Parent Only Statements - Statements of Income (Details) - USD ($) $ 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
Condensed Income Statements, Captions [Line Items]
Interest income									$ 1,935,352	$ 1,437,303	$ 1,077,246
Interest expense									522,288	308,082	185,908
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
Non-interest income (loss)									(51,895)	(2,902)	68,816
Non-interest expenses									556,976	449,089	386,313
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									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
Parent Company
Condensed Income Statements, Captions [Line Items]
Interest income									4,693	5,497	5,367
Interest expense									11,059	8,170	0
Net interest income									(6,366)	(2,673)	5,367
Non-interest income (loss)									(93,176)	(33,956)	9,396
Non-interest expenses									41,893	35,810	32,553
Loss before income tax benefit and equity in net income from subsidiaries									(141,435)	(72,439)	(17,790)
Income tax benefit									(96,170)	(40,598)	(2,839)
Equity in net income from subsidiaries (primarily Sallie Mae Bank)									532,741	320,775	265,278
Net income									487,476	288,934	250,327
Preferred stock dividends									15,640	15,714	21,204
Net income attributable to SLM Corporation common stock									$ 471,836	$ 273,220	$ 229,123