Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net income	$ 141,352	$ 128,458	$ 150,277	$ 158,189	$ 147,512	$ 103,878	$ 109,832	$ 126,254	$ 578,276	$ 487,476	$ 288,934
Preferred stock dividends	3,885	4,153	4,331	4,468	4,199	4,124	3,920	3,397	16,837	15,640	15,714
Net income attributable to SLM Corporation common stock	$ 137,467	$ 124,305	$ 145,946	$ 153,721	$ 143,313	$ 99,754	$ 105,912	$ 122,857	$ 561,439	$ 471,836	$ 273,220
Denominator:
Weighted average shares used to compute basic EPS (in shares)									427,292	435,054	431,216
Effect of dilutive securities:
Dilutive effect of stock options, restricted stock, restricted stock units and Employee Stock Purchase Plan (ESPP) (in shares)									3,382	4,627	7,335
Weighted average shares used to compute diluted EPS (in shares)									430,674	439,681	438,551
Basic earnings per common share attributable to SLM Corporation (in usd per share)	$ 0.33	$ 0.29	$ 0.34	$ 0.35	$ 0.33	$ 0.23	$ 0.24	$ 0.28	$ 1.31	$ 1.08	$ 0.63
Diluted earnings per common share attributable to SLM Corporation (in usd per share)	$ 0.32	$ 0.29	$ 0.34	$ 0.35	$ 0.33	$ 0.23	$ 0.24	$ 0.28	$ 1.30	$ 1.07	$ 0.62
Antidilutive securities excluded from computation of earnings per share (in shares)									0	1,000	0