Earnings per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net income	$ 76,371	$ 56,965	$ 241,931	$ 180,085
Preferred stock dividends	3,028	5,316	12,577	15,698
Net income attributable to SLM Corporation common stock	$ 73,343	$ 51,649	$ 229,354	$ 164,387
Denominator:
Weighted average shares used to compute basic EPS (in shares)	431,718,000	428,077,000	430,958,000	427,711,000
Effect of dilutive securities:
Dilutive effect of stock options, restricted stock and restricted stock units and Employee Stock Purchase Plan (ESPP) (in shares)	6,701,000	5,446,000	7,464,000	4,368,000
Weighted average shares used to compute diluted EPS (in shares)	438,419,000	433,523,000	438,422,000	432,079,000
Basic earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.17	$ 0.12	$ 0.53	$ 0.38
Diluted earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.17	$ 0.12	$ 0.52	$ 0.38
Securities excluded from computation of EPS (in shares)	0	1,000,000	0	1,000,000