CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Thousands	Total	Total SLM Corporation Equity	Redemption of Series A Preferred Stock	Common Stock	Treasury Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Series A Preferred Stock	Series A Preferred Stock Total SLM Corporation Equity	Series A Preferred Stock Redemption of Series A Preferred Stock	Series A Preferred Stock Retained Earnings	Series B Preferred Stock Total SLM Corporation Equity	Series B Preferred Stock Retained Earnings
Beginning balance, shares issued (in shares) at Dec. 31, 2015			7,300,000	430,677,434
Beginning balance (in shares) at Dec. 31, 2015				426,303,244	(4,374,190)
Beginning balance at Dec. 31, 2015		$ 2,096,323	$ 565,000	$ 86,136	$ (41,223)	$ 1,135,860	$ (16,059)	$ 366,609
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 250,327	250,327						250,327
Other comprehensive income, net of tax	7,388	7,388					7,388
Total comprehensive income	$ 257,715	257,715
Cash dividends:
Preferred Stock, series B ($3.91 per share)										$ (11,501)		$ (11,501)	$ (9,703)	$ (9,703)
Redemption of Series A Preferred Stock	(3,354,730)
Dividend equivalent units related to employee stock-based compensation plans		0				410		(410)
Issuance of common shares (in shares)	5,955,045			5,955,045
Issuance of common shares		19,191		$ 1,191		18,000
Tax benefit related to employee stock-based compensation		(1,650)				(1,650)
Stock-based compensation expense		22,944				22,944
Shares repurchased related to employee stock-based compensation plans				(3,354,730)	(3,354,730)
Shares repurchased related to employee stock-based compensation plans		(26,261)			$ (26,261)
Ending balance, shares issued (in shares) at Dec. 31, 2016			7,300,000	436,632,479
Ending balance (in shares) at Dec. 31, 2016				428,903,559	(7,728,920)
Ending balance at Dec. 31, 2016		2,347,058	$ 565,000	$ 87,327	$ (67,484)	1,175,564	(8,671)	595,322
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 288,934	288,934						288,934
Other comprehensive income, net of tax	11,419	11,419					11,419
Total comprehensive income	$ 300,353	300,353
Cash dividends:
Preferred Stock, series B ($3.91 per share)										(3,961)		(3,961)	$ (11,753)	$ (11,753)
Redemption of Series A Preferred Stock	(3,358,417)										(3,300,000)
Redemption of Series A Preferred Stock									$ (165,000)		$ (165,000)
Dividend equivalent units related to employee stock-based compensation plans		0				96		(96)
Issuance of common shares (in shares)	6,831,108			6,831,108
Issuance of common shares		19,655		$ 1,366		18,289
Stock-based compensation expense		27,899				27,899
Shares repurchased related to employee stock-based compensation plans				(3,358,417)	(3,358,417)
Shares repurchased related to employee stock-based compensation plans		(40,160)			$ (40,160)
Ending balance, shares issued (in shares) at Dec. 31, 2017			4,000,000	443,463,587
Ending balance (in shares) at Dec. 31, 2017				432,376,250	(11,087,337)
Ending balance at Dec. 31, 2017	$ 2,474,256	2,474,256	$ 400,000	$ 88,693	$ (107,644)	1,222,277	2,748	868,182
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification resulting from the adoption of ASU No. 2017-12	165					429		(264)
Net income	487,476	487,476						487,476
Other comprehensive income, net of tax	7,013	7,013					7,013
Total comprehensive income	$ 494,489	494,489
Reclassification resulting from the adoption of ASU No. 2018-02		0				0		(592)
Reclassification resulting from the adoption of ASU No. 2018-02							592
Cash dividends:
Preferred Stock, series B ($3.91 per share)										$ (15,640)		$ (15,640)
Redemption of Series A Preferred Stock	(3,087,396)
Issuance of common shares (in shares)	6,392,634			6,392,634
Issuance of common shares		22,113		$ 1,279		20,834
Stock-based compensation expense		31,524				31,524
Shares repurchased related to employee stock-based compensation plans				(3,087,396)	(3,087,396)
Shares repurchased related to employee stock-based compensation plans		(34,947)			$ (34,947)
Ending balance, shares issued (in shares) at Dec. 31, 2018			4,000,000	449,856,221
Ending balance (in shares) at Dec. 31, 2018				435,681,488	(14,174,733)
Ending balance at Dec. 31, 2018	$ 2,972,656	$ 2,972,656	$ 400,000	$ 89,972	$ (142,591)	$ 1,274,635	10,623	1,340,017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification resulting from the adoption of ASU No. 2017-12 | Accounting Standards Update 2017-12 [Member]	$ 861						$ 270	$ 591