Borrowings - Financing VIEs (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Debt Outstanding
Short Term	$ 0	$ 0
Long Term	4,217,119	3,275,270
Total	4,217,119	3,275,270
Carrying Amount of Assets Securing Debt Outstanding
Restricted Cash	114,659	101,836	$ 62,466
Other Assets	99,651	84,853
Variable Interest Entity, Primary Beneficiary
Debt Outstanding
Short Term	0	0
Long Term	4,020,176	3,078,731
Total	4,020,176	3,078,731
Carrying Amount of Assets Securing Debt Outstanding
Loans	4,793,290	3,691,024
Restricted Cash	105,963	96,983
Other Assets	335,039	240,369
Total	5,234,292	4,028,376
Variable Interest Entity, Primary Beneficiary \| Private Education Loan securitization
Debt Outstanding
Short Term	0	0
Long Term	4,020,176	3,078,731
Total	4,020,176	3,078,731
Carrying Amount of Assets Securing Debt Outstanding
Loans	4,793,290	3,691,024
Restricted Cash	105,963	95,966
Other Assets	334,333	240,208
Total	5,233,586	4,027,198
Variable Interest Entity, Primary Beneficiary \| ABCP Facility
Debt Outstanding
Short Term	0	0
Long Term	0	0
Total	0	0
Carrying Amount of Assets Securing Debt Outstanding
Loans	0	0
Restricted Cash	0	1,017
Other Assets	706	161
Total	$ 706	$ 1,178

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Represents the aggregate of total long-term debt, including current maturities and short-term debt.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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