Fair Value Measurements - Fair Values of Financial Assets and Liabilities, Including Derivative Financial Instruments (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Earning assets:
Accrued interest receivable, difference between carrying and fair value	$ 98,746	$ 93,861
Tax indemnification receivable	88,844	41,570
Total earning assets, difference	2,265,887	2,152,077
Interest-bearing liabilities:
Long-term borrowings, difference	(80,286)	5,373
Total interest-bearing liabilities, difference	(226,078)	63,304
Excess of net asset fair value over carrying value	2,039,809	2,215,381
Certificates of Deposit
Interest-bearing liabilities:
Certificates of deposits, difference	(117,264)	38,780
Fair Value
Earning assets:
Credit Cards	3,818
Cash and cash equivalents	5,563,877	2,559,106
Available-for-sale investments	487,669	176,245
Accrued interest receivable	1,491,471	1,285,842
Tax indemnification receivable	27,558	39,207
Derivative instruments	715	2,090
Total	34,406,223	28,391,625
Interest-bearing liabilities:
Short-term borrowings	289,230	0
Long-term borrowings	4,434,323	4,278,931
Accrued interest payable	75,158	61,341
Derivative instruments	1,164	2,032
Total interest-bearing liabilities	29,228,573	23,226,455
Fair Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	10,363,691	9,370,957
Fair Value | Certificates of Deposit
Interest-bearing liabilities:
Deposits	14,065,007	9,513,194
Carrying Value
Earning assets:
Credit Cards	3,818
Cash and cash equivalents	5,563,877	2,559,106
Available-for-sale investments	487,669	176,245
Accrued interest receivable	1,392,725	1,191,981
Tax indemnification receivable	27,558	39,207
Derivative instruments	715	2,090
Total	32,140,336	26,239,548
Interest-bearing liabilities:
Short-term borrowings	289,230	0
Long-term borrowings	4,354,037	4,284,304
Accrued interest payable	75,158	61,341
Derivative instruments	1,164	2,032
Total interest-bearing liabilities	29,002,495	23,289,759
Carrying Value | Money-market and savings accounts
Interest-bearing liabilities:
Deposits	10,335,163	9,390,108
Certificates of deposits, difference	(28,528)	19,151
Carrying Value | Certificates of Deposit
Interest-bearing liabilities:
Deposits	13,947,743	9,551,974
Private Education Loans
Earning assets:
Loans held for investment, net, difference	2,092,426	2,018,576
Private Education Loans | Fair Value
Earning assets:
Private Education Loans	24,988,941	22,313,419
Private Education Loans | Carrying Value
Earning assets:
Private Education Loans	22,896,515	20,294,843
FFELP Loans
Earning assets:
Loans held for investment, net, difference	11,239	11,296
FFELP Loans | Fair Value
Earning assets:
Private Education Loans	795,055	859,185
FFELP Loans | Carrying Value
Earning assets:
Private Education Loans	783,816	847,889
Personal Loans
Earning assets:
Loans held for investment, net, difference	63,476	28,344
Personal Loans | Fair Value
Earning assets:
Private Education Loans	1,047,119	1,156,531
Personal Loans | Carrying Value
Earning assets:
Private Education Loans	$ 983,643	$ 1,128,187