Selected Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
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
Less: provisions for credit losses	(15,309)	138,442	69,677	(225,767)	(316,372)	(3,640)	351,887	61,258	(32,957)	93,133	354,249
Net interest income after provisions for credit losses	382,659	219,076	269,107	556,881	682,909	368,207	(3,112)	338,858	1,427,723	1,386,860	1,269,066
Gains on sales of loans, net	145,535	(10)	3,679	399,111	(247)	(4)	(369)	238,935	548,315	238,315	0
Gains (losses) on derivative and hedging activities, net	(17)	44	89	28	136	(15)	3,751	45,672	144	49,544	17,825
Other income	7,243	13,879	48,580	14,288	1,043	9,646	25,412	7,487	83,990	43,590	31,102
Total operating expenses	125,495	140,649	128,010	124,499	121,743	127,490	141,557	147,298	518,653	538,088	574,253
Restructuring expenses	0	108	70	1,077	2,088	24,127	0	0	1,255	26,215	0
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	306,265	72,840	140,201	641,207	432,700	171,028	(85,211)	362,173	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 (loss) 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
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
Basic earnings per common share (in usd per share)	$ 1.06	$ 0.24	$ 0.45	$ 1.77	$ 1.15	$ 0.45	$ (0.23)	$ 0.88	$ 3.67	$ 2.27	$ 1.31
Diluted earnings per common share (in usd per share)	1.04	0.24	0.44	1.75	1.13	0.45	(0.23)	0.87	3.61	2.25	1.30
Declared dividends per common share (in usd per share)	$ 0.11	$ 0.03	$ 0.03	$ 0.03	$ 0.03	$ 0	$ 0.06	$ 0.03	$ 0.20	$ 0.12	$ 0.12