Parent Only Statements - Statements of Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Condensed Income Statements, Captions [Line Items]
Interest income									$ 1,776,844	$ 2,021,660	$ 2,331,032
Interest expense									382,078	541,667	707,717
Net interest income	$ 367,350	$ 357,518	$ 338,784	$ 331,114	$ 366,537	$ 364,567	$ 348,775	$ 400,116	1,394,766	1,479,993	1,623,315
Non-interest income (loss)									632,449	331,449	48,927
Non-interest expenses									519,908	564,303	574,253
Income tax expense (benefit)	103,660	19,392	53,174	203,525	127,310	55,189	(30,664)	121,481	379,751	273,316	165,464
Net income									1,160,513	880,690	578,276
Preferred stock dividends	1,177	1,166	1,192	1,201	1,734	2,058	2,478	3,464	4,736	9,734	16,837
Net income attributable to SLM Corporation common stock	$ 305,088	$ 71,674	$ 139,009	$ 640,006	$ 430,966	$ 168,970	$ (87,689)	$ 358,709	1,155,777	870,956	561,439
Parent Company
Condensed Income Statements, Captions [Line Items]
Interest income									392	452	2,663
Interest expense									35,208	14,896	11,060
Net interest income									(34,816)	(14,444)	(8,397)
Non-interest income (loss)									(13,078)	2,820	(10,856)
Non-interest expenses									54,352	57,945	39,423
Loss before income tax benefit and equity in net income from subsidiaries									(102,246)	(69,569)	(58,676)
Income tax expense (benefit)									8,477	(11,235)	(25,260)
Equity in net income from subsidiaries (primarily Sallie Mae Bank)									1,271,236	939,024	611,692
Net income									1,160,513	880,690	578,276
Preferred stock dividends									4,736	9,734	16,837
Net income attributable to SLM Corporation common stock									$ 1,155,777	$ 870,956	$ 561,439