Borrowings - Secured Financing (Details) - USD ($)		1 Months Ended
	Feb. 28, 2017	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Nov. 30, 2017	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total Issued		$ 1,900,500,000				$ 12,453,903,000	$ 1,448,000,000
Total loans and accrued interest amount securitized at inception		2,101,644,000					$ 1,606,804,000
Issuance 2017-A
Debt Instrument [Line Items]
Total Issued	$ 772,000,000
Weighted Average Life (in years)	4 years 2 months 36 days
Issuance 2017-B
Debt Instrument [Line Items]
Total Issued					$ 676,000,000
Weighted Average Life (in years)					4 years 25 days
Issuance 2018-A
Debt Instrument [Line Items]
Total Issued				$ 670,000,000
Weighted Average Life (in years)				4 years 5 months 4 days
Issuance 2018-B
Debt Instrument [Line Items]
Total Issued			$ 686,500,000
Weighted Average Life (in years)			4 years 4 months 25 days
Issuance 2018-C
Debt Instrument [Line Items]
Total Issued		$ 544,000,000
Weighted Average Life (in years)		4 years 3 months 26 days