Selected Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Interest income, net	$ 245,434	$ 223,275	$ 212,766	$ 209,863	$ 187,846	$ 175,442	$ 168,257	$ 170,954	$ 891,338	$ 702,499	$ 578,479
Less: provisions for credit losses	43,226	41,784	41,793	32,602	30,382	27,497	15,558	16,618	159,405	90,055	85,529
Net interest income after provisions for credit losses	202,208	181,491	170,973	177,261	157,464	147,945	152,699	154,336	731,933	612,444	492,950
Gains on sales of loans, net	230	0	0	0	58,484	0	76,874	0
(Losses) gains on derivative and hedging activities, net	(4,114)	1,368	2,142	(354)	953	(547)	1,602	3,292
Other income	13,235	21,598	13,683	21,028	12,561	10,455	10,912	8,007	69,544	41,935	39,921
Total operating expenses	98,036	99,709	94,777	92,885	85,245	92,864	89,799	81,187	385,407	349,095	274,881
Acquired intangible asset amortization expense	159	226	261	260	370	370	370	370
Restructuring and other reorganization expenses					(913)	910	744	4,657	0	5,398	38,311
Income tax benefit	43,122	47,557	34,555	38,875	54,915	17,985	60,158	31,722	164,109	164,780	139,967
Net income	70,242	56,965	57,205	65,915	89,845	45,724	91,016	47,699	250,327	274,284	194,219
Preferred stock dividends	5,506	5,316	5,243	5,139	4,989	4,913	4,870	4,823	21,204	19,595	12,933
Net income attributable to SLM Corporation common stock	$ 64,736	$ 51,649	$ 51,962	$ 60,776	$ 84,856	$ 40,811	$ 86,146	$ 42,876	$ 229,123	$ 254,689	$ 181,286
Basic earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.15	$ 0.12	$ 0.12	$ 0.14	$ 0.2	$ 0.1	$ 0.2	$ 0.1	$ 0.54	$ 0.60	$ 0.43
Diluted earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.15	$ 0.12	$ 0.12	$ 0.14	$ 0.2	$ 0.09	$ 0.2	$ 0.1	$ 0.53	$ 0.59	$ 0.42