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Bonds and Notes Payable - Outstanding Debt Obligations (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Bonds and notes payable	$ 20,910,190	$ 22,185,558	$ 22,218,740
Discount on bonds and notes payable and debt issuance costs	(284,468)		(318,359)
Warehouse facilities | FFELP warehouse facilities
Debt Instrument [Line Items]
Bonds and notes payable	739,448		986,886
Unsecured line of credit
Debt Instrument [Line Items]
Bonds and notes payable	0		310,000
Unsecured debt - Junior Subordinated Hybrid Securities
Debt Instrument [Line Items]
Bonds and notes payable	$ 20,381		$ 20,381
Interest rate range	5.46%		6.17%
Other borrowings
Debt Instrument [Line Items]
Bonds and notes payable	$ 45,465		$ 120,342
Bonds and notes payable, gross
Debt Instrument [Line Items]
Bonds and notes payable	21,194,658		22,537,099
Federally insured | Bonds and notes based on indices
Debt Instrument [Line Items]
Bonds and notes payable	18,961,410		20,192,123
Federally insured | Bonds and notes based on auction
Debt Instrument [Line Items]
Bonds and notes payable	773,726		793,476
Federally insured | Fixed rate bonds and notes
Debt Instrument [Line Items]
Bonds and notes payable	374,500
Federally insured | Variable-rate bonds and notes
Debt Instrument [Line Items]
Bonds and notes payable	19,735,136		20,985,599
Consumer loan | Warehouse facilities
Debt Instrument [Line Items]
Bonds and notes payable	$ 144,022
Interest rate range	2.23%
Private education | Fixed rate bonds and notes
Debt Instrument [Line Items]
Bonds and notes payable	$ 53,368		63,171
Private education | Variable-rate bonds and notes
Debt Instrument [Line Items]
Bonds and notes payable	$ 82,338		$ 50,720
Interest rate range			4.26%
Minimum | Warehouse facilities | FFELP warehouse facilities
Debt Instrument [Line Items]
Interest rate range	2.22%		2.65%
Minimum | Unsecured line of credit
Debt Instrument [Line Items]
Interest rate range			3.92%
Minimum | Other borrowings
Debt Instrument [Line Items]
Interest rate range	2.77%		3.05%
Minimum | Federally insured | Bonds and notes based on indices
Debt Instrument [Line Items]
Interest rate range	2.20%		2.59%
Minimum | Federally insured | Bonds and notes based on auction
Debt Instrument [Line Items]
Interest rate range	3.02%		2.84%
Minimum | Federally insured | Fixed rate bonds and notes
Debt Instrument [Line Items]
Interest rate range	2.53%
Minimum | Private education | Fixed rate bonds and notes
Debt Instrument [Line Items]
Interest rate range	3.60%		3.60%
Minimum | Private education | Variable-rate bonds and notes
Debt Instrument [Line Items]
Interest rate range	3.40%
Maximum | Warehouse facilities | FFELP warehouse facilities
Debt Instrument [Line Items]
Interest rate range	2.26%		2.71%
Maximum | Unsecured line of credit
Debt Instrument [Line Items]
Interest rate range			4.01%
Maximum | Other borrowings
Debt Instrument [Line Items]
Interest rate range	3.85%		5.22%
Maximum | Federally insured | Bonds and notes based on indices
Debt Instrument [Line Items]
Interest rate range	3.81%		4.52%
Maximum | Federally insured | Bonds and notes based on auction
Debt Instrument [Line Items]
Interest rate range	3.84%		3.55%
Maximum | Federally insured | Fixed rate bonds and notes
Debt Instrument [Line Items]
Interest rate range	3.45%
Maximum | Private education | Fixed rate bonds and notes
Debt Instrument [Line Items]
Interest rate range	5.35%		5.35%
Maximum | Private education | Variable-rate bonds and notes
Debt Instrument [Line Items]
Interest rate range	3.77%

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Amount of short-term and long-term debt and lease obligation.

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Contractual interest rate for funds borrowed, under the debt agreement.

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