Earnings per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net income	$ 150,277	$ 109,832	$ 308,466	$ 236,086
Preferred stock dividends	4,331	3,920	8,799	7,317
Net income attributable to SLM Corporation common stock	$ 145,946	$ 105,912	$ 299,667	$ 228,769
Denominator:
Weighted average shares used to compute basic EPS (in shares)	429,278,000	435,187,000	431,911,000	434,573,000
Effect of dilutive securities:
Dilutive effect of stock options, restricted stock and restricted stock units and Employee Stock Purchase Plan (ESPP) (in shares)	2,975,000	4,258,000	3,322,000	4,639,000
Weighted average shares used to compute diluted EPS (in shares)	432,253,000	439,445,000	435,233,000	439,212,000
Basic earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.34	$ 0.24	$ 0.69	$ 0.53
Diluted earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.34	$ 0.24	$ 0.69	$ 0.52
Antidilutive securities excluded from computation of earnings per share (in shares)	1,000,000	1,000,000	1,000,000	1,000,000