Earnings per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator [Abstract]
Net income attributable to SLM Corporation	$ 45,724	$ 82,926	$ 184,439	$ 174,502
Preferred stock dividends	4,913	4,850	14,606	8,078
Net income attributable to SLM Corporation common stock	$ 40,811	$ 78,076	$ 169,833	$ 166,424
Denominator [Abstract]
Weighted average shares used to compute basic EPS (in shares)	426,019	423,079	425,384	424,187
Effect of dilutive securities:
Dilutive effect of stock options, restricted stock and restricted stock units and Employee Stock Purchase Plan (ESPP) (in shares)	6,528	8,525	7,147	8,137
Weighted average shares used to compute diluted EPS (in shares)	432,547	431,604	432,531	432,324
Basic earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.10	$ 0.18	$ 0.40	$ 0.39
Diluted earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.09	$ 0.18	$ 0.39	$ 0.38
Securities excluded from computation of EPS (in shares)	2,000	3,000	2,000	3,000