Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Earning assets:
Loans held for sale	$ 0	$ 340,389
Trading investments	37,465	16,923
Accrued interest receivable, difference	100,743	140,511
Tax indemnification receivable	239,578	374,706
Total earning assets, difference	3,409,977	4,182,834
Interest-bearing liabilities:
Long-term borrowings, difference	(69,184)	(209,092)
Total interest-bearing liabilities, difference	(151,510)	(479,976)
Excess of net asset fair value over carrying value	3,258,467	3,702,858
Money-market and savings accounts
Interest-bearing liabilities:
Certificates of deposits, difference	(24,799)	(68,927)
Certificates of Deposit
Interest-bearing liabilities:
Certificates of deposits, difference	(57,527)	(201,957)
Fair Value
Earning assets:
Loans held for sale	0	3,226,029
Cash and cash equivalents	4,334,603	4,455,292
Trading investments	37,465	16,923
Available-for-sale investments	2,517,956	1,996,634
Accrued interest receivable	1,306,410	1,527,816
Tax indemnification receivable	8,047	18,492
Derivative instruments	1,322	729
Total	31,856,320	34,126,992
Interest-bearing liabilities:
Short-term borrowings	0	0
Long-term borrowings	6,000,174	5,398,309
Accrued interest payable	46,600	60,272
Derivative instruments	252	287
Total interest-bearing liabilities	26,956,044	28,394,651
Fair Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	11,457,490	11,136,560
Fair Value | Certificates of Deposit
Interest-bearing liabilities:
Deposits	9,451,528	11,799,223
Carrying Value
Earning assets:
Loans held for sale	0	2,885,640
Cash and cash equivalents	4,334,603	4,455,292
Trading investments	37,465	16,923
Available-for-sale investments	2,517,956	1,996,634
Accrued interest receivable	1,205,667	1,387,305
Tax indemnification receivable	8,047	18,492
Derivative instruments	1,322	729
Total	28,446,343	29,944,158
Interest-bearing liabilities:
Short-term borrowings	0	0
Long-term borrowings	5,930,990	5,189,217
Accrued interest payable	46,600	60,272
Derivative instruments	252	287
Total interest-bearing liabilities	26,804,534	27,914,675
Carrying Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	11,432,691	11,067,633
Carrying Value | Certificates of Deposit
Interest-bearing liabilities:
Deposits	9,394,001	11,597,266
Private Education Loans
Earning assets:
Loans held for investment, net, difference	3,294,462	3,687,203
Private Education Loans | Fair Value
Earning assets:
Private Education Loans	22,919,836	22,124,171
Private Education Loans | Carrying Value
Earning assets:
Private Education Loans	19,625,374	18,436,968
FFELP Loans
Earning assets:
Loans held for investment, net, difference	12,690	13,449
FFELP Loans | Fair Value
Earning assets:
Private Education Loans	705,644	748,657
FFELP Loans | Carrying Value
Earning assets:
Private Education Loans	692,954	735,208
Credit Cards
Earning assets:
Loans held for investment, net, difference	2,082	1,282
Credit Cards | Fair Value
Earning assets:
Private Education Loans	25,037	12,249
Credit Cards | Carrying Value
Earning assets:
Private Education Loans	$ 22,955	$ 10,967