Selected Quarterly Financial Information (unaudited) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Quarterly Financial Information Disclosure [Abstract]
Net interest income	$ 381,431,000	$ 369,510,000	$ 362,808,000	$ 375,032,000	$ 367,350,000	$ 357,518,000	$ 338,784,000	$ 331,114,000	$ 1,488,781,000	$ 1,394,766,000	$ 1,479,993,000
Less: provisions for credit losses	297,260,000	207,598,000	30,545,000	98,050,000	(15,309,000)	138,442,000	69,677,000	(225,767,000)	(633,000,000)	(33,000,000)
Net interest income (loss) after provisions for credit losses	84,171,000	161,912,000	332,263,000	276,982,000	382,659,000	219,076,000	269,107,000	556,881,000	855,328,000	1,427,723,000	1,386,860,000
Gains (losses) on sales of loans, net	2,894,000	74,978,000	239,997,000	9,881,000	145,535,000	(10,000)	3,679,000	399,111,000	327,750,000	548,315,000	238,315,000
Gains (losses) on securities, net	(58,245,000)	891,000	667,000	(3,580,000)	666,000	893,000	37,534,000	3,000	(60,267,000)	39,096,000	4,372,000
Gains (losses) on derivative and hedging activities, net	0	0	0	(5,000)	(17,000)	44,000	89,000	28,000	(5,000)	144,000	49,544,000
Other income	14,708,000	19,234,000	17,589,000	15,629,000	6,577,000	12,986,000	11,046,000	14,285,000	67,160,000	44,894,000	39,218,000
Total operating expenses	137,762,000	149,964,000	131,730,000	132,006,000	125,495,000	140,649,000	128,010,000	124,499,000
Acquired intangible assets amortization expense	2,301,000	2,328,000	2,417,000	733,000					7,779,000	0	0
Restructuring expenses					0	108,000	70,000	1,077,000	0	1,255,000	26,215,000
Income tax expense (benefit)	(19,492,000)	29,551,000	114,296,000	37,356,000	103,660,000	19,392,000	53,174,000	203,525,000	161,711,000	379,751,000	273,316,000
Net income	(77,043,000)	75,172,000	342,073,000	128,812,000	306,265,000	72,840,000	140,201,000	641,207,000	469,014,000	1,160,513,000	880,690,000
Preferred stock dividends	3,466,000	2,531,000	1,757,000	1,275,000	1,177,000	1,166,000	1,192,000	1,201,000	9,029,000	4,736,000	9,734,000
Net income attributable to SLM Corporation common stock	(80,509,000)	72,641,000	340,316,000	127,537,000	305,088,000	71,674,000	139,009,000	640,006,000	459,985,000	1,155,777,000	870,956,000
Net income attributable to SLM Corporation common stock	$ (80,509,000)	$ 72,641,000	$ 340,316,000	$ 127,537,000	$ 305,088,000	$ 71,674,000	$ 139,009,000	$ 640,006,000	$ 459,985,000	$ 1,155,777,000	$ 870,956,000
Basic earnings per common share (in usd per share)	$ (0.33)	$ 0.29	$ 1.30	$ 0.46	$ 1.06	$ 0.24	$ 0.45	$ 1.77	$ 1.78	$ 3.67	$ 2.27
Diluted earnings per common share (in usd per share)	(0.33)	0.29	1.29	0.45	1.04	0.24	0.44	1.75	1.76	3.61	2.25
Declared dividends per common share (in usd per share)	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.03	$ 0.03	$ 0.03	$ 0.44	$ 0.20	$ 0.12