Selected Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Net interest income	$ 382,867	$ 356,633	$ 340,950	$ 332,614	$ 309,191	$ 282,061	$ 269,893	$ 268,076	$ 1,413,064	$ 1,129,221	$ 891,338
Less: provisions for credit losses	57,619	70,047	63,267	53,931	55,324	54,930	50,215	25,296	244,864	185,765	159,405
Net interest income after provisions for credit losses	325,248	286,586	277,683	278,683	253,867	227,131	219,678	242,780	1,168,200	943,456	731,933
Gains on sales of loans, net	0	0	2,060	0
Losses on sales of securities, net	0	0	(1,549)	0					(1,549)	0	0
Gains (losses) on derivative and hedging activities, net	6,238	(4,949)	(5,268)	3,892	(940)	1,661	(3,609)	(5,378)
Other income (loss)	6,446	(80,702)	12,295	9,642	(21,066)	4,455	10,629	11,346	(52,319)	5,364	69,544
Total non-interest expenses	145,971	150,724	135,315	124,966	118,668	116,259	111,368	102,794
Income tax benefit	44,449	(53,667)	40,074	40,997	66,190	40,617	44,713	51,011	71,853	202,531	164,109
Net income	147,512	103,878	109,832	126,254	47,003	76,371	70,617	94,943	487,476	288,934	250,327
Preferred stock dividends	4,199	4,124	3,920	3,397	3,137	3,028	3,974	5,575	15,640	15,714	21,204
Net income attributable to SLM Corporation common stock	$ 143,313	$ 99,754	$ 105,912	$ 122,857	$ 43,866	$ 73,343	$ 66,643	$ 89,368	$ 471,836	$ 273,220	$ 229,123
Basic earnings per common share attributable to SLM Corporation	$ 0.33	$ 0.23	$ 0.24	$ 0.28	$ 0.10	$ 0.17	$ 0.15	$ 0.21	$ 1.08	$ 0.63	$ 0.54
Diluted earnings per common share attributable to SLM Corporation	$ 0.33	$ 0.23	$ 0.24	$ 0.28	$ 0.10	$ 0.17	$ 0.15	$ 0.20	$ 1.07	$ 0.62	$ 0.53