Derivative Financial Instruments - Interest Rate Caps (Details)	1 Months Ended	3 Months Ended	12 Months Ended
	Jun. 30, 2015 USD ($) contract	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Derivative [Line Items]
Notes payable			$ 21,356,573,000	$ 24,668,490,000
Payments to enter into derivative instruments			929,000	0	$ 2,936,000
Proceeds (payments) to terminate and or amend derivative instruments			$ (30,400,000)	$ 4,000,000
Interest Rate Cap [Member]
Derivative [Line Items]
Payments to enter into derivative instruments	$ 2,900,000
Derivative, Number Of Contracts | contract	2
Notional amount	$ 275,000,000.0
Proceeds (payments) to terminate and or amend derivative instruments		$ 913,000
Interest Rate Cap [Member] | Private Loan Warehouse Total [Member]
Derivative [Line Items]
Notes payable	275,000,000
Interest Rate Cap 1 [Member] | Interest Rate Cap [Member]
Derivative [Line Items]
Notional amount	$ 125,000,000.0
Derivative cap interest rate	2.50%
Interest Rate Cap 2 [Member] | Interest Rate Cap [Member]
Derivative [Line Items]
Notional amount	$ 150,000,000.0
Derivative cap interest rate	4.99%
2017 Interest Rate Cap [Member] | Interest Rate Cap [Member]
Derivative [Line Items]
Payments to enter into derivative instruments		$ 929,000