Selected Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Quarterly Financial Information Disclosure [Abstract]
Net interest income	$ 385,886	$ 384,628	$ 386,631	$ 405,068	$ 381,431	$ 369,510	$ 362,808	$ 375,032	$ 1,562,213	$ 1,488,781	$ 1,394,766
Less: provisions for credit losses	15,599	198,023	17,729	114,112	297,260	207,598	30,545	98,050	345,000	633,000
Net interest income after provisions for credit losses	370,287	186,605	368,902	290,956	84,171	161,912	332,263	276,982	1,216,750	855,328	1,427,723
Gains on sales of loans, net	35,550	(5)	124,754	(9)	2,894	74,978	239,997	9,881	160,290	327,750	548,315
Gains (losses) on securities, net	690	1,490	(1,213)	1,711	(58,245)	891	667	(3,580)	2,678	(60,267)	39,096
Gains (losses) on derivative and hedging activities, net					0	0	0	(5)	0	(5)	144
Other income	20,873	22,753	20,513	20,009	14,708	19,234	17,589	15,629	84,148	67,160	44,894
Total operating expenses	143,101	167,402	154,164	154,539	137,762	149,964	131,730	132,006
Amortized intangible assets	59,013	2,834	2,245	2,272	2,301	2,328	2,417	733	66,364	7,779	0
Income tax expense	56,843	11,242	91,482	37,338	(19,492)	29,551	114,296	37,356	196,905	161,711	379,751
Net income	168,443	29,365	265,065	118,518	(77,043)	75,172	342,073	128,812	581,391	469,014	1,160,513
Preferred stock dividends	4,726	4,642	4,274	4,063	3,466	2,531	1,757	1,275	17,705	9,029	4,736
Net income (loss) attributable to SLM Corporation common stock	163,717	24,723	260,791	114,455	(80,509)	72,641	340,316	127,537	563,686	459,985	1,155,777
Net income attributable to SLM Corporation common stock	$ 163,717	$ 24,723	$ 260,791	$ 114,455	$ (80,509)	$ 72,641	$ 340,316	$ 127,537	$ 563,686	$ 459,985	$ 1,155,777
Basic earnings per common share (in usd per share)	$ 0.73	$ 0.11	$ 1.11	$ 0.47	$ (0.33)	$ 0.29	$ 1.30	$ 0.46	$ 2.44	$ 1.78	$ 3.67
Diluted earnings per common share (in usd per share)	0.72	0.11	1.10	0.47	(0.33)	0.29	1.29	0.45	2.41	1.76	3.61
Declared dividends per common share (in usd per share)	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.44	$ 0.44	$ 0.20