CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Thousands	Total	Total SLM Corporation Equity	Preferred Stock	Common Stock	Treasury Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Series A Preferred Stock Total SLM Corporation Equity	Series A Preferred Stock 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, 2014			7,300,000	424,804,125
Beginning balance (in shares) at Dec. 31, 2014				423,438,848	(1,365,277)
Beginning balance at Dec. 31, 2014		$ 1,829,958	$ 565,000	$ 84,961	$ (12,187)	$ 1,090,511	$ (11,393)	$ 113,066
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 274,284	274,284						274,284
Other comprehensive income (loss), net of tax	(4,666)	(4,666)					(4,666)
Total comprehensive income	$ 269,618	269,618
Separation adjustments related to Spin-Off of Navient Corporation		1,660				1,660
Cash dividends:
Preferred stock									$ (11,501)		$ (11,501)	$ (8,094)	$ (8,094)
Redemption of Series A Preferred Stock (in shares)	(3,008,913)
Dividend equivalent units related to employee stock-based compensation plans		0				1,146		(1,146)
Issuance of common shares (in shares)	5,873,309			5,873,309
Issuance of common shares		15,980		$ 1,175		14,805
Tax benefit (deficiency) related to employee stock-based compensation		6,140				6,140
Stock-based compensation expense		21,598				21,598
Shares repurchased related to employee stock-based compensation plans (in shares)				(3,008,913)	(3,008,913)
Shares repurchased related to employee stock-based compensation plans		(29,036)			$ (29,036)
Ending balance, shares issued (in shares) at Dec. 31, 2015			7,300,000	430,677,434
Ending balance (in shares) at Dec. 31, 2015				426,303,244	(4,374,190)
Ending 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 (loss), net of tax	7,388	7,388					7,388
Total comprehensive income	$ 257,715	257,715
Cash dividends:
Preferred stock									(11,501)		(11,501)	(9,703)	(9,703)
Redemption of Series A Preferred Stock (in shares)	(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 (deficiency) 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 (in shares)				(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	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 (loss), net of tax	11,419	11,419					11,419
Total comprehensive income	$ 300,353	300,353
Cumulative effect of the new stock compensation standard		165				429		(264)
Cash dividends:
Preferred stock									$ (3,961)		$ (3,961)	$ (11,753)	$ (11,753)
Redemption of Series A Preferred Stock (in shares)	(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 (in shares)				(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