Arrangements with Navient Corporation Arrangements with Navient Corporation (Details) - USD ($)		12 Months Ended
	Apr. 30, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Non-compete period	5 years
Deferred taxes	$ 283,000,000		$ 9,180,000
Remaining amount of indemnification		$ 35,000,000
Remaining balance of indemnification receivable in connection with the spin-off	$ 27,000,000	25,000,000
Deferred tax asset discount		108,000,000
Private education loans		$ 17,432,167,000	14,251,675,000
Minimum days past due for spin off loan purchase		90 days
Period of hardship forbearance		6 months
Private education loans		$ 17,244,830,000	14,113,409,000
Interest income from third party		300,000	300,000	$ 600,000
Gain (loss) resulting from loans sold		0	0	0
Write-down to fair value for loans sold to third party		5,000,000	6,000,000	7,600,000
Split Loans
Related Party Transaction [Line Items]
Private education loans		48,000,000
Participated loans
Related Party Transaction [Line Items]
Loans sold to third party		12,000,000	$ 15,700,000	$ 27,000,000
FFELP Loans
Related Party Transaction [Line Items]
Private education loans		$ 48,000,000
Minimum
Related Party Transaction [Line Items]
Agreements with Navient, period of term (or less)	1 year
Maximum
Related Party Transaction [Line Items]
Agreements with Navient, period of term (or less)	2 years