Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Numerator:
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 attributable to SLM Corporation common stock, diluted	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, basic	$ 163,717	$ 24,723	$ 260,791	$ 114,455	$ (80,509)	$ 72,641	$ 340,316	$ 127,537	$ 563,686	$ 459,985	$ 1,155,777
Denominator:
Weighted average shares used to compute basic EPS (in shares)									231,411	258,439	314,993
Effect of dilutive securities:
Dilutive effect of stock options, restricted stock, restricted stock units and Employee Stock Purchase Plan (ESPP) (in shares)									2,652	3,064	4,919
Weighted average shares used to compute diluted EPS (in shares)									234,063	261,503	319,912
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
Antidilutive securities excluded from computation of earnings per share (in shares)									1,000	1,000	1,000