Parent Only Statements - Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Cash and cash equivalents	$ 1,534,339	$ 1,918,793	$ 2,416,219	$ 2,359,780
Tax indemnification receivable	168,011	259,532
Other assets	84,853	52,153
Total assets	21,779,584	18,533,003
Long-Term	3,275,270	2,167,979
Liabilities
Income taxes payable, net	102,285	184,324
Other liabilities	179,310	141,934
Total liabilities	19,305,328	16,185,945
Preferred stock, par value $0.20 per share, 20 million shares authorized:
Common stock, par value $0.20 per share, 1.125 billion shares authorized: 443.5 million and 436.6 million shares issued, respectively	88,693	87,327
Additional paid-in capital	1,222,277	1,175,564
Accumulated other comprehensive income (loss) (net of tax expense (benefit) of $1,696 and ($5,364), respectively)	2,748	(8,671)
Retained earnings	868,182	595,322
Total SLM Corporation stockholders’ equity before treasury stock	2,581,900	2,414,542
Less: Common stock held in treasury at cost: 11.1 million and 7.7 million shares, respectively	(107,644)	(67,484)
Total equity	2,474,256	2,347,058
Total liabilities and equity	21,779,584	18,533,003
Parent Company
Assets
Cash and cash equivalents	260,255	292,277	$ 282,036	$ 434,245
Total investments in subsidiaries (primarily Sallie Mae Bank)	2,401,114	2,042,015
Tax indemnification receivable	168,011	259,532
Due from subsidiaries, net	37,521	31,834
Other assets	1,279	1,561
Total assets	2,868,180	2,627,219
Long-Term	196,539	0
Liabilities
Income taxes payable, net	159,954	256,556
Payable due to Navient	10,575	2,823
Other liabilities	26,856	20,782
Total liabilities	393,924	280,161
Preferred stock, par value $0.20 per share, 20 million shares authorized:
Common stock, par value $0.20 per share, 1.125 billion shares authorized: 443.5 million and 436.6 million shares issued, respectively	88,693	87,327
Additional paid-in capital	1,222,277	1,175,564
Accumulated other comprehensive income (loss) (net of tax expense (benefit) of $1,696 and ($5,364), respectively)	2,748	(8,671)
Retained earnings	868,182	595,322
Total SLM Corporation stockholders’ equity before treasury stock	2,581,900	2,414,542
Less: Common stock held in treasury at cost: 11.1 million and 7.7 million shares, respectively	(107,644)	(67,484)
Total equity	2,474,256	2,347,058
Total liabilities and equity	2,868,180	2,627,219
Parent Company | Series A Preferred Stock
Preferred stock, par value $0.20 per share, 20 million shares authorized:
Preferred stock issued	0	165,000
Parent Company | Series B Preferred Stock
Preferred stock, par value $0.20 per share, 20 million shares authorized:
Preferred stock issued	$ 400,000	$ 400,000