Earnings per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Numerator:
Net income	$ 362,173	$ 158,189
Preferred stock dividends	3,464	4,468
Net income attributable to SLM Corporation common stock	$ 358,709	$ 153,721
Denominator:
Weighted average shares used to compute basic EPS (in shares)	409,786,000	434,574,000
Effect of dilutive securities:
Dilutive effect of stock options, restricted stock and restricted stock units and Employee Stock Purchase Plan (ESPP) (in shares)	2,969,000	3,674,000
Weighted average shares used to compute diluted EPS (in shares)	412,755,000	438,248,000
Basic earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.88	$ 0.35
Diluted earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.87	$ 0.35
Antidilutive securities excluded from computation of earnings per share (in shares)	2,000,000