Borrowings - Securitizations (Details) - USD ($)					1 Months Ended
	Feb. 28, 2017	Oct. 31, 2016	Jul. 31, 2016	May 31, 2016	Mar. 31, 2018	Nov. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Total Issued					$ 670,000,000		$ 1,448,000,000	$ 1,782,000,000
Total loan and accrued interest amount securitized at inception on-balance sheet term securitization					744,917,000		$ 1,606,804,000	$ 2,107,042,000
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
2017-B
Securities Financing Transaction [Line Items]
Total Issued						$ 676,000,000
Basis spread on variable rate						0.80%
Weighted Average Life (in years)						4 years 25 days
2018-A
Securities Financing Transaction [Line Items]
Total Issued					$ 670,000,000
Basis spread on variable rate					0.78%
Weighted Average Life (in years)					4 years 5 months 4 days
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%