Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income	$ 47,003	$ 76,371	$ 70,617	$ 94,943	$ 70,242	$ 56,965	$ 57,205	$ 65,915	$ 288,934	$ 250,327	$ 274,284
Preferred stock dividends	3,137	3,028	3,974	5,575	5,506	5,316	5,243	5,139	15,714	21,204	19,595
Net income attributable to SLM Corporation common stock	$ 43,866	$ 73,343	$ 66,643	$ 89,368	$ 64,736	$ 51,649	$ 51,962	$ 60,776	$ 273,220	$ 229,123	$ 254,689
Denominator:
Weighted average shares used to compute basic EPS (in shares)									431,216	427,876	425,574
Effect of dilutive securities:
Dilutive effect of stock options, restricted stock, restricted stock units and Employee Stock Purchase Plan (ESPP) (in shares)									7,335	5,043	6,660
Weighted average shares used to compute diluted EPS (in shares)									438,551	432,919	432,234
Basic earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.10	$ 0.17	$ 0.15	$ 0.21	$ 0.15	$ 0.12	$ 0.12	$ 0.14	$ 0.63	$ 0.54	$ 0.60
Diluted earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.10	$ 0.17	$ 0.15	$ 0.20	$ 0.15	$ 0.12	$ 0.12	$ 0.14	$ 0.62	$ 0.53	$ 0.59
Antidilutive securities excluded from computation of earnings per share (in shares)									0	1,000	2,000