Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Earning assets:
Trading investments	$ 60,473	$ 54,481
Accrued interest receivable, difference	85,186	68,862
Tax indemnification receivable	351,126	366,247
Derivative instruments	0	0
Total earning assets, difference	2,626,986	2,534,325
Interest-bearing liabilities:
Long-term borrowings, difference	251,284	353,822
Derivative instruments	116	370
Total interest-bearing liabilities, difference	343,914	489,912
Excess of net asset fair value over carrying value	2,970,900	3,024,237
Money-market and savings accounts
Interest-bearing liabilities:
Deposits, difference	73,242	68,409
Certificates of deposit
Interest-bearing liabilities:
Deposits, difference	19,388	67,681
Fair Value
Earning assets:
Cash and cash equivalents	5,262,448	4,149,838
Trading investments	60,473	54,481
Available-for-sale investments	2,283,262	2,411,622
Accrued interest receivable	1,476,267	1,448,766
Derivative instruments	0	0
Total earning assets	30,539,583	30,836,527
Interest-bearing liabilities:
Long-term borrowings	5,151,728	4,873,690
Accrued interest payable	99,493	105,066
Derivative instruments	116	370
Total interest-bearing liabilities	25,899,817	26,494,693
Fair Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	10,388,501	11,134,883
Fair Value | Certificates of deposit
Interest-bearing liabilities:
Deposits	10,259,979	10,380,684
Carrying Value
Earning assets:
Cash and cash equivalents	5,262,448	4,149,838
Trading investments	60,473	54,481
Available-for-sale investments	2,283,262	2,411,622
Accrued interest receivable	1,391,081	1,379,904
Derivative instruments	0	0
Total earning assets	27,912,597	28,302,202
Interest-bearing liabilities:
Long-term borrowings	5,403,012	5,227,512
Accrued interest payable	99,493	105,066
Derivative instruments	116	370
Total interest-bearing liabilities	26,243,731	26,984,605
Carrying Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	10,461,743	11,203,292
Carrying Value | Certificates of deposit
Interest-bearing liabilities:
Deposits	10,279,367	10,448,365
Private Education Loans
Earning assets:
Loans held for investment, net, difference	2,543,781	2,456,752
Private Education Loans | Fair Value
Earning assets:
Loans held for investment, net	20,976,381	22,229,045
Private Education Loans | Carrying Value
Earning assets:
Loans held for investment, net	18,432,600	19,772,293
FFELP Loans
Earning assets:
Loans held for investment, net, difference	(1,981)	8,711
FFELP Loans | Fair Value
Earning assets:
Loans held for investment, net	480,752	542,775
FFELP Loans | Carrying Value
Earning assets:
Loans held for investment, net	$ 482,733	$ 534,064