Selected Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Net interest income	$ 309,191	$ 282,061	$ 269,893	$ 268,076	$ 245,434	$ 223,275	$ 212,766	$ 209,863	$ 1,129,221	$ 891,338	$ 702,499
Less: provisions for credit losses	55,324	54,930	50,215	25,296	43,226	41,784	41,793	32,602	185,765	159,405	90,055
Net interest income after provisions for credit losses	253,867	227,131	219,678	242,780	202,208	181,491	170,973	177,261	943,456	731,933	612,444
Gains on sales of loans, net					230	0	0	0
(Losses) gains on derivative and hedging activities, net	(940)	1,661	(3,609)	(5,378)	(4,114)	1,368	2,142	(354)
Other income (loss)	(21,066)	4,455	10,629	11,346	13,235	21,598	13,683	21,028	5,364	69,544	41,935
Total operating expenses	118,550	116,142	111,251	102,677	98,036	99,709	94,777	92,885	448,620	385,407	349,095
Acquired intangible asset amortization expense	118	117	117	117	159	226	261	260
Restructuring and other reorganization expenses									0	0	5,398
Income tax benefit	66,190	40,617	44,713	51,011	43,122	47,557	34,555	38,875	202,531	164,109	164,780
Net income	47,003	76,371	70,617	94,943	70,242	56,965	57,205	65,915	288,934	250,327	274,284
Preferred stock dividends	3,137	3,028	3,974	5,575	5,506	5,316	5,243	5,139	15,714	21,204	19,595
Net income attributable to SLM Corporation common stock	$ 43,866	$ 73,343	$ 66,643	$ 89,368	$ 64,736	$ 51,649	$ 51,962	$ 60,776	$ 273,220	$ 229,123	$ 254,689
Basic earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.10	$ 0.17	$ 0.15	$ 0.21	$ 0.15	$ 0.12	$ 0.12	$ 0.14	$ 0.63	$ 0.54	$ 0.60
Diluted earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.10	$ 0.17	$ 0.15	$ 0.20	$ 0.15	$ 0.12	$ 0.12	$ 0.14	$ 0.62	$ 0.53	$ 0.59