Earnings per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net income	$ 128,458	$ 103,878	$ 436,924	$ 339,964
Preferred stock dividends	4,153	4,124	12,952	11,441
Net income attributable to SLM Corporation common stock	$ 124,305	$ 99,754	$ 423,972	$ 328,523
Denominator:
Weighted average shares used to compute basic EPS (in shares)	424,149,000	435,468,000	429,295,000	434,875,000
Effect of dilutive securities:
Dilutive effect of stock options, restricted stock and restricted stock units and Employee Stock Purchase Plan (ESPP) (in shares)	3,187,000	4,551,000	3,277,000	4,609,000
Weighted average shares used to compute diluted EPS (in shares)	427,336,000	440,019,000	432,572,000	439,484,000
Basic earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.29	$ 0.23	$ 0.99	$ 0.76
Diluted earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.29	$ 0.23	$ 0.98	$ 0.75
Antidilutive securities excluded from computation of earnings per share (in shares)	0	0	0	0