Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Earning assets:
Loans held for sale	$ 0	$ 0
Trading investments	54,840	54,481
Accrued interest receivable, difference	129,945	68,862
Derivative instruments	516	0
Total earning assets, difference	3,482,213	2,534,325
Interest-bearing liabilities:
Long-term borrowings, difference	146,758	353,822
Derivative instruments	0	370
Total interest-bearing liabilities, difference	86,844	489,912
Excess of net asset fair value over carrying value	3,569,057	3,024,237
Money-market and savings accounts
Interest-bearing liabilities:
Deposits, difference	6,903	68,409
Certificates of deposit
Interest-bearing liabilities:
Deposits, difference	(66,817)	67,681
Fair Value
Earning assets:
Loans held for sale	485,701	0
Cash and cash equivalents	4,489,539	4,149,838
Trading investments	54,840	54,481
Available-for-sale investments	2,022,605	2,411,622
Accrued interest receivable	1,667,539	1,448,766
Derivative instruments	600	0
Total earning assets	32,533,025	30,836,527
Interest-bearing liabilities:
Long-term borrowings	5,889,769	4,873,690
Accrued interest payable	98,945	105,066
Derivative instruments	84	370
Total interest-bearing liabilities	27,493,422	26,494,693
Fair Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	10,477,366	11,134,883
Fair Value | Certificates of deposit
Interest-bearing liabilities:
Deposits	11,027,258	10,380,684
Carrying Value
Earning assets:
Loans held for sale	485,701	0
Cash and cash equivalents	4,489,539	4,149,838
Trading investments	54,840	54,481
Available-for-sale investments	2,022,605	2,411,622
Accrued interest receivable	1,537,594	1,379,904
Derivative instruments	600	0
Total earning assets	29,050,812	28,302,202
Interest-bearing liabilities:
Long-term borrowings	6,036,527	5,227,512
Accrued interest payable	98,945	105,066
Derivative instruments	84	370
Total interest-bearing liabilities	27,580,266	26,984,605
Carrying Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	10,484,269	11,203,292
Carrying Value | Certificates of deposit
Interest-bearing liabilities:
Deposits	10,960,441	10,448,365
Private Education Loans
Earning assets:
Loans held for investment, net, difference	3,352,268	2,456,752
Private Education Loans | Fair Value
Earning assets:
Loans held for investment, net	23,812,201	22,229,045
Private Education Loans | Carrying Value
Earning assets:
Loans held for investment, net	20,459,933	19,772,293
FFELP Loans
Earning assets:
Loans held for investment, net, difference	0	8,711
FFELP Loans | Fair Value
Earning assets:
Loans held for investment, net	0	542,775
FFELP Loans | Carrying Value
Earning assets:
Loans held for investment, net	$ 0	$ 534,064