Borrowings - Securitizations (Details) - USD ($)		1 Months Ended				3 Months Ended
	Feb. 28, 2017	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Nov. 30, 2017	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Total Issued						$ 453,000,000	$ 1,900,500,000	$ 1,448,000,000
Total loan and accrued interest amount securitized at inception on-balance sheet term securitization						498,087,000	$ 2,101,644,000	$ 1,606,804,000
2017-A
Securities Financing Transaction [Line Items]
Total Issued	$ 772,000,000
Weighted Average Life (in years)	4 years 3 months 7 days
Basis spread on variable rate	0.93%
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%
2018-C
Securities Financing Transaction [Line Items]
Total Issued		$ 544,000,000
Weighted Average Life (in years)		4 years 3 months 26 days
Basis spread on variable rate		0.77%
2019-A
Securities Financing Transaction [Line Items]
Total Issued						$ 453,000,000
Weighted Average Life (in years)						4 years 3 months 3 days
Basis spread on variable rate						0.92%