Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net income	$ 147,512	$ 103,878	$ 109,832	$ 126,254	$ 47,003	$ 76,371	$ 70,617	$ 94,943	$ 487,476	$ 288,934	$ 250,327
Preferred stock dividends	4,199	4,124	3,920	3,397	3,137	3,028	3,974	5,575	15,640	15,714	21,204
Net income attributable to SLM Corporation common stock	$ 143,313	$ 99,754	$ 105,912	$ 122,857	$ 43,866	$ 73,343	$ 66,643	$ 89,368	$ 471,836	$ 273,220	$ 229,123
Denominator:
Weighted average shares used to compute basic EPS (in shares)									435,054	431,216	427,876
Effect of dilutive securities:
Dilutive effect of stock options, restricted stock, restricted stock units and Employee Stock Purchase Plan (ESPP) (in shares)									4,627	7,335	5,043
Weighted average shares used to compute diluted EPS (in shares)									439,681	438,551	432,919
Basic earnings per common share attributable to SLM Corporation	$ 0.33	$ 0.23	$ 0.24	$ 0.28	$ 0.10	$ 0.17	$ 0.15	$ 0.21	$ 1.08	$ 0.63	$ 0.54
Diluted earnings per common share attributable to SLM Corporation	$ 0.33	$ 0.23	$ 0.24	$ 0.28	$ 0.10	$ 0.17	$ 0.15	$ 0.20	$ 1.07	$ 0.62	$ 0.53
Antidilutive securities excluded from computation of earnings per share (in shares)									1,000	0	1,000