Earnings per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Numerator:
Net income	$ 641,207	$ 362,173
Preferred stock dividends	1,201	3,464
Net income attributable to SLM Corporation common stock	$ 640,006	$ 358,709
Denominator:
Weighted average shares used to compute basic EPS (in shares)	361,042,000	409,786,000
Effect of dilutive securities:
Dilutive effect of stock options, restricted stock and restricted stock units and Employee Stock Purchase Plan (ESPP) (in shares)	5,198,000	2,969,000
Weighted average shares used to compute diluted EPS (in shares)	366,240,000	412,755,000
Basic earnings per common share attributable to SLM Corporation (in dollars per share)	$ 1.77	$ 0.88
Diluted earnings per common share attributable to SLM Corporation (in dollars per share)	$ 1.75	$ 0.87
Antidilutive securities excluded from computation of earnings per share (in shares)	1,000,000	2,000,000