Parent Only Statements - Statements of Income (Details) - USD ($) $ 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
Condensed Income Statements, Captions [Line Items]
Interest income									$ 2,331,032	$ 1,935,352	$ 1,437,303
Interest expense									707,717	522,288	308,082
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
Non-interest loss									48,927	(51,895)	(2,902)
Non-interest expenses									574,253	556,976	449,089
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									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
Parent Company
Condensed Income Statements, Captions [Line Items]
Interest income									2,663	4,693	5,497
Interest expense									11,060	11,059	8,170
Net interest income									(8,397)	(6,366)	(2,673)
Non-interest loss									(10,856)	(93,176)	(33,956)
Non-interest expenses									39,423	41,893	35,810
Loss before income tax benefit and equity in net income from subsidiaries									(58,676)	(141,435)	(72,439)
Income tax benefit									(25,260)	(96,170)	(40,598)
Equity in net income from subsidiaries (primarily Sallie Mae Bank)									611,692	532,741	320,775
Net income									578,276	487,476	288,934
Preferred stock dividends									16,837	15,640	15,714
Net income attributable to SLM Corporation common stock									$ 561,439	$ 471,836	$ 273,220