Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net income attributable to SLM Corporation	$ 89,845	$ 45,724	$ 91,016	$ 47,699	$ 19,717	$ 82,926	$ 44,128	$ 47,448	$ 274,284	$ 194,219	$ 258,945
Preferred stock dividends	4,989	4,913	4,870	4,823	4,855	4,850	3,228	0	19,595	12,933	0
Net income attributable to SLM Corporation common stock	$ 84,856	$ 40,811	$ 86,146	$ 42,876	$ 14,862	$ 78,076	$ 40,900	$ 47,448	$ 254,689	$ 181,286	$ 258,945
Denominator:
Weighted average shares used to compute basic EPS (in shares)									425,574	423,970	440,108
Effect of dilutive securities:
Dilutive effect of stock options, restricted stock, restricted stock units and Employee Stock Purchase Plan (ESPP) (in shares)									6,660	8,299	8,441
Weighted average shares used to compute diluted EPS (in shares)									432,234	432,269	448,549
Basic earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.20	$ 0.10	$ 0.20	$ 0.10	$ 0.04	$ 0.18	$ 0.10	$ 0.11	$ 0.60	$ 0.43	$ 0.59
Diluted earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.20	$ 0.09	$ 0.20	$ 0.10	$ 0.03	$ 0.18	$ 0.09	$ 0.11	$ 0.59	$ 0.42	$ 0.58
Antidilutive securities excluded from computation of earnings per share									2,000	3,000	3,000