Earnings per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net income	$ 171,028	$ 128,458	$ 447,990	$ 436,924
Preferred stock dividends	2,058	4,153	8,000	12,952
Net income attributable to SLM Corporation common stock	$ 168,970	$ 124,305	$ 439,990	$ 423,972
Denominator:
Weighted average shares used to compute basic EPS (in shares)	375,094,000	424,149,000	386,587,000	429,295,000
Effect of dilutive securities:
Dilutive effect of stock options, restricted stock and restricted stock units and Employee Stock Purchase Plan (ESPP) (in shares)	2,824,000	3,187,000	2,804,000	3,277,000
Weighted average shares used to compute diluted EPS (in shares)	377,918,000	427,336,000	389,391,000	432,572,000
Basic earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.45	$ 0.29	$ 1.14	$ 0.99
Diluted earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.45	$ 0.29	$ 1.13	$ 0.98
Antidilutive securities excluded from computation of earnings per share (in shares)	2,000,000	0	0	0