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Bonds and Notes Payable - Schedule of Asset-Backed Securitizations (Details)	9 Months Ended
	Sep. 30, 2019 USD ($)
2019-1, Class A-1 Notes
Debt Instrument [Line Items]
Principal amount	$ 35,700,000
2019-1, Class A-1 Notes | Senior notes
Debt Instrument [Line Items]
Principal amount	35,700,000
Bond discount	0
Issue price	$ 35,700,000
2019-1, Class A-1 Notes | Senior notes | LIBOR
Debt Instrument [Line Items]
Cost of funds, spread on variable interest rate	0.30%
2019-1, Class A-2 Notes
Debt Instrument [Line Items]
Principal amount	$ 448,000,000
2019-1, Class A-2 Notes | Senior notes
Debt Instrument [Line Items]
Principal amount	448,000,000
Bond discount	0
Issue price	$ 448,000,000
2019-1, Class A-2 Notes | Senior notes | LIBOR
Debt Instrument [Line Items]
Cost of funds, spread on variable interest rate	0.75%
2019-1
Debt Instrument [Line Items]
Principal amount	$ 496,800,000
2019-1, Class A Notes | Senior notes
Debt Instrument [Line Items]
Principal amount	483,700,000
Bond discount	0
Issue price	483,700,000
2019-1, Class B Notes | Subordinated debt
Debt Instrument [Line Items]
Principal amount	13,100,000
Bond discount	0
Issue price	$ 13,100,000
2019-1, Class B Notes | Subordinated debt | LIBOR
Debt Instrument [Line Items]
Cost of funds, spread on variable interest rate	1.40%
2019-2
Debt Instrument [Line Items]
Principal amount	$ 416,100,000
2019-2, Class A Notes | Senior notes
Debt Instrument [Line Items]
Principal amount	405,000,000
Bond discount	0
Issue price	$ 405,000,000
2019-2, Class A Notes | Senior notes | LIBOR
Debt Instrument [Line Items]
Cost of funds, spread on variable interest rate	0.90%
2019-2, Class B Notes | Subordinated debt
Debt Instrument [Line Items]
Principal amount	$ 11,100,000
Bond discount	0
Issue price	$ 11,100,000
2019-2, Class B Notes | Subordinated debt | LIBOR
Debt Instrument [Line Items]
Cost of funds, spread on variable interest rate	1.50%
Private education loan 2019-A
Debt Instrument [Line Items]
Principal amount	$ 47,159,000
Private education loan 2019-A | Senior notes
Debt Instrument [Line Items]
Principal amount	47,159,000
Bond discount	0
Issue price	$ 47,159,000
Private education loan 2019-A | Senior notes | Prime Rate
Debt Instrument [Line Items]
Cost of funds, spread on variable interest rate	1.60%
2019-3
Debt Instrument [Line Items]
Principal amount	$ 498,300,000
2019-3, Class A Notes | Senior notes
Debt Instrument [Line Items]
Principal amount	485,800,000
Bond discount	0
Issue price	$ 485,800,000
2019-3, Class A Notes | Senior notes | LIBOR
Debt Instrument [Line Items]
Cost of funds, spread on variable interest rate	0.80%
2019-3, Class B Notes | Subordinated debt
Debt Instrument [Line Items]
Principal amount	$ 12,500,000
Bond discount	0
Issue price	$ 12,500,000
2019-3, Class B Notes | Subordinated debt | LIBOR
Debt Instrument [Line Items]
Cost of funds, spread on variable interest rate	1.55%
2019-4
Debt Instrument [Line Items]
Principal amount	$ 418,600,000
2019-4, Class A Notes | Senior notes
Debt Instrument [Line Items]
Principal amount	408,000,000
Bond discount	0
Issue price	$ 408,000,000
2019-4, Class A Notes | Senior notes | LIBOR
Debt Instrument [Line Items]
Cost of funds, spread on variable interest rate	0.87%
2019-4, Class B Notes | Subordinated debt
Debt Instrument [Line Items]
Principal amount	$ 10,600,000
Bond discount	0
Issue price	$ 10,600,000
2019-4, Class B Notes | Subordinated debt | LIBOR
Debt Instrument [Line Items]
Cost of funds, spread on variable interest rate	1.65%
2019-5
Debt Instrument [Line Items]
Principal amount	$ 374,500,000
2019-5, Class A Notes | Senior notes
Debt Instrument [Line Items]
Principal amount	364,500,000
Bond discount	(114,000)
Issue price	$ 364,386,000
Cost of funds	2.53%
2019-5, Class B Notes | Subordinated debt
Debt Instrument [Line Items]
Principal amount	$ 10,000,000
Bond discount	(4,000)
Issue price	$ 9,996,000
Cost of funds	3.45%
2019 Total
Debt Instrument [Line Items]
Principal amount	$ 2,251,459,000
2019 Class A | Senior notes
Debt Instrument [Line Items]
Principal amount	2,194,159,000
Bond discount	(114,000)
Issue price	2,194,045,000
2019 Class B | Subordinated debt
Debt Instrument [Line Items]
Principal amount	57,300,000
Bond discount	(4,000)
Issue price	$ 57,296,000

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