Borrowings - Securitizations (Details) - USD ($)					1 Months Ended
	Feb. 28, 2017	Oct. 31, 2016	Jul. 31, 2016	May 31, 2016	Jun. 30, 2018	Mar. 31, 2018	Nov. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Total Issued					$ 1,356,500,000			$ 1,448,000,000	$ 1,782,000,000
Total loan and accrued interest amount securitized at inception on-balance sheet term securitization					1,504,121,000			$ 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
Weighted Average Life (in years)							4 years 25 days
Basis spread on variable rate							0.80%
2018-A
Securities Financing Transaction [Line Items]
Total Issued						$ 670,000,000
Weighted Average Life (in years)						4 years 5 months 4 days
Basis spread on variable rate						0.78%
2018-B
Securities Financing Transaction [Line Items]
Total Issued					$ 686,500,000
Weighted Average Life (in years)					4 years 4 months 25 days
Basis spread on variable rate					0.76%
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%