Defined Contribution Plans (Details) - Sallie Mae 401(k) Savings Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items]
Type 1 of defined benefit contribution (up to)	3.00%
Type 2 of defined contribution plan	2.00%
Percent of core employer contribution	1.00%
Employer contribution amount	$ 5.0	$ 4.4	$ 3.8
Maximum
Defined Contribution Plan Disclosure [Line Items]
Employer matching contribution percentage	100.00%
Minimum
Defined Contribution Plan Disclosure [Line Items]
Employer matching contribution percentage	50.00%