Earnings (Loss) per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Numerator:
Net income (loss)	$ 140,201	$ (85,211)	$ 781,408	$ 276,962
Preferred stock dividends	1,192	2,478	2,393	5,942
Net income (loss) attributable to SLM Corporation common stock	139,009	(87,689)	779,015	271,020
Net income (loss) attributable to SLM Corporation common stock	$ 139,009	$ (87,689)	$ 779,015	$ 271,020
Denominator:
Weighted average shares used to compute basic EPS (in shares)	312,183,000	375,009,000	336,478,000	392,397,000
Effect of dilutive securities:
Dilutive effect of stock options, restricted stock, restricted stock units, performance stock units and Employee Stock Purchase Plan (“ESPP”) (in shares)	4,936,000	0	5,066,000	2,794,000
Weighted average shares used to compute diluted EPS (in shares)	317,119,000	375,009,000	341,544,000	395,191,000
Basic earnings (loss) per common share attributable to SLM Corporation (in dollars per share)	$ 0.45	$ (0.23)	$ 2.32	$ 0.69
Diluted earnings (loss) per common share attributable to SLM Corporation (in dollars per share)	$ 0.44	$ (0.23)	$ 2.28	$ 0.69
Antidilutive securities excluded from computation of earnings per share (in shares)	1,000,000	7,000,000	1,000,000	3,000,000