Borrowings - Securitizations (Details) - USD ($)	Jul. 31, 2016	May 31, 2016	Jul. 31, 2015	Sep. 30, 2016	Dec. 31, 2015
Securities Financing Transaction [Line Items]
Total bonds issued				$ 1,108,000,000	$ 630,800,000
Total loan and accrued interest amount securitized at inception on-balance sheet term securitization		$ 1,364,481,000	$ 745,580,000
2015-B
Securities Financing Transaction [Line Items]
Total bonds issued			$ 630,800,000
Weighted Average Life (in years)			4 years 9 months 26 days
2016-A
Securities Financing Transaction [Line Items]
Total bonds issued		$ 501,000,000
Weighted Average Life (in years)		4 years 3 days
2016-B
Securities Financing Transaction [Line Items]
Total bonds issued	$ 607,000,000
Weighted Average Life (in years)	4 years 3 days
LIBOR | 2015-B
Securities Financing Transaction [Line Items]
Basis spread on variable rate			1.53%
LIBOR | 2016-A
Securities Financing Transaction [Line Items]
Basis spread on variable rate		1.38%
LIBOR | 2016-B
Securities Financing Transaction [Line Items]
Basis spread on variable rate	1.36%