Selected Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Net interest income	$ 419,101	$ 405,065	$ 396,868	$ 402,281	$ 382,867	$ 356,633	$ 340,950	$ 332,614	$ 1,623,315	$ 1,413,064	$ 1,129,221
Less: provisions for credit losses	97,558	99,526	93,375	63,790	57,619	70,047	63,267	53,931	354,249	244,864	185,765
Net interest income after provisions for credit losses	321,543	305,539	303,493	338,491	325,248	286,586	277,683	278,683	1,269,066	1,168,200	943,456
Gains on sales of loans, net					0	0	2,060	0
Losses on sales of securities, net					0	0	(1,549)	0	0	(1,549)	0
Gains (losses) on derivative and hedging activities, net	(3,635)	1,961	16,736	2,763	6,238	(4,949)	(5,268)	3,892
Other income (loss)	(211)	15,280	2,655	13,378	6,446	(80,702)	12,295	9,642	31,102	(52,319)	5,364
Total non-interest expenses	141,679	153,621	138,806	140,147	145,971	150,724	135,315	124,966
Income tax benefit	34,666	40,701	33,801	56,296	44,449	(53,667)	40,074	40,997	165,464	71,853	202,531
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
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
Declared dividends per common share attributable to SLM Corporation (in usd per share)	$ 0.03	$ 0	$ 0.06	$ 0.03					$ 0.12	$ 0	$ 0