Borrowings - Securitizations (Details) - USD ($)	Feb. 28, 2017	Oct. 31, 2016	Jul. 31, 2016	May 31, 2016	Jul. 31, 2015	Sep. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Securities Financing Transaction [Line Items]
Total Issued						$ 772,000,000	$ 1,782,000,000	$ 630,800,000
Total loan and accrued interest amount securitized at inception on-balance sheet term securitization						$ 856,253,000	$ 2,107,042,000	$ 745,580,000
2015-B
Securities Financing Transaction [Line Items]
Total Issued					$ 630,800,000
Weighted Average Life (in years)					4 years 9 months 26 days
2016-A
Securities Financing Transaction [Line Items]
Total Issued				$ 501,000,000
Weighted Average Life (in years)				4 years 3 days
2016-B
Securities Financing Transaction [Line Items]
Total Issued			$ 607,000,000
Weighted Average Life (in years)			4 years 3 days
2016-C
Securities Financing Transaction [Line Items]
Total Issued		$ 674,000,000
Weighted Average Life (in years)		4 years 3 months 7 days
2017-A
Securities Financing Transaction [Line Items]
Total Issued	$ 772,000,000
Weighted Average Life (in years)	4 years 3 months 7 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%
LIBOR | 2016-C
Securities Financing Transaction [Line Items]
Basis spread on variable rate		1.15%
LIBOR | 2017-A
Securities Financing Transaction [Line Items]
Basis spread on variable rate	0.93%