Parent Only Statements - Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Income Statements, Captions [Line Items]
Interest income									$ 1,437,303	$ 1,077,246	$ 831,118
Interest expense									308,082	185,908	128,619
Net interest income	$ 309,191	$ 282,061	$ 269,893	$ 268,076	$ 245,434	$ 223,275	$ 212,766	$ 209,863	1,129,221	891,338	702,499
Non-interest income (loss)									(2,902)	68,816	182,593
Non-interest expenses									449,089	386,313	355,973
Income tax benefit	66,190	40,617	44,713	51,011	43,122	47,557	34,555	38,875	202,531	164,109	164,780
Net income									288,934	250,327	274,284
Preferred stock dividends	3,137	3,028	3,974	5,575	5,506	5,316	5,243	5,139	15,714	21,204	19,595
Net income attributable to SLM Corporation common stock	$ 43,866	$ 73,343	$ 66,643	$ 89,368	$ 64,736	$ 51,649	$ 51,962	$ 60,776	273,220	229,123	254,689
Parent Company
Condensed Income Statements, Captions [Line Items]
Interest income									5,497	5,367	6,414
Interest expense									8,170	0	0
Net interest income									(2,673)	5,367	6,414
Non-interest income (loss)									(33,956)	9,396	(239)
Non-interest expenses									35,810	32,553	36,141
Loss before income tax benefit and equity in net income from subsidiaries									(72,439)	(17,790)	(29,966)
Income tax benefit									(40,598)	(2,839)	(8,612)
Equity in net income from subsidiaries (primarily Sallie Mae Bank)									320,775	265,278	295,638
Net income									288,934	250,327	274,284
Preferred stock dividends									15,714	21,204	19,595
Net income attributable to SLM Corporation common stock									$ 273,220	$ 229,123	$ 254,689