Borrowings - Securitizations (Details) - USD ($)		1 Months Ended				3 Months Ended	6 Months Ended
	Feb. 28, 2017	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Nov. 30, 2017	Mar. 31, 2019	Jun. 30, 2019	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Securities Financing Transaction [Line Items]
Total Issued								$ 1,110,000,000	$ 1,900,500,000	$ 1,448,000,000
Total loan and accrued interest amount securitized at inception on-balance sheet term securitization								$ 1,208,963,000	$ 2,101,644,000	$ 1,606,804,000
2017-A
Securities Financing Transaction [Line Items]
Total Issued	$ 772,000,000
Weighted Average Life	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					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				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			4 years 4 months 24 days
Basis spread on variable rate			0.76%
2018-C
Securities Financing Transaction [Line Items]
Total Issued		$ 544,000,000
Weighted Average Life		4 years 3 months 25 days
Basis spread on variable rate		0.77%
2019-A
Securities Financing Transaction [Line Items]
Total Issued						$ 453,000,000
Weighted Average Life						4 years 3 months 3 days
Basis spread on variable rate						0.92%
2019-B
Securities Financing Transaction [Line Items]
Total Issued							$ 657,000,000
Weighted Average Life							4 years 4 months 28 days
Basis spread on variable rate							1.01%