Selected Quarterly Financial Information (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Interest income, net	$ 187,846	$ 175,442	$ 168,257	$ 170,954	$ 150,676	$ 144,026	$ 144,539	$ 139,238	$ 702,499	$ 578,479	$ 462,115
Less: provisions for credit losses	30,382	27,497	15,558	16,618	30,458	14,898	1,014	39,159	90,055	85,529	69,339
Net interest income after provisions for credit losses	157,464	147,945	152,699	154,336	120,218	129,128	143,525	100,079	612,444	492,950	392,776
Gains on sales of loans, net	58,484	0	76,874	0	396	85,147	1,928	33,888
Gains (losses) on derivative and hedging activities, net	953	(547)	1,602	3,292	825	5,401	(9,458)	(764)	13,765	7,892	29,308
Other income	12,561	10,455	10,912	8,007	11,095	5,461	15,229	8,136	41,935	39,921	37,222
Operating expenses	85,245	92,864	89,799	81,187	78,724	72,079	60,479	63,671	349,095	274,881	270,474
Acquired intangible asset impairment and amortization expense	370	370	370	370	(855)	1,150	1,156	1,767	1,480	3,290	3,317
Restructuring and other reorganization expenses	(913)	910	744	4,657	10,483	14,079	13,520	229	5,398	38,311	726
Income tax (benefit) expense	54,915	17,985	60,158	31,722	24,465	54,903	31,941	28,658	164,780	139,967	158,934
Net income attributable to SLM Corporation	89,845	45,724	91,016	47,699	19,717	82,926	44,128	47,448	274,284	194,219	258,945
Less: Net loss attributable to noncontrolling interest					0	0	0	(434)	0	(434)	(1,352)
Net income attributable to SLM Corporation common stock	84,856	40,811	86,146	42,876	14,862	78,076	40,900	47,448	254,689	181,286	258,945
Preferred stock dividends	$ 4,989	$ 4,913	$ 4,870	$ 4,823	$ 4,855	$ 4,850	$ 3,228	$ 0	$ 19,595	$ 12,933	$ 0
Basic earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.20	$ 0.10	$ 0.20	$ 0.10	$ 0.04	$ 0.18	$ 0.10	$ 0.11	$ 0.60	$ 0.43	$ 0.59
Diluted earnings per common share attributable to SLM Corporation (in dollars per share)	$ 0.20	$ 0.09	$ 0.20	$ 0.10	$ 0.03	$ 0.18	$ 0.09	$ 0.11	$ 0.59	$ 0.42	$ 0.58