Earnings per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator:
Net income	$ 109,832	$ 70,617	$ 236,086	$ 165,560
Preferred stock dividends	3,920	3,974	7,317	9,549
Net income attributable to SLM Corporation common stock	$ 105,912	$ 66,643	$ 228,769	$ 156,011
Denominator:
Weighted average shares used to compute basic EPS (in shares)	435,187,000	431,245,000	434,573,000	430,572,000
Effect of dilutive securities:
Dilutive effect of stock options, restricted stock and restricted stock units and Employee Stock Purchase Plan (ESPP) (in shares)	4,258,000	6,870,000	4,639,000	7,852,000
Weighted average shares used to compute diluted EPS (in shares)	439,445,000	438,115,000	439,212,000	438,424,000
Basic earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.24	$ 0.15	$ 0.53	$ 0.36
Diluted earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.24	$ 0.15	$ 0.52	$ 0.35
Securities excluded from computation of EPS (in shares)	0	0	0	0