Earnings Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Numerator:
Net income	$ 306,265	$ 72,840	$ 140,201	$ 641,207	$ 432,700	$ 171,028	$ (85,211)	$ 362,173	$ 1,160,513	$ 880,690	$ 578,276
Preferred stock dividends	1,177	1,166	1,192	1,201	1,734	2,058	2,478	3,464	4,736	9,734	16,837
Net income (loss) attributable to SLM Corporation common stock	305,088	71,674	139,009	640,006	430,966	168,970	(87,689)	358,709	1,155,777	870,956	561,439
Net income attributable to SLM Corporation common stock	$ 305,088	$ 71,674	$ 139,009	$ 640,006	$ 430,966	$ 168,970	$ (87,689)	$ 358,709	$ 1,155,777	$ 870,956	$ 561,439
Denominator:
Weighted average shares used to compute basic EPS (in shares)									314,993,000	383,705,000	427,292,000
Effect of dilutive securities:
Dilutive effect of stock options, restricted stock, restricted stock units and Employee Stock Purchase Plan (ESPP) (in shares)									4,919,000	3,490,000	3,382,000
Weighted average shares used to compute diluted EPS (in shares)									319,912,000	387,195,000	430,674,000
Basic earnings per common share (in usd per share)	$ 1.06	$ 0.24	$ 0.45	$ 1.77	$ 1.15	$ 0.45	$ (0.23)	$ 0.88	$ 3.67	$ 2.27	$ 1.31
Diluted earnings per common share (in usd per share)	$ 1.04	$ 0.24	$ 0.44	$ 1.75	$ 1.13	$ 0.45	$ (0.23)	$ 0.87	$ 3.61	$ 2.25	$ 1.30
Antidilutive securities excluded from computation of earnings per share (in shares)									1,000,000	0	0