interest-bearing liabilities:
Deposits	10,483,580	10,448,365
Private Education Loans
Earning assets:
Loans held for investment, net, difference	2,654,655	2,456,752
Private Education Loans | Fair Value
Earning assets:
Loans held for investment, net	22,342,438	22,229,045
Private Education Loans | Carrying Value
Earning assets:
Loans held for investment, net	19,687,783	19,772,293
FFELP Loans
Earning assets:
Loans held for investment, net, difference	8,521	8,711
FFELP Loans | Fair Value
Earning assets:
Loans held for investment, net	521,527	542,775
FFELP Loans | Carrying Value
Earning assets:
Loans held for investment, net	$ 513,006	$ 534,064