CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Thousands	Total	Total SLM Corporation Equity	Total SLM Corporation Equity Cumulative Effect, Period of Adoption, Adjustment	Total SLM Corporation Equity Cumulative Effect, Period Of Adoption, Adjusted Balance	Total SLM Corporation Equity Series B Preferred Stock	Preferred Stock	Preferred Stock Cumulative Effect, Period Of Adoption, Adjusted Balance	Preferred Stock Series B Preferred Stock	Common Stock	Common Stock Cumulative Effect, Period Of Adoption, Adjusted Balance	Treasury Stock	Treasury Stock Cumulative Effect, Period Of Adoption, Adjusted Balance	Additional Paid-In Capital	Additional Paid-In Capital Cumulative Effect, Period Of Adoption, Adjusted Balance	Additional Paid-In Capital Series B Preferred Stock	Accumulated Other Comprehensive Income	Accumulated Other Comprehensive Income Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income Cumulative Effect, Period Of Adoption, Adjusted Balance	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings Cumulative Effect, Period Of Adoption, Adjusted Balance	Retained Earnings Series B Preferred Stock
Beginning balance, shares issued (in shares) at Dec. 31, 2017						4,000,000			443,463,587
Beginning balance (in shares) at Dec. 31, 2017									432,376,250		(11,087,337)
Beginning balance at Dec. 31, 2017		$ 2,474,256				$ 400,000			$ 88,693		$ (107,644)		$ 1,222,277			$ 2,748			$ 868,182
Beginning balance (Accounting Standards Update 2018-02) at Dec. 31, 2017			$ 0														$ 592			$ (592)
Beginning balance (Accounting Standards Update 2017-12) at Dec. 31, 2017			861														$ 270			591
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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
Cash dividends:
Preferred stock					$ (15,640)																	$ (15,640)
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 (in shares)									(3,087,396)		(3,087,396)
Shares repurchased related to employee stock-based compensation plans		(34,947)									$ (34,947)
Common stock repurchased (in shares)	0
Repurchase of Preferred Stock, series B (in shares)	(3,087,396)
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				$ 400,000			$ 89,972		$ (142,591)		1,274,635			10,623			1,340,017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 578,276	578,276																	578,276
Other comprehensive income, net of tax	(22,990)	(22,990)														(22,990)
Total comprehensive income	$ 555,286	555,286
Cash dividends:
Preferred stock					(16,837)																	(16,837)
Common stock		(51,114)																	(51,114)
Issuance of common shares (in shares)	3,743,705								3,743,705
Issuance of common shares		3,375							$ 748				2,627
Stock-based compensation expense		30,538											30,363						175
Shares repurchased related to employee stock-based compensation plans (in shares)									(1,369,630)		(1,369,630)
Shares repurchased related to employee stock-based compensation plans		(14,867)									$ (14,867)
Dividend equivalent units related to employee stock-based compensation plans		0											5						(5)
Common stock repurchased (in shares)	(16,962,199)								(16,962,199)		(16,962,199)
Common stock repurchased		(167,201)									$ (167,201)
Repurchase of Preferred Stock, series B (in shares)	(1,369,630)
Ending balance, shares issued (in shares) at Dec. 31, 2019						4,000,000	4,000,000		453,599,926	453,599,926
Ending balance (in shares) at Dec. 31, 2019									421,093,364	421,093,364	(32,506,562)	(32,506,562)
Ending balance at Dec. 31, 2019	$ 3,311,836	3,311,836	$ (952,639)	$ 2,359,197		$ 400,000	$ 400,000		$ 90,720	$ 90,720	$ (324,659)	$ (324,659)	1,307,630	$ 1,307,630		(12,367)		$ (12,367)	1,850,512	$ (952,639)	$ 897,873
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	880,690	880,690																	880,690
Other comprehensive income, net of tax	(21,833)	(21,833)														(21,833)
Total comprehensive income	$ 858,857	858,857
Cash dividends:
Preferred stock					(9,734)																	$ (9,734)
Common stock		(46,351)																	(46,351)
Issuance of common shares (in shares)	3,129,325								3,129,325
Issuance of common shares		3,602							$ 626				2,976
Stock-based compensation expense		36,586											36,418						168
Shares repurchased related to employee stock-based compensation plans (in shares)									(1,197,843)		(1,197,843)
Shares repurchased related to employee stock-based compensation plans		(13,090)									$ (13,090)
Dividend equivalent units related to employee stock-based compensation plans		(10)											271						(281)
Common stock repurchased (in shares)	(47,736,847)								(47,736,847)		(47,736,847)
Common stock repurchased	$ (75,000)	(558,167)									$ (461,244)		(96,923)
Repurchase of Preferred Stock, series B (in shares)	(1,197,843)							(1,489,304)
Repurchase of Preferred Stock, series B					$ (68,055)			$ (148,930)							$ 80,875
Ending balance, shares issued (in shares) at Dec. 31, 2020						2,510,696			456,729,251
Ending balance (in shares) at Dec. 31, 2020									375,287,999		(81,441,252)
Ending balance at Dec. 31, 2020	$ 2,562,835	$ 2,562,835				$ 251,070			$ 91,346		$ (798,993)		$ 1,331,247			$ (34,200)			$ 1,722,365