Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2023	Dec. 31, 2022
Earning assets:
Loans held for sale	$ 0	$ 0
Trading investments	52,146	55,903
Accrued interest receivable, difference	57,224	35,015
Tax indemnification receivable	325,063	380,058
Derivative instruments	0	405
Total earning assets, difference	2,358,899	2,088,881
Interest-bearing liabilities:
Long-term borrowings, difference	377,679	421,881
Derivative instruments	210	0
Total interest-bearing liabilities, difference	794,396	838,340
Excess of net asset fair value over carrying value	3,153,295	2,927,221
Money-market and savings accounts
Interest-bearing liabilities:
Deposits, difference	106,258	104,979
Certificates of deposit
Interest-bearing liabilities:
Deposits, difference	310,459	311,480
Fair Value
Earning assets:
Loans held for sale	0	29,448
Cash and cash equivalents	3,875,758	4,616,117
Trading investments	52,146	55,903
Available-for-sale investments	2,372,846	2,342,089
Accrued interest receivable	1,357,360	1,237,074
Tax indemnification receivable	2,902	2,816
Derivative instruments	212	972
Total earning assets	29,182,417	29,965,153
Interest-bearing liabilities:
Long-term borrowings	4,836,019	4,813,233
Accrued interest payable	82,019	71,586
Derivative instruments	422	567
Total interest-bearing liabilities	24,860,704	25,915,574
Fair Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	11,063,242	11,854,849
Fair Value | Certificates of deposit
Interest-bearing liabilities:
Deposits	8,879,002	9,175,339
Carrying Value
Earning assets:
Loans held for sale	0	29,448
Cash and cash equivalents	3,875,758	4,616,117
Trading investments	52,146	55,903
Available-for-sale investments	2,372,846	2,342,089
Accrued interest receivable	1,300,136	1,202,059
Tax indemnification receivable	2,902	2,816
Derivative instruments	212	972
Total earning assets	26,823,518	27,876,272
Interest-bearing liabilities:
Long-term borrowings	5,213,698	5,235,114
Accrued interest payable	82,019	71,586
Derivative instruments	422	567
Total interest-bearing liabilities	25,655,100	26,753,914
Carrying Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	11,169,500	11,959,828
Carrying Value | Certificates of deposit
Interest-bearing liabilities:
Deposits	9,189,461	9,486,819
Private Education Loans
Earning assets:
Loans held for investment, net, difference	2,292,956	2,042,835
Private Education Loans | Fair Value
Earning assets:
Loans held for investment, net	20,941,860	21,062,548
Private Education Loans | Carrying Value
Earning assets:
Loans held for investment, net	18,648,904	19,019,713
FFELP Loans
Earning assets:
Loans held for investment, net, difference	8,719	11,031
FFELP Loans | Fair Value
Earning assets:
Loans held for investment, net	579,333	618,186
FFELP Loans | Carrying Value
Earning assets:
Loans held for investment, net	$ 570,614	$ 607,155